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Somatosensory Evoked Potentials (SSEP)

2024-10-04T01:47:29Z

Wdoyon: /* Intraoperative Monitoring */

Somatosensory evoked potentials (SSEP) are recorded from the central nervous system following stimulation of peripheral nerves.

== Introduction==

Somatosensory Evoked Potentials (SSEPs) are electric signals recorded from the scalp or spine following stimulation to the peripheral nerves. They are time-locked responses, representing the function of the ascending sensory pathways. Early in the 1960s Larson et al introduced the use of somatosensory evoked potentials to monitor neural structure during neurosurgical procedures. It was utilized as a supplement to the wake-up test during correctional spinal surgeries for spinal deformities such as scoliosis to provide warning of compromised spinal cord function to the spine surgeons, as reported by McCallum et al and Nash et al in the 1970s. Since then SSEP has become one of the earliest and primary tools for intraoperative neurophysiological monitoring.

== Somatosensory Pathways ==
[[Image:Ascending Pathways.jpg|right|100px|200px]]

Distal peripheral nerves are stimulated for SSEP recordings; typically the median or ulnar nerve at the wrist for upper extremity SSEPs, or posterior tibial nerve at the ankle for lower extremity SSEPs. The ascending sensory volley enters the spinal cord through dorsal nerve roots. Multiple dorsal or posterior column spinal pathways such as the gracile fasciculus (legs and trunk) and the cuneate fasciculus (arms and trunk) mediate the SSEP responses. They arrive at the medulla and make synaptic connection at the medullary nuclei (nucleus cuneatus and nucleus gracilis). From there they cross and ascend in the medial lemniscal pathways to thalamic nuclei, which in turn project up to the somatosenory cortex.

There is no synapses between the peripheral stimulation sites and the medullary nuclei. Synapses are the sites of action for inhalational anesthetic agents. Therefore any SSEP responses recorded below the level of medullary nuclei are affected only minimally by general anesthesia. The cortical SSEP responses, however, are greatly affected by inhalational agents; whichever recorded anesthetic management is very much required.

== Stimulation ==
#'''Stimulation Sites''': SSEPs may be elicited by electrical stimulation to major nerve trunks or dermatomes. Upper extremity mixed or major nerve SSEPs are typically obtained by stimulating the median nerve or ulnar nerve near the wrist; or sometimes the ulnar nerve at the elbow. Lower extremity SSEPs are normally recorded to stimulation of the posterior tibial nerve at the ankle, or peroneal nerve near the head of the fibula at the knee. Usually the anode electrode should be placed 2-4 cm distal to the cathode electrode to avoid anodal block.
#'''Electrodes''': Subdermal needle electrodes or adhesive surface electrodes are most commonly used for intraoperative SSEP recordings. Some other types of electrodes such as bar electrodes and EEG metal disc electrodes are more suitable for diagnostic SSEP recordings in the lab. Needle electrodes can be placed closer to the underlying nerves than surface electrodes; however they are associated with additional risks of infection, bleeding, and inadvertent needle sticks.
#'''Constant Current vs Constant Voltage''': Constant current stimulation is recommended for consistent and reliable stimulation for optimal SSEP recording in the OR, especially for long surgical procedures. Constant current stimuli may compensate for any changes in electrical conductivity, or electrode contact resistance.
#'''Stimulation Parameters''': a series of rectangular pulses with certain pulse width and frequency are typically used as electrical stimuli for SSEP recording.
##'''Pulse width (or pulse duration)''': 200-300 microsecond is suggested
##'''Frequency (or rep rate)''': Higher stimulus rate is desired for quick acquisition of SSEP responses in the operating room. However increasing the rate too high may result in degradation of the responses; also it is limited when interleaving stimulation of 2 or 4 limbs. In general a frequency between 2 and 5 Hz are recommended. To avoid synchronization between the responses and the underlying electrical noise, most commonly 60Hz or 50Hz in different countries, the stimulus rate should not be a submultiple of the noise frequency. Sometimes a slight change of stimulus rate, for example from 4.80 to 4.13, may improve the quality of the evoked responses.
##'''Intensity''': Supramaximal stimulation should be used to produce repeatable responses. Some factors, such as pathology of the peripheral nerves, large or edematous extremities, distance of the electrodes to the underlying nerves, types of stimulating electrodes, may limit the effectiveness of stimulation. A stimulus of 50mA or greater is required sometimes. There is concern of tissue damage from high current densities at the stimulation site which is rare.

== Recording Techniques ==

#'''Electrodes''': Metal surface cup electrodes are non-invasive electrodes, which can be applied with conductive gel after skin preparation. Subdermal needles are also commonly used in the operating room; some are available with angled or hooked tips. A corkscrew version can be screwed into the scalp for prolonged recording, however excessive bleeding can be a concern. For direct cortical mapping and recording, a strip or grid electrode array can be used for cranial surgeries.
#'''Recording Sites'''
##Cortical recording of upper extremity SSEP: It is the post-central gyrus of somatosensory cortex, contralateral to the stimulated limb. The locations are called CP3 and CP4 which are 2 cm posterior to the C3 and C4 positions of the 10-20 International System of EEG electrode placement. The recording montage can be CP3-Fz or CP3-CP4 for right arm stimulation; CP4-Fz or CP4-CP3 for left arm stimulation.
##Cortical recording of lower extremity SSEP: CPz is the active electrode site, which is 2 cm posterior to Cz. The traditional derivation of recording is CPz-Fz; however it should not be the only standard recording derivation, according to studies for optimized lower extremity SSEP recording by MacDonald DB et al and many others. Often time CPz-CPc (which is CPz-CP4 for left leg stimulation, and CPz-CP3 for right) may produce higher amplitude and more reliable signals.
##Subcortical: The recording can be made at posterior cervical spine, one or linked earlobes, or mastoid. Subcortical responses are less affected by inhalational agents. However, the subcortical SSEP response is normally very well defined for upper extremity stimulation, it is often poorly defined for lower extremity stimulation.
##Peripheral nerve: Usually it is the ipsilateral Erb's point for upper extremity stimulation, and ipsilateral popliteal fossa for lower extremity stimulation. This is done to verify the status of the peripheral stimulation.
#'''Averaging:''' The SSEP amplitudes tend to be low; as high as only several microvolts, or as low as less than a microvolts especially with pathological subjects. Averaging is required to record the signal against biological and ambient noise. SSEP signal is time-locked to the stimulus and most of the noise occurs randomly, allowing the noise to be averaged out with averaging of repeated responses. Some earlier guidelines such as that from American EEG Society suggested acquiring 500-2000 trails per averaged response. However for surgical monitoring well defined evoked potentials should be acquired as quickly as possible. With good preparation of the recording sites to reduce impedance, and optimized recording montages, clean signals could be recorded as few as 50-200 trials.
#'''Recording Parameters'''
##Filters: More environmental noise in the operating room and necessity of quick acquisition make it important to choose optimal filter settings, different from laboratory SSEP diagnostic studies. The majority of the energy contained in cortical SSEP is present in the frequency bandpass above 30Hz and below 500Hz; filters can be set to (10-30) Hz to (250-1000) Hz. The relative frequency content of the subcortical or peripheral responses is much higher, thus the filters can be set to (30-100)Hz to (500-2000) Hz.
##Timebase: The timebase is usually set at 50 milliseconds for upper extremity SSEPs, and 100 milliseconds for upper extremity SSEPs. It is based on the normal conduction time between the stimulation and recording sites. It may need adjustment depending on the age and size of the individual, and any pathological conditions.
##Sensitivity: The median amplitude of SSEP is about 1 microvolt. The recording sensitivity could range from 0.1 to 5 microvolts/unit. With direct cortical recording during cranial surgeries, the amplitude can be high and the sensitivity my be set to 20-50 microvolts/unit.
##Sweep delay: Normally it should be avoided to use sweep delay. However it may be necessary when large stimulus artifact is present.
##Interleaving recording: Alternating recording of SSEP responses to stimulation of 2 or 4 limbs at the same time is available with modern recording devices from different manufacturers.

== Waveforms ==
'''Upper Extremity SSEPs'''
#N9: Recorded from ipsilateral Erb's point, and sometimes it's called EP. It is a compound action potential from the axons to the median nerve or ulnar nerve stimulation.
#N13-P14: Cervical potentials, recorded near C5 spinal process. The origin is thought to be dorsal horn neurons, and the caudal medial lemniscus.
#N20-P23: Scalp potentials, generated from the thalamocortical radiations.

'''Lower Extremity SSEPs'''
#LP: Recorded from the T12 spinal process, arise from the afferent nerve volley in the dorsal roots and dorsal root entry zone.
#N34: Recorded at the cervical spine; the amplitude can be very low and difficult to identify. The origin is thought to be brainstem or perhaps thalamus.
#P37-N45: Scalp potentials, reflect activation of the primary somatosensory cortex.

[[Image:SSEP Waveforms.jpg|center|2000px|750px]]

== Intraoperative Monitoring ==
The basic principle of intraoperative monitoring with mixed nerve SSEP is to stimulate nerves distal to the surgical site, and to record responses proximal to the surgical site. In most cases, these recording sites should include one cortical and one subcortical recording site.

#'''Data acquisition''': Baseline SSEP responses should be established, ideally after induction and before patient positioning and surgical incision. Data should be collected throughout the surgical procedure, with corresponding documentation of surgical events such as incision, exposure, decompression, instrumentation, closure, etc. In addition, relevant physiological variables such as blood pressure and temperature changes, anesthetic agents being used and the levels, significant signal changes during the surgery and any communication and interventions should be documented.
#'''Alert criteria''': Significant signal changes, with 50% decrease of peak to peak amplitude and 10% increase of latency, may be indication of nervous system functional changes and should be reported to the operating surgeon, and call for possible intervention.

Depending on the specific surgery, planning is required to monitor the part of the nervous system at risk.
#'''Peripheral nerve and plexus.'''

As mentioned above, the lower extremity somatosensory evoked potentials are typically monitored via the posterior tibial nerve. The tibial nerve is a major component of the sciatic nerve and originates from the L4-S3 spinal nerve roots. The tibial nerve provides sensory information from the posterolateral leg, lateral foot and the bottom of the foot.

Saphenous nerve SSEPs are useful for monitoring the femoral nerve during transpsoas (lateral) lumbar fusions (Silverstein et al., 2014; Spine). The femoral nerve and lumbar plexus are at risk for injury with the transpsoas approach because of the lateral angle of the approach and because of nerve compression from the retractors. The femoral nerve is the major branch of the lumbar plexus and originates from the L2-4 spinal nerve roots; and the saphenous nerve is a sensory branch of the femoral nerve and innervates the medial side of the leg and foot. The saphenous nerve is not superficial and is surrounded by the vastus medialis and sartorius on the medial part of the thigh. Therefore, it is advised to use needle electrodes for deeper stimulation through the tissue. Higher current intensities and pulse durations relative to posterior tibial nerve SSEPs may be required for a good signal to noise ratio. The cortical responses are likely to be shorter in latency compared to posterior tibial nerve SSEPs.

Superficial peroneal nerve SSEPs have been used to monitor lower lumbar nerve root dysfunction during L4-5 lumbar decompressions (Yue and Martinez, Spine J.). The superficial peroneal nerve is one of two branches of the common peroneal nerve and originates from the L4-S1 spinal nerve roots. The superficial peroneal nerve provides sensory innervation from the anterolateral aspect of the lower leg and foot. Yue and Martinez showed that superficial peroneal nerve SSEPs were more sensitive than posterior tibial nerve SSEPs at detecting L4-5 nerve root dysfunction.

#'''Spinal nerve root'''
#'''Spinal cord'''
#'''Brainstem and thalamus'''
#'''Brain'''

== Systemic and Anesthesia Factors ==
#'''Temperature'''. The temperature of the patient can increase or decrease SSEP latencies but does not necessarily affect amplitudes.
#'''Blood pressure'''. An intraoperative decline in blood pressure is associated with a loss in SSEP signals.
#'''Halogenated inhalational agents'''. Higher levels of gas anesthesia (> Half MAC) can increase SSEP latencies and reduce SSEP amplitudes.
#'''Nitrous oxide'''. Nitrous oxide decreases SSEP amplitudes but has no effect on latencies.
#'''Intravenous analgesic agents'''. Intravenous anesthetics, such as propofol, do significantly influence SSEP latencies or amplitudes.
#'''Muscle relaxants'''. Neuromuscular blockers do not influence SSEP latencies or amplitudes.

==References==
# Brown RH, Nash CL, Berilla JA, Amaddio MD. Cortical evoked potential monitoring. A system for intraoperative monitoring of spinal cord function. Spine 1984; 9: 256-261.
# Celesia GG. Somatosensory evoked potentials recorded directly from human thalamus and Sm I cortical area. Arch Neurol 1979; 36: 399-405.
# Cohen AR, Young W, Ransohoff J. Intraspinal localization of the somatosensory evoked potential. Neurosurgery 1981; 9: 157-62.
# Kelly DL Jr, Goldring S, O’Leary JL. Averaged evoked somatosensory responses from exposed cortex of man. Arch Neurol 1965; 13: 1-9.
# Larson SJ, Sances A. Evoked potentials in man: neurosurgical applications. Am J Surg 1966; 111: 857-861.
# MacDonald DB. Individually optimizing posterior tibial somatosensory evoked potential P37 scalp derivations for intraoperative monitoring. J Clin Neurophysiol. 2001 Jul;18(4):364-71.
# Mauguiere F. Anatomic origin of the cervical N13 potential evoked by upper extremity stimulation. J of Clin Neurophysiology 2000; 17: 236-245.
# McCallum JE, Bennett MH. Electrophysiologic monitoring of spinal cord function during intraspinal surgery. Surg Forum 1975; 26: 469-471.
# Nash CL, Long RA, Schatzinger LA, Brown RH. Spinal cord monitoring during operative treatment of the spine. Clin Orthop 1977; 126: 100-105.
# Nuwer MR, Dawson EG, Carlson LG, Kanim LEA, Sherman JE. Somatosensory evoked potential spinal cord monitoring reduces neurologic deficits after scoliosis surgery: results of a large multicenter survey. Electroencepholog and Clin Neurophysiol 1995; 96: 6-11.
# Nuwer MR, et al. IFCN recommended standards for short latency somatosensory evoked potentials. Report of an IFCN committee. Electroencephalography and Clinical Neurophysiology 1994; 91: 6-11.
# Powers SK, Bolger CA, Edwards MSB. Spinal cord pathways mediating somatosensory evoked potentials. J Neurosurg 1982; 57: 472-82.
# Simpson RK JR, Blackburn JG, Martin HF III, Katz S. Peripheral nerve fibers and spinal cord pathway contribution to the somatosensory evoked potentials. Exp Neurol 1981; 73: 700-15.
# Sloan TB. Anesthetic Effects on Electrophysiologic Recordings. Journal of Clinical Neurophysiology 1998; 15: 217-226.
# Toleikis JR. Intraoperative monitoring using somatosensory evoked potentials: A position statement by the American Society of Neurophysiological Monitoring. J Clin Monit Comput. 2005; 19(3):241-58.
# Wang BP, Turner LA, Kamp AM, Venier LH. CPz-CP4 and CPz-CP3 are superior to CPz-FPz for recording left and right tibial nerve somatosensory evoked potentials for intraoperative monitoring. A review study in 264 patients. 2006 The 17th Annual Meeting of American Society of Neurophysiological Monitoring.
# Yanni DS, Ulkatan S, Deletis V, Barrenechea IJ, Sen C, Perin NI. Utility of neurophysiological monitoring using dorsal column mapping in intramedullary spinal cord surgery. J Neurosurg Spine. 2010;12:623-628.
# York DH, Chabot RJ, Gaines RW. Response variability of somatosensory evoked potentials during scoliosis surgery. Spine 1987; 12: 864-876.

Neurophysiological Diagnostics

2024-09-25T23:20:18Z

Wdoyon: /* Spinal nerve roots and radiculopathy */

==Introduction==

==Spinal nerve roots and radiculopathy==

'''Upper extremities'''

Nerve conduction studies. Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

Signs of cervical radiculopathy include a positive shoulder abduction test. This test is done by putting the hand of the patient above the shoulder, usually above the head, which will relieve the pain.

'''Lower extremities'''

The bulbocavernosus reflex or Osinski reflex can be monitored to test for S2-4 nerve root function. The bulbocavernosus reflex
is a reflex arc. The dorsal nerve (a branch of the pudendal nerve) carries sensory information from the genitals to the spinal cord via S2-4 nerve roots. Some fibers of the dorsal nerve synapse onto interneurons of the spinal cord that make synaptic connections with motor neurons that innervate the bulbocavernosus muscle, causing the rectum to contract. This pathway can be monitored by placing stimulating electrodes along the sensory nerve of the genitals and recording electrodes on the rectum.

==Spinal cord and myelopathy==
Signs of cervical myelopathy include a positive Hoffman's sign, Babinski reflex, clonus sign, finger escape sign, or Lhermitte's sign. Individually, none of these signs can be used as a sole predictor of myelopathy, but they do provide evidence to make a diagnosis.

1. Hoffman's sign is a reflex test that is an involuntary movement of the thumb or index finger when the nail of the middle finger is flicked. It is a sign of upper motor neuron disease.

2. Lhermitte's sign is produced by neck movement or forward flexion, causing a transient feeling of electric shock down the spine or extremities.

3. Clonus sign

Signs of cervical radiculopathy include a positive shoulder abduction test. This test is done by putting the hand of the patient above the shoulder, usually above the head, which will relieve the pain.

==Epilepsy==

==Sleep disorders==

==Movement disorders==

==Neuromuscular disorders==

==Others==

==References==

Neurophysiological Diagnostics

2024-09-25T20:51:11Z

Wdoyon: /* Spinal cord and myelopathy */

==Introduction==

==Spinal nerve roots and radiculopathy==

'''Upper extremities'''

Nerve conduction studies. Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''Lower extremities'''

The bulbocavernosus reflex or Osinski reflex can be monitored to test for S2-4 nerve root function. The bulbocavernosus reflex
is a reflex arc. The dorsal nerve (a branch of the pudendal nerve) carries sensory information from the genitals to the spinal cord via S2-4 nerve roots. Some fibers of the dorsal nerve synapse onto interneurons of the spinal cord that make synaptic connections with motor neurons that innervate the bulbocavernosus muscle, causing the rectum to contract. This pathway can be monitored by placing stimulating electrodes along the sensory nerve of the genitals and recording electrodes on the rectum.

==Spinal cord and myelopathy==
Signs of cervical myelopathy include a positive Hoffman's sign, Babinski reflex, clonus sign, finger escape sign, or Lhermitte's sign. Individually, none of these signs can be used as a sole predictor of myelopathy, but they do provide evidence to make a diagnosis.

1. Hoffman's sign is a reflex test that is an involuntary movement of the thumb or index finger when the nail of the middle finger is flicked. It is a sign of upper motor neuron disease.

2. Lhermitte's sign is produced by neck movement or forward flexion, causing a transient feeling of electric shock down the spine or extremities.

3. Clonus sign

Signs of cervical radiculopathy include a positive shoulder abduction test. This test is done by putting the hand of the patient above the shoulder, usually above the head, which will relieve the pain.

==Epilepsy==

==Sleep disorders==

==Movement disorders==

==Neuromuscular disorders==

==Others==

==References==

Neurophysiological Diagnostics

2024-09-25T20:49:42Z

Wdoyon: /* Spinal cord and myelopathy */

==Introduction==

==Spinal nerve roots and radiculopathy==

'''Upper extremities'''

Nerve conduction studies. Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''Lower extremities'''

The bulbocavernosus reflex or Osinski reflex can be monitored to test for S2-4 nerve root function. The bulbocavernosus reflex
is a reflex arc. The dorsal nerve (a branch of the pudendal nerve) carries sensory information from the genitals to the spinal cord via S2-4 nerve roots. Some fibers of the dorsal nerve synapse onto interneurons of the spinal cord that make synaptic connections with motor neurons that innervate the bulbocavernosus muscle, causing the rectum to contract. This pathway can be monitored by placing stimulating electrodes along the sensory nerve of the genitals and recording electrodes on the rectum.

==Spinal cord and myelopathy==
Signs of cervical myelopathy include a positive Hoffman's sign, Babinski reflex, clonus sign, finger escape sign, or Lhermitte's sign. Individually, none of these signs can be used as a sole predictor of myelopathy, but they do provide evidence to make a diagnosis.

1. Hoffman's sign is a reflex test that is an involuntary movement of the thumb or index finger when the nail of the middle finger is flicked. It is a sign of upper motor neuron disease.
2. Lhermitte's sign is produced by neck movement or forward flexion, causing a transient feeling of electric shock down the spine or extremities.

Signs of cervical radiculopathy include a positive shoulder abduction test. This test is done by putting the hand of the patient above the shoulder, usually above the head, which will relieve the pain.

==Epilepsy==

==Sleep disorders==

==Movement disorders==

==Neuromuscular disorders==

==Others==

==References==

Neurophysiological Diagnostics

2024-09-25T20:48:39Z

Wdoyon: /* Spinal cord and myelopathy */

==Introduction==

==Spinal nerve roots and radiculopathy==

'''Upper extremities'''

Nerve conduction studies. Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''Lower extremities'''

The bulbocavernosus reflex or Osinski reflex can be monitored to test for S2-4 nerve root function. The bulbocavernosus reflex
is a reflex arc. The dorsal nerve (a branch of the pudendal nerve) carries sensory information from the genitals to the spinal cord via S2-4 nerve roots. Some fibers of the dorsal nerve synapse onto interneurons of the spinal cord that make synaptic connections with motor neurons that innervate the bulbocavernosus muscle, causing the rectum to contract. This pathway can be monitored by placing stimulating electrodes along the sensory nerve of the genitals and recording electrodes on the rectum.

==Spinal cord and myelopathy==
Signs of cervical myelopathy include a positive Hoffman's sign, Babinski reflex, clonus sign, finger escape sign, or Lhermitte's sign. Individually, none of these signs can be used as a sole predictor of myelopathy, but they do provide evidence to make a diagnosis.
1. Hoffman's sign is a reflex test that is an involuntary movement of the thumb or index finger when the nail of the middle finger is flicked. It is a sign of upper motor neuron disease.
1. Lhermitte's sign is produced by neck movement or flexion, causing a transient feeling of electric shock down the spine or extremities.

Signs of cervical radiculopathy include a positive shoulder abduction test. This test is done by putting the hand of the patient above the shoulder, usually above the head, which will relieve the pain.

==Epilepsy==

==Sleep disorders==

==Movement disorders==

==Neuromuscular disorders==

==Others==

==References==

Somatosensory Evoked Potentials (SSEP)

2024-09-18T18:13:09Z

Wdoyon: /* Intraoperative Monitoring */

Somatosensory evoked potentials (SSEP) are recorded from the central nervous system following stimulation of peripheral nerves.

== Introduction==

Somatosensory Evoked Potentials (SSEPs) are electric signals recorded from the scalp or spine following stimulation to the peripheral nerves. They are time-locked responses, representing the function of the ascending sensory pathways. Early in the 1960s Larson et al introduced the use of somatosensory evoked potentials to monitor neural structure during neurosurgical procedures. It was utilized as a supplement to the wake-up test during correctional spinal surgeries for spinal deformities such as scoliosis to provide warning of compromised spinal cord function to the spine surgeons, as reported by McCallum et al and Nash et al in the 1970s. Since then SSEP has become one of the earliest and primary tools for intraoperative neurophysiological monitoring.

== Somatosensory Pathways ==
[[Image:Ascending Pathways.jpg|right|100px|200px]]

Distal peripheral nerves are stimulated for SSEP recordings; typically the median or ulnar nerve at the wrist for upper extremity SSEPs, or posterior tibial nerve at the ankle for lower extremity SSEPs. The ascending sensory volley enters the spinal cord through dorsal nerve roots. Multiple dorsal or posterior column spinal pathways such as the gracile fasciculus (legs and trunk) and the cuneate fasciculus (arms and trunk) mediate the SSEP responses. They arrive at the medulla and make synaptic connection at the medullary nuclei (nucleus cuneatus and nucleus gracilis). From there they cross and ascend in the medial lemniscal pathways to thalamic nuclei, which in turn project up to the somatosenory cortex.

There is no synapses between the peripheral stimulation sites and the medullary nuclei. Synapses are the sites of action for inhalational anesthetic agents. Therefore any SSEP responses recorded below the level of medullary nuclei are affected only minimally by general anesthesia. The cortical SSEP responses, however, are greatly affected by inhalational agents; whichever recorded anesthetic management is very much required.

== Stimulation ==
#'''Stimulation Sites''': SSEPs may be elicited by electrical stimulation to major nerve trunks or dermatomes. Upper extremity mixed or major nerve SSEPs are typically obtained by stimulating the median nerve or ulnar nerve near the wrist; or sometimes the ulnar nerve at the elbow. Lower extremity SSEPs are normally recorded to stimulation of the posterior tibial nerve at the ankle, or peroneal nerve near the head of the fibula at the knee. Usually the anode electrode should be placed 2-4 cm distal to the cathode electrode to avoid anodal block.
#'''Electrodes''': Subdermal needle electrodes or adhesive surface electrodes are most commonly used for intraoperative SSEP recordings. Some other types of electrodes such as bar electrodes and EEG metal disc electrodes are more suitable for diagnostic SSEP recordings in the lab. Needle electrodes can be placed closer to the underlying nerves than surface electrodes; however they are associated with additional risks of infection, bleeding, and inadvertent needle sticks.
#'''Constant Current vs Constant Voltage''': Constant current stimulation is recommended for consistent and reliable stimulation for optimal SSEP recording in the OR, especially for long surgical procedures. Constant current stimuli may compensate for any changes in electrical conductivity, or electrode contact resistance.
#'''Stimulation Parameters''': a series of rectangular pulses with certain pulse width and frequency are typically used as electrical stimuli for SSEP recording.
##'''Pulse width (or pulse duration)''': 200-300 microsecond is suggested
##'''Frequency (or rep rate)''': Higher stimulus rate is desired for quick acquisition of SSEP responses in the operating room. However increasing the rate too high may result in degradation of the responses; also it is limited when interleaving stimulation of 2 or 4 limbs. In general a frequency between 2 and 5 Hz are recommended. To avoid synchronization between the responses and the underlying electrical noise, most commonly 60Hz or 50Hz in different countries, the stimulus rate should not be a submultiple of the noise frequency. Sometimes a slight change of stimulus rate, for example from 4.80 to 4.13, may improve the quality of the evoked responses.
##'''Intensity''': Supramaximal stimulation should be used to produce repeatable responses. Some factors, such as pathology of the peripheral nerves, large or edematous extremities, distance of the electrodes to the underlying nerves, types of stimulating electrodes, may limit the effectiveness of stimulation. A stimulus of 50mA or greater is required sometimes. There is concern of tissue damage from high current densities at the stimulation site which is rare.

== Recording Techniques ==

#'''Electrodes''': Metal surface cup electrodes are non-invasive electrodes, which can be applied with conductive gel after skin preparation. Subdermal needles are also commonly used in the operating room; some are available with angled or hooked tips. A corkscrew version can be screwed into the scalp for prolonged recording, however excessive bleeding can be a concern. For direct cortical mapping and recording, a strip or grid electrode array can be used for cranial surgeries.
#'''Recording Sites'''
##Cortical recording of upper extremity SSEP: It is the post-central gyrus of somatosensory cortex, contralateral to the stimulated limb. The locations are called CP3 and CP4 which are 2 cm posterior to the C3 and C4 positions of the 10-20 International System of EEG electrode placement. The recording montage can be CP3-Fz or CP3-CP4 for right arm stimulation; CP4-Fz or CP4-CP3 for left arm stimulation.
##Cortical recording of lower extremity SSEP: CPz is the active electrode site, which is 2 cm posterior to Cz. The traditional derivation of recording is CPz-Fz; however it should not be the only standard recording derivation, according to studies for optimized lower extremity SSEP recording by MacDonald DB et al and many others. Often time CPz-CPc (which is CPz-CP4 for left leg stimulation, and CPz-CP3 for right) may produce higher amplitude and more reliable signals.
##Subcortical: The recording can be made at posterior cervical spine, one or linked earlobes, or mastoid. Subcortical responses are less affected by inhalational agents. However, the subcortical SSEP response is normally very well defined for upper extremity stimulation, it is often poorly defined for lower extremity stimulation.
##Peripheral nerve: Usually it is the ipsilateral Erb's point for upper extremity stimulation, and ipsilateral popliteal fossa for lower extremity stimulation. This is done to verify the status of the peripheral stimulation.
#'''Averaging:''' The SSEP amplitudes tend to be low; as high as only several microvolts, or as low as less than a microvolts especially with pathological subjects. Averaging is required to record the signal against biological and ambient noise. SSEP signal is time-locked to the stimulus and most of the noise occurs randomly, allowing the noise to be averaged out with averaging of repeated responses. Some earlier guidelines such as that from American EEG Society suggested acquiring 500-2000 trails per averaged response. However for surgical monitoring well defined evoked potentials should be acquired as quickly as possible. With good preparation of the recording sites to reduce impedance, and optimized recording montages, clean signals could be recorded as few as 50-200 trials.
#'''Recording Parameters'''
##Filters: More environmental noise in the operating room and necessity of quick acquisition make it important to choose optimal filter settings, different from laboratory SSEP diagnostic studies. The majority of the energy contained in cortical SSEP is present in the frequency bandpass above 30Hz and below 500Hz; filters can be set to (10-30) Hz to (250-1000) Hz. The relative frequency content of the subcortical or peripheral responses is much higher, thus the filters can be set to (30-100)Hz to (500-2000) Hz.
##Timebase: The timebase is usually set at 50 milliseconds for upper extremity SSEPs, and 100 milliseconds for upper extremity SSEPs. It is based on the normal conduction time between the stimulation and recording sites. It may need adjustment depending on the age and size of the individual, and any pathological conditions.
##Sensitivity: The median amplitude of SSEP is about 1 microvolt. The recording sensitivity could range from 0.1 to 5 microvolts/unit. With direct cortical recording during cranial surgeries, the amplitude can be high and the sensitivity my be set to 20-50 microvolts/unit.
##Sweep delay: Normally it should be avoided to use sweep delay. However it may be necessary when large stimulus artifact is present.
##Interleaving recording: Alternating recording of SSEP responses to stimulation of 2 or 4 limbs at the same time is available with modern recording devices from different manufacturers.

== Waveforms ==
'''Upper Extremity SSEPs'''
#N9: Recorded from ipsilateral Erb's point, and sometimes it's called EP. It is a compound action potential from the axons to the median nerve or ulnar nerve stimulation.
#N13-P14: Cervical potentials, recorded near C5 spinal process. The origin is thought to be dorsal horn neurons, and the caudal medial lemniscus.
#N20-P23: Scalp potentials, generated from the thalamocortical radiations.

'''Lower Extremity SSEPs'''
#LP: Recorded from the T12 spinal process, arise from the afferent nerve volley in the dorsal roots and dorsal root entry zone.
#N34: Recorded at the cervical spine; the amplitude can be very low and difficult to identify. The origin is thought to be brainstem or perhaps thalamus.
#P37-N45: Scalp potentials, reflect activation of the primary somatosensory cortex.

[[Image:SSEP Waveforms.jpg|center|2000px|750px]]

== Intraoperative Monitoring ==
The basic principle of intraoperative monitoring with mixed nerve SSEP is to stimulate nerves distal to the surgical site, and to record responses proximal to the surgical site. In most cases, these recording sites should include one cortical and one subcortical recording site.

#'''Data acquisition''': Baseline SSEP responses should be established, ideally after induction and before patient positioning and surgical incision. Data should be collected throughout the surgical procedure, with corresponding documentation of surgical events such as incision, exposure, decompression, instrumentation, closure, etc. In addition, relevant physiological variables such as blood pressure and temperature changes, anesthetic agents being used and the levels, significant signal changes during the surgery and any communication and interventions should be documented.
#'''Alert criteria''': Significant signal changes, with 50% decrease of peak to peak amplitude and 10% increase of latency, may be indication of nervous system functional changes and should be reported to the operating surgeon, and call for possible intervention.

Depending on the specific surgery, planning is required to monitor the part of the nervous system at risk.
#'''Peripheral nerve and plexus.'''

As mentioned above, the lower extremity somatosensory evoked potentials are typically monitored via the posterior tibial nerve. The tibial nerve is a major component of the sciatic nerve and originates from the L4-S3 spinal nerve roots. The tibial nerve provides sensory information from the posterolateral leg, lateral foot and the bottom of the foot.

Saphenous nerve SSEPs are useful for monitoring the femoral nerve during transpsoas (lateral) lumbar fusions (Silverstein et al., 2014; Spine). The femoral nerve and lumbar plexus are at risk for injury with the transpsoas approach because of the lateral approach itself as well as retractor related nerve compression. The femoral nerve is the major branch of the lumbar plexus and originates from the L2-4 spinal nerve roots. The saphenous nerve is a sensory branch of the femoral nerve and innervates the medial side of the leg and foot. The saphenous nerve can be found between the vastus medialis and sartorius on the medial part of the thigh. Because it is surrounded by these large muscles and is not superficial, it is best to use needle electrodes for stimulation. Higher current intensities and pulse durations relative to posterior tibial nerve SSEPs may be required for a good signal to noise ratio. The cortical responses should be shorter in latency compared to posterior tibial nerve SSEPs.

Superficial peroneal nerve SSEPs have been used to monitor lower lumbar nerve root dysfunction during L4-5 lumbar decompressions (Yue and Martinez, Spine J.). The superficial peroneal nerve is one of two branches of the common peroneal nerve and originates from the L4-S1 spinal nerve roots. The superficial peroneal nerve provides sensory innervation from the anterolateral aspect of the lower leg and foot. Yue and Martinez showed that superficial peroneal nerve SSEPs were more sensitive than posterior tibial nerve SSEPs at detecting L4-5 nerve root dysfunction.

#'''Spinal nerve root'''
#'''Spinal cord'''
#'''Brainstem and thalamus'''
#'''Brain'''

== Systemic and Anesthesia Factors ==
#'''Temperature'''. The temperature of the patient can increase or decrease SSEP latencies but does not necessarily affect amplitudes.
#'''Blood pressure'''. An intraoperative decline in blood pressure is associated with a loss in SSEP signals.
#'''Halogenated inhalational agents'''. Higher levels of gas anesthesia (> Half MAC) can increase SSEP latencies and reduce SSEP amplitudes.
#'''Nitrous oxide'''. Nitrous oxide decreases SSEP amplitudes but has no effect on latencies.
#'''Intravenous analgesic agents'''. Intravenous anesthetics, such as propofol, do significantly influence SSEP latencies or amplitudes.
#'''Muscle relaxants'''. Neuromuscular blockers do not influence SSEP latencies or amplitudes.

==References==
# Brown RH, Nash CL, Berilla JA, Amaddio MD. Cortical evoked potential monitoring. A system for intraoperative monitoring of spinal cord function. Spine 1984; 9: 256-261.
# Celesia GG. Somatosensory evoked potentials recorded directly from human thalamus and Sm I cortical area. Arch Neurol 1979; 36: 399-405.
# Cohen AR, Young W, Ransohoff J. Intraspinal localization of the somatosensory evoked potential. Neurosurgery 1981; 9: 157-62.
# Kelly DL Jr, Goldring S, O’Leary JL. Averaged evoked somatosensory responses from exposed cortex of man. Arch Neurol 1965; 13: 1-9.
# Larson SJ, Sances A. Evoked potentials in man: neurosurgical applications. Am J Surg 1966; 111: 857-861.
# MacDonald DB. Individually optimizing posterior tibial somatosensory evoked potential P37 scalp derivations for intraoperative monitoring. J Clin Neurophysiol. 2001 Jul;18(4):364-71.
# Mauguiere F. Anatomic origin of the cervical N13 potential evoked by upper extremity stimulation. J of Clin Neurophysiology 2000; 17: 236-245.
# McCallum JE, Bennett MH. Electrophysiologic monitoring of spinal cord function during intraspinal surgery. Surg Forum 1975; 26: 469-471.
# Nash CL, Long RA, Schatzinger LA, Brown RH. Spinal cord monitoring during operative treatment of the spine. Clin Orthop 1977; 126: 100-105.
# Nuwer MR, Dawson EG, Carlson LG, Kanim LEA, Sherman JE. Somatosensory evoked potential spinal cord monitoring reduces neurologic deficits after scoliosis surgery: results of a large multicenter survey. Electroencepholog and Clin Neurophysiol 1995; 96: 6-11.
# Nuwer MR, et al. IFCN recommended standards for short latency somatosensory evoked potentials. Report of an IFCN committee. Electroencephalography and Clinical Neurophysiology 1994; 91: 6-11.
# Powers SK, Bolger CA, Edwards MSB. Spinal cord pathways mediating somatosensory evoked potentials. J Neurosurg 1982; 57: 472-82.
# Simpson RK JR, Blackburn JG, Martin HF III, Katz S. Peripheral nerve fibers and spinal cord pathway contribution to the somatosensory evoked potentials. Exp Neurol 1981; 73: 700-15.
# Sloan TB. Anesthetic Effects on Electrophysiologic Recordings. Journal of Clinical Neurophysiology 1998; 15: 217-226.
# Toleikis JR. Intraoperative monitoring using somatosensory evoked potentials: A position statement by the American Society of Neurophysiological Monitoring. J Clin Monit Comput. 2005; 19(3):241-58.
# Wang BP, Turner LA, Kamp AM, Venier LH. CPz-CP4 and CPz-CP3 are superior to CPz-FPz for recording left and right tibial nerve somatosensory evoked potentials for intraoperative monitoring. A review study in 264 patients. 2006 The 17th Annual Meeting of American Society of Neurophysiological Monitoring.
# Yanni DS, Ulkatan S, Deletis V, Barrenechea IJ, Sen C, Perin NI. Utility of neurophysiological monitoring using dorsal column mapping in intramedullary spinal cord surgery. J Neurosurg Spine. 2010;12:623-628.
# York DH, Chabot RJ, Gaines RW. Response variability of somatosensory evoked potentials during scoliosis surgery. Spine 1987; 12: 864-876.

Somatosensory Evoked Potentials (SSEP)

2024-09-18T18:09:44Z

Wdoyon: /* Intraoperative Monitoring */

Somatosensory evoked potentials (SSEP) are recorded from the central nervous system following stimulation of peripheral nerves.

== Introduction==

Somatosensory Evoked Potentials (SSEPs) are electric signals recorded from the scalp or spine following stimulation to the peripheral nerves. They are time-locked responses, representing the function of the ascending sensory pathways. Early in the 1960s Larson et al introduced the use of somatosensory evoked potentials to monitor neural structure during neurosurgical procedures. It was utilized as a supplement to the wake-up test during correctional spinal surgeries for spinal deformities such as scoliosis to provide warning of compromised spinal cord function to the spine surgeons, as reported by McCallum et al and Nash et al in the 1970s. Since then SSEP has become one of the earliest and primary tools for intraoperative neurophysiological monitoring.

== Somatosensory Pathways ==
[[Image:Ascending Pathways.jpg|right|100px|200px]]

Distal peripheral nerves are stimulated for SSEP recordings; typically the median or ulnar nerve at the wrist for upper extremity SSEPs, or posterior tibial nerve at the ankle for lower extremity SSEPs. The ascending sensory volley enters the spinal cord through dorsal nerve roots. Multiple dorsal or posterior column spinal pathways such as the gracile fasciculus (legs and trunk) and the cuneate fasciculus (arms and trunk) mediate the SSEP responses. They arrive at the medulla and make synaptic connection at the medullary nuclei (nucleus cuneatus and nucleus gracilis). From there they cross and ascend in the medial lemniscal pathways to thalamic nuclei, which in turn project up to the somatosenory cortex.

There is no synapses between the peripheral stimulation sites and the medullary nuclei. Synapses are the sites of action for inhalational anesthetic agents. Therefore any SSEP responses recorded below the level of medullary nuclei are affected only minimally by general anesthesia. The cortical SSEP responses, however, are greatly affected by inhalational agents; whichever recorded anesthetic management is very much required.

== Stimulation ==
#'''Stimulation Sites''': SSEPs may be elicited by electrical stimulation to major nerve trunks or dermatomes. Upper extremity mixed or major nerve SSEPs are typically obtained by stimulating the median nerve or ulnar nerve near the wrist; or sometimes the ulnar nerve at the elbow. Lower extremity SSEPs are normally recorded to stimulation of the posterior tibial nerve at the ankle, or peroneal nerve near the head of the fibula at the knee. Usually the anode electrode should be placed 2-4 cm distal to the cathode electrode to avoid anodal block.
#'''Electrodes''': Subdermal needle electrodes or adhesive surface electrodes are most commonly used for intraoperative SSEP recordings. Some other types of electrodes such as bar electrodes and EEG metal disc electrodes are more suitable for diagnostic SSEP recordings in the lab. Needle electrodes can be placed closer to the underlying nerves than surface electrodes; however they are associated with additional risks of infection, bleeding, and inadvertent needle sticks.
#'''Constant Current vs Constant Voltage''': Constant current stimulation is recommended for consistent and reliable stimulation for optimal SSEP recording in the OR, especially for long surgical procedures. Constant current stimuli may compensate for any changes in electrical conductivity, or electrode contact resistance.
#'''Stimulation Parameters''': a series of rectangular pulses with certain pulse width and frequency are typically used as electrical stimuli for SSEP recording.
##'''Pulse width (or pulse duration)''': 200-300 microsecond is suggested
##'''Frequency (or rep rate)''': Higher stimulus rate is desired for quick acquisition of SSEP responses in the operating room. However increasing the rate too high may result in degradation of the responses; also it is limited when interleaving stimulation of 2 or 4 limbs. In general a frequency between 2 and 5 Hz are recommended. To avoid synchronization between the responses and the underlying electrical noise, most commonly 60Hz or 50Hz in different countries, the stimulus rate should not be a submultiple of the noise frequency. Sometimes a slight change of stimulus rate, for example from 4.80 to 4.13, may improve the quality of the evoked responses.
##'''Intensity''': Supramaximal stimulation should be used to produce repeatable responses. Some factors, such as pathology of the peripheral nerves, large or edematous extremities, distance of the electrodes to the underlying nerves, types of stimulating electrodes, may limit the effectiveness of stimulation. A stimulus of 50mA or greater is required sometimes. There is concern of tissue damage from high current densities at the stimulation site which is rare.

== Recording Techniques ==

#'''Electrodes''': Metal surface cup electrodes are non-invasive electrodes, which can be applied with conductive gel after skin preparation. Subdermal needles are also commonly used in the operating room; some are available with angled or hooked tips. A corkscrew version can be screwed into the scalp for prolonged recording, however excessive bleeding can be a concern. For direct cortical mapping and recording, a strip or grid electrode array can be used for cranial surgeries.
#'''Recording Sites'''
##Cortical recording of upper extremity SSEP: It is the post-central gyrus of somatosensory cortex, contralateral to the stimulated limb. The locations are called CP3 and CP4 which are 2 cm posterior to the C3 and C4 positions of the 10-20 International System of EEG electrode placement. The recording montage can be CP3-Fz or CP3-CP4 for right arm stimulation; CP4-Fz or CP4-CP3 for left arm stimulation.
##Cortical recording of lower extremity SSEP: CPz is the active electrode site, which is 2 cm posterior to Cz. The traditional derivation of recording is CPz-Fz; however it should not be the only standard recording derivation, according to studies for optimized lower extremity SSEP recording by MacDonald DB et al and many others. Often time CPz-CPc (which is CPz-CP4 for left leg stimulation, and CPz-CP3 for right) may produce higher amplitude and more reliable signals.
##Subcortical: The recording can be made at posterior cervical spine, one or linked earlobes, or mastoid. Subcortical responses are less affected by inhalational agents. However, the subcortical SSEP response is normally very well defined for upper extremity stimulation, it is often poorly defined for lower extremity stimulation.
##Peripheral nerve: Usually it is the ipsilateral Erb's point for upper extremity stimulation, and ipsilateral popliteal fossa for lower extremity stimulation. This is done to verify the status of the peripheral stimulation.
#'''Averaging:''' The SSEP amplitudes tend to be low; as high as only several microvolts, or as low as less than a microvolts especially with pathological subjects. Averaging is required to record the signal against biological and ambient noise. SSEP signal is time-locked to the stimulus and most of the noise occurs randomly, allowing the noise to be averaged out with averaging of repeated responses. Some earlier guidelines such as that from American EEG Society suggested acquiring 500-2000 trails per averaged response. However for surgical monitoring well defined evoked potentials should be acquired as quickly as possible. With good preparation of the recording sites to reduce impedance, and optimized recording montages, clean signals could be recorded as few as 50-200 trials.
#'''Recording Parameters'''
##Filters: More environmental noise in the operating room and necessity of quick acquisition make it important to choose optimal filter settings, different from laboratory SSEP diagnostic studies. The majority of the energy contained in cortical SSEP is present in the frequency bandpass above 30Hz and below 500Hz; filters can be set to (10-30) Hz to (250-1000) Hz. The relative frequency content of the subcortical or peripheral responses is much higher, thus the filters can be set to (30-100)Hz to (500-2000) Hz.
##Timebase: The timebase is usually set at 50 milliseconds for upper extremity SSEPs, and 100 milliseconds for upper extremity SSEPs. It is based on the normal conduction time between the stimulation and recording sites. It may need adjustment depending on the age and size of the individual, and any pathological conditions.
##Sensitivity: The median amplitude of SSEP is about 1 microvolt. The recording sensitivity could range from 0.1 to 5 microvolts/unit. With direct cortical recording during cranial surgeries, the amplitude can be high and the sensitivity my be set to 20-50 microvolts/unit.
##Sweep delay: Normally it should be avoided to use sweep delay. However it may be necessary when large stimulus artifact is present.
##Interleaving recording: Alternating recording of SSEP responses to stimulation of 2 or 4 limbs at the same time is available with modern recording devices from different manufacturers.

== Waveforms ==
'''Upper Extremity SSEPs'''
#N9: Recorded from ipsilateral Erb's point, and sometimes it's called EP. It is a compound action potential from the axons to the median nerve or ulnar nerve stimulation.
#N13-P14: Cervical potentials, recorded near C5 spinal process. The origin is thought to be dorsal horn neurons, and the caudal medial lemniscus.
#N20-P23: Scalp potentials, generated from the thalamocortical radiations.

'''Lower Extremity SSEPs'''
#LP: Recorded from the T12 spinal process, arise from the afferent nerve volley in the dorsal roots and dorsal root entry zone.
#N34: Recorded at the cervical spine; the amplitude can be very low and difficult to identify. The origin is thought to be brainstem or perhaps thalamus.
#P37-N45: Scalp potentials, reflect activation of the primary somatosensory cortex.

[[Image:SSEP Waveforms.jpg|center|2000px|750px]]

== Intraoperative Monitoring ==
The basic principle of intraoperative monitoring with mixed nerve SSEP is to stimulate nerves distal to the surgical site, and to record responses proximal to the surgical site. In most cases, these recording sites should include one cortical and one subcortical recording site.

#'''Data acquisition''': Baseline SSEP responses should be established, ideally after induction and before patient positioning and surgical incision. Data should be collected throughout the surgical procedure, with corresponding documentation of surgical events such as incision, exposure, decompression, instrumentation, closure, etc. In addition, relevant physiological variables such as blood pressure and temperature changes, anesthetic agents being used and the levels, significant signal changes during the surgery and any communication and interventions should be documented.
#'''Alert criteria''': Significant signal changes, with 50% decrease of peak to peak amplitude and 10% increase of latency, may be indication of nervous system functional changes and should be reported to the operating surgeon, and call for possible intervention.

Depending on the specific surgery, planning is required to monitor the part of the nervous system at risk.
#'''Peripheral nerve and plexus.'''

As mentioned above, the lower extremity somatosensory evoked potentials are typically monitored via the posterior tibial nerve. The tibial nerve is a major component of the sciatic nerve and originates from the L4-S3 spinal nerve roots. The tibial nerve provides sensory information from the posterolateral leg, lateral foot and the bottom of the foot.

Saphenous nerve SSEPs are useful for monitoring the femoral nerve during transpsoas (lateral) lumbar fusions (Silverstein et al., 2014; Spine). The femoral nerve and lumbar plexus are at risk for injury with the transpsoas approach because of the lateral approach itself as well as retractor related nerve compression. The femoral nerve is the major branch of the lumbar plexus and originates from the L2-4 spinal nerve roots. The saphenous nerve is a sensory branch of the femoral nerve and innervates the medial side of the leg and foot. The saphenous nerve can be found between the vastus medialis and sartorius on the medial part of the thigh. Because it is surrounded by these large muscles and is not superficial, it is best to use needle electrodes for stimulation. Higher current intensities and pulse durations relative to posterior tibial nerve SSEPs may be required for a good signal to noise ratio. The cortical responses should be shorter in latency compared to posterior tibial nerve SSEPs.

Superficial peroneal nerve SSEPs have been used to monitor lower lumbar nerve root dysfunction during L4-5 lumbar decompressions (Yue and Martinez, Spine J.). The superficial peroneal nerve is one of two branches of the common peroneal nerve and originates from the L4-S1 spinal nerve roots. The superficial peroneal nerve provides sensory innervation from the anterolateral aspect of the lower leg and foot.

#'''Spinal nerve root'''
#'''Spinal cord'''
#'''Brainstem and thalamus'''
#'''Brain'''

== Systemic and Anesthesia Factors ==
#'''Temperature'''. The temperature of the patient can increase or decrease SSEP latencies but does not necessarily affect amplitudes.
#'''Blood pressure'''. An intraoperative decline in blood pressure is associated with a loss in SSEP signals.
#'''Halogenated inhalational agents'''. Higher levels of gas anesthesia (> Half MAC) can increase SSEP latencies and reduce SSEP amplitudes.
#'''Nitrous oxide'''. Nitrous oxide decreases SSEP amplitudes but has no effect on latencies.
#'''Intravenous analgesic agents'''. Intravenous anesthetics, such as propofol, do significantly influence SSEP latencies or amplitudes.
#'''Muscle relaxants'''. Neuromuscular blockers do not influence SSEP latencies or amplitudes.

==References==
# Brown RH, Nash CL, Berilla JA, Amaddio MD. Cortical evoked potential monitoring. A system for intraoperative monitoring of spinal cord function. Spine 1984; 9: 256-261.
# Celesia GG. Somatosensory evoked potentials recorded directly from human thalamus and Sm I cortical area. Arch Neurol 1979; 36: 399-405.
# Cohen AR, Young W, Ransohoff J. Intraspinal localization of the somatosensory evoked potential. Neurosurgery 1981; 9: 157-62.
# Kelly DL Jr, Goldring S, O’Leary JL. Averaged evoked somatosensory responses from exposed cortex of man. Arch Neurol 1965; 13: 1-9.
# Larson SJ, Sances A. Evoked potentials in man: neurosurgical applications. Am J Surg 1966; 111: 857-861.
# MacDonald DB. Individually optimizing posterior tibial somatosensory evoked potential P37 scalp derivations for intraoperative monitoring. J Clin Neurophysiol. 2001 Jul;18(4):364-71.
# Mauguiere F. Anatomic origin of the cervical N13 potential evoked by upper extremity stimulation. J of Clin Neurophysiology 2000; 17: 236-245.
# McCallum JE, Bennett MH. Electrophysiologic monitoring of spinal cord function during intraspinal surgery. Surg Forum 1975; 26: 469-471.
# Nash CL, Long RA, Schatzinger LA, Brown RH. Spinal cord monitoring during operative treatment of the spine. Clin Orthop 1977; 126: 100-105.
# Nuwer MR, Dawson EG, Carlson LG, Kanim LEA, Sherman JE. Somatosensory evoked potential spinal cord monitoring reduces neurologic deficits after scoliosis surgery: results of a large multicenter survey. Electroencepholog and Clin Neurophysiol 1995; 96: 6-11.
# Nuwer MR, et al. IFCN recommended standards for short latency somatosensory evoked potentials. Report of an IFCN committee. Electroencephalography and Clinical Neurophysiology 1994; 91: 6-11.
# Powers SK, Bolger CA, Edwards MSB. Spinal cord pathways mediating somatosensory evoked potentials. J Neurosurg 1982; 57: 472-82.
# Simpson RK JR, Blackburn JG, Martin HF III, Katz S. Peripheral nerve fibers and spinal cord pathway contribution to the somatosensory evoked potentials. Exp Neurol 1981; 73: 700-15.
# Sloan TB. Anesthetic Effects on Electrophysiologic Recordings. Journal of Clinical Neurophysiology 1998; 15: 217-226.
# Toleikis JR. Intraoperative monitoring using somatosensory evoked potentials: A position statement by the American Society of Neurophysiological Monitoring. J Clin Monit Comput. 2005; 19(3):241-58.
# Wang BP, Turner LA, Kamp AM, Venier LH. CPz-CP4 and CPz-CP3 are superior to CPz-FPz for recording left and right tibial nerve somatosensory evoked potentials for intraoperative monitoring. A review study in 264 patients. 2006 The 17th Annual Meeting of American Society of Neurophysiological Monitoring.
# Yanni DS, Ulkatan S, Deletis V, Barrenechea IJ, Sen C, Perin NI. Utility of neurophysiological monitoring using dorsal column mapping in intramedullary spinal cord surgery. J Neurosurg Spine. 2010;12:623-628.
# York DH, Chabot RJ, Gaines RW. Response variability of somatosensory evoked potentials during scoliosis surgery. Spine 1987; 12: 864-876.

User:Wdoyon

2024-09-18T16:04:37Z

Wdoyon:

EDUCATION
Ph.D. Pharmacology and Toxicology, University of Texas (2005)
B.S. Biology, New Mexico State University (1999)
B.A. Psychology, New Mexico State University (1999)

POSITIONS
Clinical Neurophysiologist, Neurological Monitoring Services (2019-present)
Assistant Professor (Research Track), University of Pennsylvania, Dept. of Neuroscience (2013-2019)
Assistant Professor (Non-Tenure Track), Baylor College of Medicine, Dept. of Neuroscience (2010-2013)
Postdoctoral Fellow, Baylor College of Medicine (2006-2010)

BACKGROUND
My training is in neurophysiology and neuropharmacology.

Somatosensory Evoked Potentials (SSEP)

2024-09-17T23:47:49Z

Wdoyon: /* Intraoperative Monitoring */

Somatosensory evoked potentials (SSEP) are recorded from the central nervous system following stimulation of peripheral nerves.

== Introduction==

Somatosensory Evoked Potentials (SSEPs) are electric signals recorded from the scalp or spine following stimulation to the peripheral nerves. They are time-locked responses, representing the function of the ascending sensory pathways. Early in the 1960s Larson et al introduced the use of somatosensory evoked potentials to monitor neural structure during neurosurgical procedures. It was utilized as a supplement to the wake-up test during correctional spinal surgeries for spinal deformities such as scoliosis to provide warning of compromised spinal cord function to the spine surgeons, as reported by McCallum et al and Nash et al in the 1970s. Since then SSEP has become one of the earliest and primary tools for intraoperative neurophysiological monitoring.

== Somatosensory Pathways ==
[[Image:Ascending Pathways.jpg|right|100px|200px]]

Distal peripheral nerves are stimulated for SSEP recordings; typically the median or ulnar nerve at the wrist for upper extremity SSEPs, or posterior tibial nerve at the ankle for lower extremity SSEPs. The ascending sensory volley enters the spinal cord through dorsal nerve roots. Multiple dorsal or posterior column spinal pathways such as the gracile fasciculus (legs and trunk) and the cuneate fasciculus (arms and trunk) mediate the SSEP responses. They arrive at the medulla and make synaptic connection at the medullary nuclei (nucleus cuneatus and nucleus gracilis). From there they cross and ascend in the medial lemniscal pathways to thalamic nuclei, which in turn project up to the somatosenory cortex.

There is no synapses between the peripheral stimulation sites and the medullary nuclei. Synapses are the sites of action for inhalational anesthetic agents. Therefore any SSEP responses recorded below the level of medullary nuclei are affected only minimally by general anesthesia. The cortical SSEP responses, however, are greatly affected by inhalational agents; whichever recorded anesthetic management is very much required.

== Stimulation ==
#'''Stimulation Sites''': SSEPs may be elicited by electrical stimulation to major nerve trunks or dermatomes. Upper extremity mixed or major nerve SSEPs are typically obtained by stimulating the median nerve or ulnar nerve near the wrist; or sometimes the ulnar nerve at the elbow. Lower extremity SSEPs are normally recorded to stimulation of the posterior tibial nerve at the ankle, or peroneal nerve near the head of the fibula at the knee. Usually the anode electrode should be placed 2-4 cm distal to the cathode electrode to avoid anodal block.
#'''Electrodes''': Subdermal needle electrodes or adhesive surface electrodes are most commonly used for intraoperative SSEP recordings. Some other types of electrodes such as bar electrodes and EEG metal disc electrodes are more suitable for diagnostic SSEP recordings in the lab. Needle electrodes can be placed closer to the underlying nerves than surface electrodes; however they are associated with additional risks of infection, bleeding, and inadvertent needle sticks.
#'''Constant Current vs Constant Voltage''': Constant current stimulation is recommended for consistent and reliable stimulation for optimal SSEP recording in the OR, especially for long surgical procedures. Constant current stimuli may compensate for any changes in electrical conductivity, or electrode contact resistance.
#'''Stimulation Parameters''': a series of rectangular pulses with certain pulse width and frequency are typically used as electrical stimuli for SSEP recording.
##'''Pulse width (or pulse duration)''': 200-300 microsecond is suggested
##'''Frequency (or rep rate)''': Higher stimulus rate is desired for quick acquisition of SSEP responses in the operating room. However increasing the rate too high may result in degradation of the responses; also it is limited when interleaving stimulation of 2 or 4 limbs. In general a frequency between 2 and 5 Hz are recommended. To avoid synchronization between the responses and the underlying electrical noise, most commonly 60Hz or 50Hz in different countries, the stimulus rate should not be a submultiple of the noise frequency. Sometimes a slight change of stimulus rate, for example from 4.80 to 4.13, may improve the quality of the evoked responses.
##'''Intensity''': Supramaximal stimulation should be used to produce repeatable responses. Some factors, such as pathology of the peripheral nerves, large or edematous extremities, distance of the electrodes to the underlying nerves, types of stimulating electrodes, may limit the effectiveness of stimulation. A stimulus of 50mA or greater is required sometimes. There is concern of tissue damage from high current densities at the stimulation site which is rare.

== Recording Techniques ==

#'''Electrodes''': Metal surface cup electrodes are non-invasive electrodes, which can be applied with conductive gel after skin preparation. Subdermal needles are also commonly used in the operating room; some are available with angled or hooked tips. A corkscrew version can be screwed into the scalp for prolonged recording, however excessive bleeding can be a concern. For direct cortical mapping and recording, a strip or grid electrode array can be used for cranial surgeries.
#'''Recording Sites'''
##Cortical recording of upper extremity SSEP: It is the post-central gyrus of somatosensory cortex, contralateral to the stimulated limb. The locations are called CP3 and CP4 which are 2 cm posterior to the C3 and C4 positions of the 10-20 International System of EEG electrode placement. The recording montage can be CP3-Fz or CP3-CP4 for right arm stimulation; CP4-Fz or CP4-CP3 for left arm stimulation.
##Cortical recording of lower extremity SSEP: CPz is the active electrode site, which is 2 cm posterior to Cz. The traditional derivation of recording is CPz-Fz; however it should not be the only standard recording derivation, according to studies for optimized lower extremity SSEP recording by MacDonald DB et al and many others. Often time CPz-CPc (which is CPz-CP4 for left leg stimulation, and CPz-CP3 for right) may produce higher amplitude and more reliable signals.
##Subcortical: The recording can be made at posterior cervical spine, one or linked earlobes, or mastoid. Subcortical responses are less affected by inhalational agents. However, the subcortical SSEP response is normally very well defined for upper extremity stimulation, it is often poorly defined for lower extremity stimulation.
##Peripheral nerve: Usually it is the ipsilateral Erb's point for upper extremity stimulation, and ipsilateral popliteal fossa for lower extremity stimulation. This is done to verify the status of the peripheral stimulation.
#'''Averaging:''' The SSEP amplitudes tend to be low; as high as only several microvolts, or as low as less than a microvolts especially with pathological subjects. Averaging is required to record the signal against biological and ambient noise. SSEP signal is time-locked to the stimulus and most of the noise occurs randomly, allowing the noise to be averaged out with averaging of repeated responses. Some earlier guidelines such as that from American EEG Society suggested acquiring 500-2000 trails per averaged response. However for surgical monitoring well defined evoked potentials should be acquired as quickly as possible. With good preparation of the recording sites to reduce impedance, and optimized recording montages, clean signals could be recorded as few as 50-200 trials.
#'''Recording Parameters'''
##Filters: More environmental noise in the operating room and necessity of quick acquisition make it important to choose optimal filter settings, different from laboratory SSEP diagnostic studies. The majority of the energy contained in cortical SSEP is present in the frequency bandpass above 30Hz and below 500Hz; filters can be set to (10-30) Hz to (250-1000) Hz. The relative frequency content of the subcortical or peripheral responses is much higher, thus the filters can be set to (30-100)Hz to (500-2000) Hz.
##Timebase: The timebase is usually set at 50 milliseconds for upper extremity SSEPs, and 100 milliseconds for upper extremity SSEPs. It is based on the normal conduction time between the stimulation and recording sites. It may need adjustment depending on the age and size of the individual, and any pathological conditions.
##Sensitivity: The median amplitude of SSEP is about 1 microvolt. The recording sensitivity could range from 0.1 to 5 microvolts/unit. With direct cortical recording during cranial surgeries, the amplitude can be high and the sensitivity my be set to 20-50 microvolts/unit.
##Sweep delay: Normally it should be avoided to use sweep delay. However it may be necessary when large stimulus artifact is present.
##Interleaving recording: Alternating recording of SSEP responses to stimulation of 2 or 4 limbs at the same time is available with modern recording devices from different manufacturers.

== Waveforms ==
'''Upper Extremity SSEPs'''
#N9: Recorded from ipsilateral Erb's point, and sometimes it's called EP. It is a compound action potential from the axons to the median nerve or ulnar nerve stimulation.
#N13-P14: Cervical potentials, recorded near C5 spinal process. The origin is thought to be dorsal horn neurons, and the caudal medial lemniscus.
#N20-P23: Scalp potentials, generated from the thalamocortical radiations.

'''Lower Extremity SSEPs'''
#LP: Recorded from the T12 spinal process, arise from the afferent nerve volley in the dorsal roots and dorsal root entry zone.
#N34: Recorded at the cervical spine; the amplitude can be very low and difficult to identify. The origin is thought to be brainstem or perhaps thalamus.
#P37-N45: Scalp potentials, reflect activation of the primary somatosensory cortex.

[[Image:SSEP Waveforms.jpg|center|2000px|750px]]

== Intraoperative Monitoring ==
The basic principle of intraoperative monitoring with mixed nerve SSEP is to stimulate nerves distal to the surgical site, and to record responses proximal to the surgical site. In most cases, these recording sites should include one cortical and one subcortical recording site.

#'''Data acquisition''': Baseline SSEP responses should be established, ideally after induction and before patient positioning and surgical incision. Data should be collected throughout the surgical procedure, with corresponding documentation of surgical events such as incision, exposure, decompression, instrumentation, closure, etc. In addition, relevant physiological variables such as blood pressure and temperature changes, anesthetic agents being used and the levels, significant signal changes during the surgery and any communication and interventions should be documented.
#'''Alert criteria''': Significant signal changes, with 50% decrease of peak to peak amplitude and 10% increase of latency, may be indication of nervous system functional changes and should be reported to the operating surgeon, and call for possible intervention.

Depending on the specific surgery, planning is required to monitor the part of the nervous system at risk.
#'''Peripheral nerve and plexus.'''
Saphenous nerve SSEPs are useful for monitoring the femoral nerve during transpsoas (lateral) lumbar fusions (Silverstein et al., 2014; Spine). The femoral nerve and lumbar plexus are at risk for injury with the transpsoas approach because of the lateral approach itself as well as retractor related nerve compression. The femoral nerve is the major branch of the lumbar plexus and exits at the L2-4 level. The saphenous nerve is a sensory branch of the femoral nerve and innervates the medial side of the leg and foot. The saphenous nerve can be found between the vastus medialis and sartorius on the medial part of the thigh. Because it is surrounded by these large muscles and is not superficial, it is best to use needle electrodes for stimulation. Higher current intensities and pulse durations relative to posterior tibial nerve SSEPs may be required for a good signal to noise ratio. The cortical responses should be shorter in latency compared to posterior tibial nerve SSEPs.

Superficial peroneal nerve SSEPs have been used to monitor lower lumbar nerve root dysfunction during L4-5 lumbar decompressions (Yue and Martinez, Spine J.). The superficial peroneal nerve is one of two branches of the common peroneal nerve and exits the spine at the L4-S1 level. The superficial peroneal nerve provides sensory innervation to the anterolateral aspect of the lower leg and foot.

#'''Spinal nerve root'''
#'''Spinal cord'''
#'''Brainstem and thalamus'''
#'''Brain'''

== Systemic and Anesthesia Factors ==
#'''Temperature'''. The temperature of the patient can increase or decrease SSEP latencies but does not necessarily affect amplitudes.
#'''Blood pressure'''. An intraoperative decline in blood pressure is associated with a loss in SSEP signals.
#'''Halogenated inhalational agents'''. Higher levels of gas anesthesia (> Half MAC) can increase SSEP latencies and reduce SSEP amplitudes.
#'''Nitrous oxide'''. Nitrous oxide decreases SSEP amplitudes but has no effect on latencies.
#'''Intravenous analgesic agents'''. Intravenous anesthetics, such as propofol, do significantly influence SSEP latencies or amplitudes.
#'''Muscle relaxants'''. Neuromuscular blockers do not influence SSEP latencies or amplitudes.

==References==
# Brown RH, Nash CL, Berilla JA, Amaddio MD. Cortical evoked potential monitoring. A system for intraoperative monitoring of spinal cord function. Spine 1984; 9: 256-261.
# Celesia GG. Somatosensory evoked potentials recorded directly from human thalamus and Sm I cortical area. Arch Neurol 1979; 36: 399-405.
# Cohen AR, Young W, Ransohoff J. Intraspinal localization of the somatosensory evoked potential. Neurosurgery 1981; 9: 157-62.
# Kelly DL Jr, Goldring S, O’Leary JL. Averaged evoked somatosensory responses from exposed cortex of man. Arch Neurol 1965; 13: 1-9.
# Larson SJ, Sances A. Evoked potentials in man: neurosurgical applications. Am J Surg 1966; 111: 857-861.
# MacDonald DB. Individually optimizing posterior tibial somatosensory evoked potential P37 scalp derivations for intraoperative monitoring. J Clin Neurophysiol. 2001 Jul;18(4):364-71.
# Mauguiere F. Anatomic origin of the cervical N13 potential evoked by upper extremity stimulation. J of Clin Neurophysiology 2000; 17: 236-245.
# McCallum JE, Bennett MH. Electrophysiologic monitoring of spinal cord function during intraspinal surgery. Surg Forum 1975; 26: 469-471.
# Nash CL, Long RA, Schatzinger LA, Brown RH. Spinal cord monitoring during operative treatment of the spine. Clin Orthop 1977; 126: 100-105.
# Nuwer MR, Dawson EG, Carlson LG, Kanim LEA, Sherman JE. Somatosensory evoked potential spinal cord monitoring reduces neurologic deficits after scoliosis surgery: results of a large multicenter survey. Electroencepholog and Clin Neurophysiol 1995; 96: 6-11.
# Nuwer MR, et al. IFCN recommended standards for short latency somatosensory evoked potentials. Report of an IFCN committee. Electroencephalography and Clinical Neurophysiology 1994; 91: 6-11.
# Powers SK, Bolger CA, Edwards MSB. Spinal cord pathways mediating somatosensory evoked potentials. J Neurosurg 1982; 57: 472-82.
# Simpson RK JR, Blackburn JG, Martin HF III, Katz S. Peripheral nerve fibers and spinal cord pathway contribution to the somatosensory evoked potentials. Exp Neurol 1981; 73: 700-15.
# Sloan TB. Anesthetic Effects on Electrophysiologic Recordings. Journal of Clinical Neurophysiology 1998; 15: 217-226.
# Toleikis JR. Intraoperative monitoring using somatosensory evoked potentials: A position statement by the American Society of Neurophysiological Monitoring. J Clin Monit Comput. 2005; 19(3):241-58.
# Wang BP, Turner LA, Kamp AM, Venier LH. CPz-CP4 and CPz-CP3 are superior to CPz-FPz for recording left and right tibial nerve somatosensory evoked potentials for intraoperative monitoring. A review study in 264 patients. 2006 The 17th Annual Meeting of American Society of Neurophysiological Monitoring.
# Yanni DS, Ulkatan S, Deletis V, Barrenechea IJ, Sen C, Perin NI. Utility of neurophysiological monitoring using dorsal column mapping in intramedullary spinal cord surgery. J Neurosurg Spine. 2010;12:623-628.
# York DH, Chabot RJ, Gaines RW. Response variability of somatosensory evoked potentials during scoliosis surgery. Spine 1987; 12: 864-876.

Somatosensory Evoked Potentials (SSEP)

2024-09-17T18:04:29Z

Wdoyon: /* Intraoperative Monitoring */

Somatosensory evoked potentials (SSEP) are recorded from the central nervous system following stimulation of peripheral nerves.

== Introduction==

Somatosensory Evoked Potentials (SSEPs) are electric signals recorded from the scalp or spine following stimulation to the peripheral nerves. They are time-locked responses, representing the function of the ascending sensory pathways. Early in the 1960s Larson et al introduced the use of somatosensory evoked potentials to monitor neural structure during neurosurgical procedures. It was utilized as a supplement to the wake-up test during correctional spinal surgeries for spinal deformities such as scoliosis to provide warning of compromised spinal cord function to the spine surgeons, as reported by McCallum et al and Nash et al in the 1970s. Since then SSEP has become one of the earliest and primary tools for intraoperative neurophysiological monitoring.

== Somatosensory Pathways ==
[[Image:Ascending Pathways.jpg|right|100px|200px]]

Distal peripheral nerves are stimulated for SSEP recordings; typically the median or ulnar nerve at the wrist for upper extremity SSEPs, or posterior tibial nerve at the ankle for lower extremity SSEPs. The ascending sensory volley enters the spinal cord through dorsal nerve roots. Multiple dorsal or posterior column spinal pathways such as the gracile fasciculus (legs and trunk) and the cuneate fasciculus (arms and trunk) mediate the SSEP responses. They arrive at the medulla and make synaptic connection at the medullary nuclei (nucleus cuneatus and nucleus gracilis). From there they cross and ascend in the medial lemniscal pathways to thalamic nuclei, which in turn project up to the somatosenory cortex.

There is no synapses between the peripheral stimulation sites and the medullary nuclei. Synapses are the sites of action for inhalational anesthetic agents. Therefore any SSEP responses recorded below the level of medullary nuclei are affected only minimally by general anesthesia. The cortical SSEP responses, however, are greatly affected by inhalational agents; whichever recorded anesthetic management is very much required.

== Stimulation ==
#'''Stimulation Sites''': SSEPs may be elicited by electrical stimulation to major nerve trunks or dermatomes. Upper extremity mixed or major nerve SSEPs are typically obtained by stimulating the median nerve or ulnar nerve near the wrist; or sometimes the ulnar nerve at the elbow. Lower extremity SSEPs are normally recorded to stimulation of the posterior tibial nerve at the ankle, or peroneal nerve near the head of the fibula at the knee. Usually the anode electrode should be placed 2-4 cm distal to the cathode electrode to avoid anodal block.
#'''Electrodes''': Subdermal needle electrodes or adhesive surface electrodes are most commonly used for intraoperative SSEP recordings. Some other types of electrodes such as bar electrodes and EEG metal disc electrodes are more suitable for diagnostic SSEP recordings in the lab. Needle electrodes can be placed closer to the underlying nerves than surface electrodes; however they are associated with additional risks of infection, bleeding, and inadvertent needle sticks.
#'''Constant Current vs Constant Voltage''': Constant current stimulation is recommended for consistent and reliable stimulation for optimal SSEP recording in the OR, especially for long surgical procedures. Constant current stimuli may compensate for any changes in electrical conductivity, or electrode contact resistance.
#'''Stimulation Parameters''': a series of rectangular pulses with certain pulse width and frequency are typically used as electrical stimuli for SSEP recording.
##'''Pulse width (or pulse duration)''': 200-300 microsecond is suggested
##'''Frequency (or rep rate)''': Higher stimulus rate is desired for quick acquisition of SSEP responses in the operating room. However increasing the rate too high may result in degradation of the responses; also it is limited when interleaving stimulation of 2 or 4 limbs. In general a frequency between 2 and 5 Hz are recommended. To avoid synchronization between the responses and the underlying electrical noise, most commonly 60Hz or 50Hz in different countries, the stimulus rate should not be a submultiple of the noise frequency. Sometimes a slight change of stimulus rate, for example from 4.80 to 4.13, may improve the quality of the evoked responses.
##'''Intensity''': Supramaximal stimulation should be used to produce repeatable responses. Some factors, such as pathology of the peripheral nerves, large or edematous extremities, distance of the electrodes to the underlying nerves, types of stimulating electrodes, may limit the effectiveness of stimulation. A stimulus of 50mA or greater is required sometimes. There is concern of tissue damage from high current densities at the stimulation site which is rare.

== Recording Techniques ==

#'''Electrodes''': Metal surface cup electrodes are non-invasive electrodes, which can be applied with conductive gel after skin preparation. Subdermal needles are also commonly used in the operating room; some are available with angled or hooked tips. A corkscrew version can be screwed into the scalp for prolonged recording, however excessive bleeding can be a concern. For direct cortical mapping and recording, a strip or grid electrode array can be used for cranial surgeries.
#'''Recording Sites'''
##Cortical recording of upper extremity SSEP: It is the post-central gyrus of somatosensory cortex, contralateral to the stimulated limb. The locations are called CP3 and CP4 which are 2 cm posterior to the C3 and C4 positions of the 10-20 International System of EEG electrode placement. The recording montage can be CP3-Fz or CP3-CP4 for right arm stimulation; CP4-Fz or CP4-CP3 for left arm stimulation.
##Cortical recording of lower extremity SSEP: CPz is the active electrode site, which is 2 cm posterior to Cz. The traditional derivation of recording is CPz-Fz; however it should not be the only standard recording derivation, according to studies for optimized lower extremity SSEP recording by MacDonald DB et al and many others. Often time CPz-CPc (which is CPz-CP4 for left leg stimulation, and CPz-CP3 for right) may produce higher amplitude and more reliable signals.
##Subcortical: The recording can be made at posterior cervical spine, one or linked earlobes, or mastoid. Subcortical responses are less affected by inhalational agents. However, the subcortical SSEP response is normally very well defined for upper extremity stimulation, it is often poorly defined for lower extremity stimulation.
##Peripheral nerve: Usually it is the ipsilateral Erb's point for upper extremity stimulation, and ipsilateral popliteal fossa for lower extremity stimulation. This is done to verify the status of the peripheral stimulation.
#'''Averaging:''' The SSEP amplitudes tend to be low; as high as only several microvolts, or as low as less than a microvolts especially with pathological subjects. Averaging is required to record the signal against biological and ambient noise. SSEP signal is time-locked to the stimulus and most of the noise occurs randomly, allowing the noise to be averaged out with averaging of repeated responses. Some earlier guidelines such as that from American EEG Society suggested acquiring 500-2000 trails per averaged response. However for surgical monitoring well defined evoked potentials should be acquired as quickly as possible. With good preparation of the recording sites to reduce impedance, and optimized recording montages, clean signals could be recorded as few as 50-200 trials.
#'''Recording Parameters'''
##Filters: More environmental noise in the operating room and necessity of quick acquisition make it important to choose optimal filter settings, different from laboratory SSEP diagnostic studies. The majority of the energy contained in cortical SSEP is present in the frequency bandpass above 30Hz and below 500Hz; filters can be set to (10-30) Hz to (250-1000) Hz. The relative frequency content of the subcortical or peripheral responses is much higher, thus the filters can be set to (30-100)Hz to (500-2000) Hz.
##Timebase: The timebase is usually set at 50 milliseconds for upper extremity SSEPs, and 100 milliseconds for upper extremity SSEPs. It is based on the normal conduction time between the stimulation and recording sites. It may need adjustment depending on the age and size of the individual, and any pathological conditions.
##Sensitivity: The median amplitude of SSEP is about 1 microvolt. The recording sensitivity could range from 0.1 to 5 microvolts/unit. With direct cortical recording during cranial surgeries, the amplitude can be high and the sensitivity my be set to 20-50 microvolts/unit.
##Sweep delay: Normally it should be avoided to use sweep delay. However it may be necessary when large stimulus artifact is present.
##Interleaving recording: Alternating recording of SSEP responses to stimulation of 2 or 4 limbs at the same time is available with modern recording devices from different manufacturers.

== Waveforms ==
'''Upper Extremity SSEPs'''
#N9: Recorded from ipsilateral Erb's point, and sometimes it's called EP. It is a compound action potential from the axons to the median nerve or ulnar nerve stimulation.
#N13-P14: Cervical potentials, recorded near C5 spinal process. The origin is thought to be dorsal horn neurons, and the caudal medial lemniscus.
#N20-P23: Scalp potentials, generated from the thalamocortical radiations.

'''Lower Extremity SSEPs'''
#LP: Recorded from the T12 spinal process, arise from the afferent nerve volley in the dorsal roots and dorsal root entry zone.
#N34: Recorded at the cervical spine; the amplitude can be very low and difficult to identify. The origin is thought to be brainstem or perhaps thalamus.
#P37-N45: Scalp potentials, reflect activation of the primary somatosensory cortex.

[[Image:SSEP Waveforms.jpg|center|2000px|750px]]

== Intraoperative Monitoring ==
The basic principle of intraoperative monitoring with mixed nerve SSEP is to stimulate nerves distal to the surgical site, and to record responses proximal to the surgical site. In most cases, these recording sites should include one cortical and one subcortical recording site.

#'''Data acquisition''': Baseline SSEP responses should be established, ideally after induction and before patient positioning and surgical incision. Data should be collected throughout the surgical procedure, with corresponding documentation of surgical events such as incision, exposure, decompression, instrumentation, closure, etc. In addition, relevant physiological variables such as blood pressure and temperature changes, anesthetic agents being used and the levels, significant signal changes during the surgery and any communication and interventions should be documented.
#'''Alert criteria''': Significant signal changes, with 50% decrease of peak to peak amplitude and 10% increase of latency, may be indication of nervous system functional changes and should be reported to the operating surgeon, and call for possible intervention.

Depending on the specific surgery, planning is required to monitor the part of the nervous system at risk.
#'''Peripheral nerve and plexus.'''
Saphenous nerve SSEPs are useful for monitoring the femoral nerve during transpsoas (lateral) lumbar fusions (Silverstein et al., 2014; Spine). The femoral nerve and lumbar plexus are at risk for injury with the transpsoas approach because of the lateral approach itself as well as retractor related nerve compression. The femoral nerve is the major branch of the lumbar plexus and exits at the L2-4 level. The saphenous nerve is a sensory branch of the femoral nerve and innervates the medial side of the leg and foot. The saphenous nerve can be found between the vastus medialis and sartorius on the medial part of the thigh. Because it is surrounded by these large muscles and is not superficial, it is best to use needle electrodes for stimulation. Higher current intensities and pulse durations relative to posterior tibial nerve SSEPs may be required for a good signal to noise ratio. The cortical responses should be shorter in latency compared to posterior tibial nerve SSEPs.

Peroneal nerve SSEPs have been used to monitor L4-5 nerve root dysfunction during L4-5 lumbar decompressions (Yue and Martinez, Spine J.).

#'''Spinal nerve root'''
#'''Spinal cord'''
#'''Brainstem and thalamus'''
#'''Brain'''

== Systemic and Anesthesia Factors ==
#'''Temperature'''. The temperature of the patient can increase or decrease SSEP latencies but does not necessarily affect amplitudes.
#'''Blood pressure'''. An intraoperative decline in blood pressure is associated with a loss in SSEP signals.
#'''Halogenated inhalational agents'''. Higher levels of gas anesthesia (> Half MAC) can increase SSEP latencies and reduce SSEP amplitudes.
#'''Nitrous oxide'''. Nitrous oxide decreases SSEP amplitudes but has no effect on latencies.
#'''Intravenous analgesic agents'''. Intravenous anesthetics, such as propofol, do significantly influence SSEP latencies or amplitudes.
#'''Muscle relaxants'''. Neuromuscular blockers do not influence SSEP latencies or amplitudes.

==References==
# Brown RH, Nash CL, Berilla JA, Amaddio MD. Cortical evoked potential monitoring. A system for intraoperative monitoring of spinal cord function. Spine 1984; 9: 256-261.
# Celesia GG. Somatosensory evoked potentials recorded directly from human thalamus and Sm I cortical area. Arch Neurol 1979; 36: 399-405.
# Cohen AR, Young W, Ransohoff J. Intraspinal localization of the somatosensory evoked potential. Neurosurgery 1981; 9: 157-62.
# Kelly DL Jr, Goldring S, O’Leary JL. Averaged evoked somatosensory responses from exposed cortex of man. Arch Neurol 1965; 13: 1-9.
# Larson SJ, Sances A. Evoked potentials in man: neurosurgical applications. Am J Surg 1966; 111: 857-861.
# MacDonald DB. Individually optimizing posterior tibial somatosensory evoked potential P37 scalp derivations for intraoperative monitoring. J Clin Neurophysiol. 2001 Jul;18(4):364-71.
# Mauguiere F. Anatomic origin of the cervical N13 potential evoked by upper extremity stimulation. J of Clin Neurophysiology 2000; 17: 236-245.
# McCallum JE, Bennett MH. Electrophysiologic monitoring of spinal cord function during intraspinal surgery. Surg Forum 1975; 26: 469-471.
# Nash CL, Long RA, Schatzinger LA, Brown RH. Spinal cord monitoring during operative treatment of the spine. Clin Orthop 1977; 126: 100-105.
# Nuwer MR, Dawson EG, Carlson LG, Kanim LEA, Sherman JE. Somatosensory evoked potential spinal cord monitoring reduces neurologic deficits after scoliosis surgery: results of a large multicenter survey. Electroencepholog and Clin Neurophysiol 1995; 96: 6-11.
# Nuwer MR, et al. IFCN recommended standards for short latency somatosensory evoked potentials. Report of an IFCN committee. Electroencephalography and Clinical Neurophysiology 1994; 91: 6-11.
# Powers SK, Bolger CA, Edwards MSB. Spinal cord pathways mediating somatosensory evoked potentials. J Neurosurg 1982; 57: 472-82.
# Simpson RK JR, Blackburn JG, Martin HF III, Katz S. Peripheral nerve fibers and spinal cord pathway contribution to the somatosensory evoked potentials. Exp Neurol 1981; 73: 700-15.
# Sloan TB. Anesthetic Effects on Electrophysiologic Recordings. Journal of Clinical Neurophysiology 1998; 15: 217-226.
# Toleikis JR. Intraoperative monitoring using somatosensory evoked potentials: A position statement by the American Society of Neurophysiological Monitoring. J Clin Monit Comput. 2005; 19(3):241-58.
# Wang BP, Turner LA, Kamp AM, Venier LH. CPz-CP4 and CPz-CP3 are superior to CPz-FPz for recording left and right tibial nerve somatosensory evoked potentials for intraoperative monitoring. A review study in 264 patients. 2006 The 17th Annual Meeting of American Society of Neurophysiological Monitoring.
# Yanni DS, Ulkatan S, Deletis V, Barrenechea IJ, Sen C, Perin NI. Utility of neurophysiological monitoring using dorsal column mapping in intramedullary spinal cord surgery. J Neurosurg Spine. 2010;12:623-628.
# York DH, Chabot RJ, Gaines RW. Response variability of somatosensory evoked potentials during scoliosis surgery. Spine 1987; 12: 864-876.

Somatosensory Evoked Potentials (SSEP)

2024-09-16T16:43:34Z

Wdoyon: /* Intraoperative Monitoring */

Somatosensory evoked potentials (SSEP) are recorded from the central nervous system following stimulation of peripheral nerves.

== Introduction==

Somatosensory Evoked Potentials (SSEPs) are electric signals recorded from the scalp or spine following stimulation to the peripheral nerves. They are time-locked responses, representing the function of the ascending sensory pathways. Early in the 1960s Larson et al introduced the use of somatosensory evoked potentials to monitor neural structure during neurosurgical procedures. It was utilized as a supplement to the wake-up test during correctional spinal surgeries for spinal deformities such as scoliosis to provide warning of compromised spinal cord function to the spine surgeons, as reported by McCallum et al and Nash et al in the 1970s. Since then SSEP has become one of the earliest and primary tools for intraoperative neurophysiological monitoring.

== Somatosensory Pathways ==
[[Image:Ascending Pathways.jpg|right|100px|200px]]

Distal peripheral nerves are stimulated for SSEP recordings; typically the median or ulnar nerve at the wrist for upper extremity SSEPs, or posterior tibial nerve at the ankle for lower extremity SSEPs. The ascending sensory volley enters the spinal cord through dorsal nerve roots. Multiple dorsal or posterior column spinal pathways such as the gracile fasciculus (legs and trunk) and the cuneate fasciculus (arms and trunk) mediate the SSEP responses. They arrive at the medulla and make synaptic connection at the medullary nuclei (nucleus cuneatus and nucleus gracilis). From there they cross and ascend in the medial lemniscal pathways to thalamic nuclei, which in turn project up to the somatosenory cortex.

There is no synapses between the peripheral stimulation sites and the medullary nuclei. Synapses are the sites of action for inhalational anesthetic agents. Therefore any SSEP responses recorded below the level of medullary nuclei are affected only minimally by general anesthesia. The cortical SSEP responses, however, are greatly affected by inhalational agents; whichever recorded anesthetic management is very much required.

== Stimulation ==
#'''Stimulation Sites''': SSEPs may be elicited by electrical stimulation to major nerve trunks or dermatomes. Upper extremity mixed or major nerve SSEPs are typically obtained by stimulating the median nerve or ulnar nerve near the wrist; or sometimes the ulnar nerve at the elbow. Lower extremity SSEPs are normally recorded to stimulation of the posterior tibial nerve at the ankle, or peroneal nerve near the head of the fibula at the knee. Usually the anode electrode should be placed 2-4 cm distal to the cathode electrode to avoid anodal block.
#'''Electrodes''': Subdermal needle electrodes or adhesive surface electrodes are most commonly used for intraoperative SSEP recordings. Some other types of electrodes such as bar electrodes and EEG metal disc electrodes are more suitable for diagnostic SSEP recordings in the lab. Needle electrodes can be placed closer to the underlying nerves than surface electrodes; however they are associated with additional risks of infection, bleeding, and inadvertent needle sticks.
#'''Constant Current vs Constant Voltage''': Constant current stimulation is recommended for consistent and reliable stimulation for optimal SSEP recording in the OR, especially for long surgical procedures. Constant current stimuli may compensate for any changes in electrical conductivity, or electrode contact resistance.
#'''Stimulation Parameters''': a series of rectangular pulses with certain pulse width and frequency are typically used as electrical stimuli for SSEP recording.
##'''Pulse width (or pulse duration)''': 200-300 microsecond is suggested
##'''Frequency (or rep rate)''': Higher stimulus rate is desired for quick acquisition of SSEP responses in the operating room. However increasing the rate too high may result in degradation of the responses; also it is limited when interleaving stimulation of 2 or 4 limbs. In general a frequency between 2 and 5 Hz are recommended. To avoid synchronization between the responses and the underlying electrical noise, most commonly 60Hz or 50Hz in different countries, the stimulus rate should not be a submultiple of the noise frequency. Sometimes a slight change of stimulus rate, for example from 4.80 to 4.13, may improve the quality of the evoked responses.
##'''Intensity''': Supramaximal stimulation should be used to produce repeatable responses. Some factors, such as pathology of the peripheral nerves, large or edematous extremities, distance of the electrodes to the underlying nerves, types of stimulating electrodes, may limit the effectiveness of stimulation. A stimulus of 50mA or greater is required sometimes. There is concern of tissue damage from high current densities at the stimulation site which is rare.

== Recording Techniques ==

#'''Electrodes''': Metal surface cup electrodes are non-invasive electrodes, which can be applied with conductive gel after skin preparation. Subdermal needles are also commonly used in the operating room; some are available with angled or hooked tips. A corkscrew version can be screwed into the scalp for prolonged recording, however excessive bleeding can be a concern. For direct cortical mapping and recording, a strip or grid electrode array can be used for cranial surgeries.
#'''Recording Sites'''
##Cortical recording of upper extremity SSEP: It is the post-central gyrus of somatosensory cortex, contralateral to the stimulated limb. The locations are called CP3 and CP4 which are 2 cm posterior to the C3 and C4 positions of the 10-20 International System of EEG electrode placement. The recording montage can be CP3-Fz or CP3-CP4 for right arm stimulation; CP4-Fz or CP4-CP3 for left arm stimulation.
##Cortical recording of lower extremity SSEP: CPz is the active electrode site, which is 2 cm posterior to Cz. The traditional derivation of recording is CPz-Fz; however it should not be the only standard recording derivation, according to studies for optimized lower extremity SSEP recording by MacDonald DB et al and many others. Often time CPz-CPc (which is CPz-CP4 for left leg stimulation, and CPz-CP3 for right) may produce higher amplitude and more reliable signals.
##Subcortical: The recording can be made at posterior cervical spine, one or linked earlobes, or mastoid. Subcortical responses are less affected by inhalational agents. However, the subcortical SSEP response is normally very well defined for upper extremity stimulation, it is often poorly defined for lower extremity stimulation.
##Peripheral nerve: Usually it is the ipsilateral Erb's point for upper extremity stimulation, and ipsilateral popliteal fossa for lower extremity stimulation. This is done to verify the status of the peripheral stimulation.
#'''Averaging:''' The SSEP amplitudes tend to be low; as high as only several microvolts, or as low as less than a microvolts especially with pathological subjects. Averaging is required to record the signal against biological and ambient noise. SSEP signal is time-locked to the stimulus and most of the noise occurs randomly, allowing the noise to be averaged out with averaging of repeated responses. Some earlier guidelines such as that from American EEG Society suggested acquiring 500-2000 trails per averaged response. However for surgical monitoring well defined evoked potentials should be acquired as quickly as possible. With good preparation of the recording sites to reduce impedance, and optimized recording montages, clean signals could be recorded as few as 50-200 trials.
#'''Recording Parameters'''
##Filters: More environmental noise in the operating room and necessity of quick acquisition make it important to choose optimal filter settings, different from laboratory SSEP diagnostic studies. The majority of the energy contained in cortical SSEP is present in the frequency bandpass above 30Hz and below 500Hz; filters can be set to (10-30) Hz to (250-1000) Hz. The relative frequency content of the subcortical or peripheral responses is much higher, thus the filters can be set to (30-100)Hz to (500-2000) Hz.
##Timebase: The timebase is usually set at 50 milliseconds for upper extremity SSEPs, and 100 milliseconds for upper extremity SSEPs. It is based on the normal conduction time between the stimulation and recording sites. It may need adjustment depending on the age and size of the individual, and any pathological conditions.
##Sensitivity: The median amplitude of SSEP is about 1 microvolt. The recording sensitivity could range from 0.1 to 5 microvolts/unit. With direct cortical recording during cranial surgeries, the amplitude can be high and the sensitivity my be set to 20-50 microvolts/unit.
##Sweep delay: Normally it should be avoided to use sweep delay. However it may be necessary when large stimulus artifact is present.
##Interleaving recording: Alternating recording of SSEP responses to stimulation of 2 or 4 limbs at the same time is available with modern recording devices from different manufacturers.

== Waveforms ==
'''Upper Extremity SSEPs'''
#N9: Recorded from ipsilateral Erb's point, and sometimes it's called EP. It is a compound action potential from the axons to the median nerve or ulnar nerve stimulation.
#N13-P14: Cervical potentials, recorded near C5 spinal process. The origin is thought to be dorsal horn neurons, and the caudal medial lemniscus.
#N20-P23: Scalp potentials, generated from the thalamocortical radiations.

'''Lower Extremity SSEPs'''
#LP: Recorded from the T12 spinal process, arise from the afferent nerve volley in the dorsal roots and dorsal root entry zone.
#N34: Recorded at the cervical spine; the amplitude can be very low and difficult to identify. The origin is thought to be brainstem or perhaps thalamus.
#P37-N45: Scalp potentials, reflect activation of the primary somatosensory cortex.

[[Image:SSEP Waveforms.jpg|center|2000px|750px]]

== Intraoperative Monitoring ==
The basic principle of intraoperative monitoring with mixed nerve SSEP is to stimulate nerves distal to the surgical site, and to record responses proximal to the surgical site. In most cases, these recording sites should include one cortical and one subcortical recording site.

#'''Data acquisition''': Baseline SSEP responses should be established, ideally after induction and before patient positioning and surgical incision. Data should be collected throughout the surgical procedure, with corresponding documentation of surgical events such as incision, exposure, decompression, instrumentation, closure, etc. In addition, relevant physiological variables such as blood pressure and temperature changes, anesthetic agents being used and the levels, significant signal changes during the surgery and any communication and interventions should be documented.
#'''Alert criteria''': Significant signal changes, with 50% decrease of peak to peak amplitude and 10% increase of latency, may be indication of nervous system functional changes and should be reported to the operating surgeon, and call for possible intervention.

Depending on the specific surgery, planning is required to monitor the part of the nervous system at risk.
#'''Peripheral nerve and plexus.'''
For example, saphenous nerve SSEPs are useful for monitoring the femoral nerve during transpsoas (lateral) lumbar fusions. The femoral nerve and lumbar plexus are at risk for injury with the transpsoas approach, which is due to the approach itself as well as retractor related nerve damage. The femoral nerve is the major branch of the lumbar plexus and exits at the L2-4 level. The saphenous nerve is a sensory branch of the femoral nerve and innervates the medial side of the leg and foot. The saphenous nerve can be found between the vastus medialis and sartorius on the medial part of the thigh. Because it is surrounded by these large muscles and not superficial, it is best to use needle electrodes for stimulation. Higher current intensities and pulse durations relative to posterior tibial nerve SSEPs may be required for a good signal to noise ratio. The cortical responses should be shorter in latency compared to posterior tibial nerve SSEPs.
#'''Spinal nerve root'''
#'''Spinal cord'''
#'''Brainstem and thalamus'''
#'''Brain'''

== Systemic and Anesthesia Factors ==
#'''Temperature'''. The temperature of the patient can increase or decrease SSEP latencies but does not necessarily affect amplitudes.
#'''Blood pressure'''. An intraoperative decline in blood pressure is associated with a loss in SSEP signals.
#'''Halogenated inhalational agents'''. Higher levels of gas anesthesia (> Half MAC) can increase SSEP latencies and reduce SSEP amplitudes.
#'''Nitrous oxide'''. Nitrous oxide decreases SSEP amplitudes but has no effect on latencies.
#'''Intravenous analgesic agents'''. Intravenous anesthetics, such as propofol, do significantly influence SSEP latencies or amplitudes.
#'''Muscle relaxants'''. Neuromuscular blockers do not influence SSEP latencies or amplitudes.

==References==
# Brown RH, Nash CL, Berilla JA, Amaddio MD. Cortical evoked potential monitoring. A system for intraoperative monitoring of spinal cord function. Spine 1984; 9: 256-261.
# Celesia GG. Somatosensory evoked potentials recorded directly from human thalamus and Sm I cortical area. Arch Neurol 1979; 36: 399-405.
# Cohen AR, Young W, Ransohoff J. Intraspinal localization of the somatosensory evoked potential. Neurosurgery 1981; 9: 157-62.
# Kelly DL Jr, Goldring S, O’Leary JL. Averaged evoked somatosensory responses from exposed cortex of man. Arch Neurol 1965; 13: 1-9.
# Larson SJ, Sances A. Evoked potentials in man: neurosurgical applications. Am J Surg 1966; 111: 857-861.
# MacDonald DB. Individually optimizing posterior tibial somatosensory evoked potential P37 scalp derivations for intraoperative monitoring. J Clin Neurophysiol. 2001 Jul;18(4):364-71.
# Mauguiere F. Anatomic origin of the cervical N13 potential evoked by upper extremity stimulation. J of Clin Neurophysiology 2000; 17: 236-245.
# McCallum JE, Bennett MH. Electrophysiologic monitoring of spinal cord function during intraspinal surgery. Surg Forum 1975; 26: 469-471.
# Nash CL, Long RA, Schatzinger LA, Brown RH. Spinal cord monitoring during operative treatment of the spine. Clin Orthop 1977; 126: 100-105.
# Nuwer MR, Dawson EG, Carlson LG, Kanim LEA, Sherman JE. Somatosensory evoked potential spinal cord monitoring reduces neurologic deficits after scoliosis surgery: results of a large multicenter survey. Electroencepholog and Clin Neurophysiol 1995; 96: 6-11.
# Nuwer MR, et al. IFCN recommended standards for short latency somatosensory evoked potentials. Report of an IFCN committee. Electroencephalography and Clinical Neurophysiology 1994; 91: 6-11.
# Powers SK, Bolger CA, Edwards MSB. Spinal cord pathways mediating somatosensory evoked potentials. J Neurosurg 1982; 57: 472-82.
# Simpson RK JR, Blackburn JG, Martin HF III, Katz S. Peripheral nerve fibers and spinal cord pathway contribution to the somatosensory evoked potentials. Exp Neurol 1981; 73: 700-15.
# Sloan TB. Anesthetic Effects on Electrophysiologic Recordings. Journal of Clinical Neurophysiology 1998; 15: 217-226.
# Toleikis JR. Intraoperative monitoring using somatosensory evoked potentials: A position statement by the American Society of Neurophysiological Monitoring. J Clin Monit Comput. 2005; 19(3):241-58.
# Wang BP, Turner LA, Kamp AM, Venier LH. CPz-CP4 and CPz-CP3 are superior to CPz-FPz for recording left and right tibial nerve somatosensory evoked potentials for intraoperative monitoring. A review study in 264 patients. 2006 The 17th Annual Meeting of American Society of Neurophysiological Monitoring.
# Yanni DS, Ulkatan S, Deletis V, Barrenechea IJ, Sen C, Perin NI. Utility of neurophysiological monitoring using dorsal column mapping in intramedullary spinal cord surgery. J Neurosurg Spine. 2010;12:623-628.
# York DH, Chabot RJ, Gaines RW. Response variability of somatosensory evoked potentials during scoliosis surgery. Spine 1987; 12: 864-876.

Somatosensory Evoked Potentials (SSEP)

2024-09-16T16:42:55Z

Wdoyon: /* Intraoperative Monitoring */

Somatosensory evoked potentials (SSEP) are recorded from the central nervous system following stimulation of peripheral nerves.

== Introduction==

Somatosensory Evoked Potentials (SSEPs) are electric signals recorded from the scalp or spine following stimulation to the peripheral nerves. They are time-locked responses, representing the function of the ascending sensory pathways. Early in the 1960s Larson et al introduced the use of somatosensory evoked potentials to monitor neural structure during neurosurgical procedures. It was utilized as a supplement to the wake-up test during correctional spinal surgeries for spinal deformities such as scoliosis to provide warning of compromised spinal cord function to the spine surgeons, as reported by McCallum et al and Nash et al in the 1970s. Since then SSEP has become one of the earliest and primary tools for intraoperative neurophysiological monitoring.

== Somatosensory Pathways ==
[[Image:Ascending Pathways.jpg|right|100px|200px]]

Distal peripheral nerves are stimulated for SSEP recordings; typically the median or ulnar nerve at the wrist for upper extremity SSEPs, or posterior tibial nerve at the ankle for lower extremity SSEPs. The ascending sensory volley enters the spinal cord through dorsal nerve roots. Multiple dorsal or posterior column spinal pathways such as the gracile fasciculus (legs and trunk) and the cuneate fasciculus (arms and trunk) mediate the SSEP responses. They arrive at the medulla and make synaptic connection at the medullary nuclei (nucleus cuneatus and nucleus gracilis). From there they cross and ascend in the medial lemniscal pathways to thalamic nuclei, which in turn project up to the somatosenory cortex.

There is no synapses between the peripheral stimulation sites and the medullary nuclei. Synapses are the sites of action for inhalational anesthetic agents. Therefore any SSEP responses recorded below the level of medullary nuclei are affected only minimally by general anesthesia. The cortical SSEP responses, however, are greatly affected by inhalational agents; whichever recorded anesthetic management is very much required.

== Stimulation ==
#'''Stimulation Sites''': SSEPs may be elicited by electrical stimulation to major nerve trunks or dermatomes. Upper extremity mixed or major nerve SSEPs are typically obtained by stimulating the median nerve or ulnar nerve near the wrist; or sometimes the ulnar nerve at the elbow. Lower extremity SSEPs are normally recorded to stimulation of the posterior tibial nerve at the ankle, or peroneal nerve near the head of the fibula at the knee. Usually the anode electrode should be placed 2-4 cm distal to the cathode electrode to avoid anodal block.
#'''Electrodes''': Subdermal needle electrodes or adhesive surface electrodes are most commonly used for intraoperative SSEP recordings. Some other types of electrodes such as bar electrodes and EEG metal disc electrodes are more suitable for diagnostic SSEP recordings in the lab. Needle electrodes can be placed closer to the underlying nerves than surface electrodes; however they are associated with additional risks of infection, bleeding, and inadvertent needle sticks.
#'''Constant Current vs Constant Voltage''': Constant current stimulation is recommended for consistent and reliable stimulation for optimal SSEP recording in the OR, especially for long surgical procedures. Constant current stimuli may compensate for any changes in electrical conductivity, or electrode contact resistance.
#'''Stimulation Parameters''': a series of rectangular pulses with certain pulse width and frequency are typically used as electrical stimuli for SSEP recording.
##'''Pulse width (or pulse duration)''': 200-300 microsecond is suggested
##'''Frequency (or rep rate)''': Higher stimulus rate is desired for quick acquisition of SSEP responses in the operating room. However increasing the rate too high may result in degradation of the responses; also it is limited when interleaving stimulation of 2 or 4 limbs. In general a frequency between 2 and 5 Hz are recommended. To avoid synchronization between the responses and the underlying electrical noise, most commonly 60Hz or 50Hz in different countries, the stimulus rate should not be a submultiple of the noise frequency. Sometimes a slight change of stimulus rate, for example from 4.80 to 4.13, may improve the quality of the evoked responses.
##'''Intensity''': Supramaximal stimulation should be used to produce repeatable responses. Some factors, such as pathology of the peripheral nerves, large or edematous extremities, distance of the electrodes to the underlying nerves, types of stimulating electrodes, may limit the effectiveness of stimulation. A stimulus of 50mA or greater is required sometimes. There is concern of tissue damage from high current densities at the stimulation site which is rare.

== Recording Techniques ==

#'''Electrodes''': Metal surface cup electrodes are non-invasive electrodes, which can be applied with conductive gel after skin preparation. Subdermal needles are also commonly used in the operating room; some are available with angled or hooked tips. A corkscrew version can be screwed into the scalp for prolonged recording, however excessive bleeding can be a concern. For direct cortical mapping and recording, a strip or grid electrode array can be used for cranial surgeries.
#'''Recording Sites'''
##Cortical recording of upper extremity SSEP: It is the post-central gyrus of somatosensory cortex, contralateral to the stimulated limb. The locations are called CP3 and CP4 which are 2 cm posterior to the C3 and C4 positions of the 10-20 International System of EEG electrode placement. The recording montage can be CP3-Fz or CP3-CP4 for right arm stimulation; CP4-Fz or CP4-CP3 for left arm stimulation.
##Cortical recording of lower extremity SSEP: CPz is the active electrode site, which is 2 cm posterior to Cz. The traditional derivation of recording is CPz-Fz; however it should not be the only standard recording derivation, according to studies for optimized lower extremity SSEP recording by MacDonald DB et al and many others. Often time CPz-CPc (which is CPz-CP4 for left leg stimulation, and CPz-CP3 for right) may produce higher amplitude and more reliable signals.
##Subcortical: The recording can be made at posterior cervical spine, one or linked earlobes, or mastoid. Subcortical responses are less affected by inhalational agents. However, the subcortical SSEP response is normally very well defined for upper extremity stimulation, it is often poorly defined for lower extremity stimulation.
##Peripheral nerve: Usually it is the ipsilateral Erb's point for upper extremity stimulation, and ipsilateral popliteal fossa for lower extremity stimulation. This is done to verify the status of the peripheral stimulation.
#'''Averaging:''' The SSEP amplitudes tend to be low; as high as only several microvolts, or as low as less than a microvolts especially with pathological subjects. Averaging is required to record the signal against biological and ambient noise. SSEP signal is time-locked to the stimulus and most of the noise occurs randomly, allowing the noise to be averaged out with averaging of repeated responses. Some earlier guidelines such as that from American EEG Society suggested acquiring 500-2000 trails per averaged response. However for surgical monitoring well defined evoked potentials should be acquired as quickly as possible. With good preparation of the recording sites to reduce impedance, and optimized recording montages, clean signals could be recorded as few as 50-200 trials.
#'''Recording Parameters'''
##Filters: More environmental noise in the operating room and necessity of quick acquisition make it important to choose optimal filter settings, different from laboratory SSEP diagnostic studies. The majority of the energy contained in cortical SSEP is present in the frequency bandpass above 30Hz and below 500Hz; filters can be set to (10-30) Hz to (250-1000) Hz. The relative frequency content of the subcortical or peripheral responses is much higher, thus the filters can be set to (30-100)Hz to (500-2000) Hz.
##Timebase: The timebase is usually set at 50 milliseconds for upper extremity SSEPs, and 100 milliseconds for upper extremity SSEPs. It is based on the normal conduction time between the stimulation and recording sites. It may need adjustment depending on the age and size of the individual, and any pathological conditions.
##Sensitivity: The median amplitude of SSEP is about 1 microvolt. The recording sensitivity could range from 0.1 to 5 microvolts/unit. With direct cortical recording during cranial surgeries, the amplitude can be high and the sensitivity my be set to 20-50 microvolts/unit.
##Sweep delay: Normally it should be avoided to use sweep delay. However it may be necessary when large stimulus artifact is present.
##Interleaving recording: Alternating recording of SSEP responses to stimulation of 2 or 4 limbs at the same time is available with modern recording devices from different manufacturers.

== Waveforms ==
'''Upper Extremity SSEPs'''
#N9: Recorded from ipsilateral Erb's point, and sometimes it's called EP. It is a compound action potential from the axons to the median nerve or ulnar nerve stimulation.
#N13-P14: Cervical potentials, recorded near C5 spinal process. The origin is thought to be dorsal horn neurons, and the caudal medial lemniscus.
#N20-P23: Scalp potentials, generated from the thalamocortical radiations.

'''Lower Extremity SSEPs'''
#LP: Recorded from the T12 spinal process, arise from the afferent nerve volley in the dorsal roots and dorsal root entry zone.
#N34: Recorded at the cervical spine; the amplitude can be very low and difficult to identify. The origin is thought to be brainstem or perhaps thalamus.
#P37-N45: Scalp potentials, reflect activation of the primary somatosensory cortex.

[[Image:SSEP Waveforms.jpg|center|2000px|750px]]

== Intraoperative Monitoring ==
The basic principle of intraoperative monitoring with mixed nerve SSEP is to stimulate nerves distal to the surgical site, and to record responses proximal to the surgical site. In most cases, these recording sites should include one cortical and one subcortical recording site.

#'''Data acquisition''': Baseline SSEP responses should be established, ideally after induction and before patient positioning and surgical incision. Data should be collected throughout the surgical procedure, with corresponding documentation of surgical events such as incision, exposure, decompression, instrumentation, closure, etc. In addition, relevant physiological variables such as blood pressure and temperature changes, anesthetic agents being used and the levels, significant signal changes during the surgery and any communication and interventions should be documented.
#'''Alert criteria''': Significant signal changes, with 50% decrease of peak to peak amplitude and 10% increase of latency, may be indication of nervous system functional changes and should be reported to the operating surgeon, and call for possible intervention.

Depending on the specific surgery, planning is required to monitor the part of the nervous system at risk.
#'''Peripheral nerve and plexus.''' For example, saphenous nerve SSEPs are useful for monitoring the femoral nerve during transpsoas (lateral) lumbar fusions. The femoral nerve and lumbar plexus are at risk for injury with the transpsoas approach, which is due to the approach itself as well as retractor related nerve damage. The femoral nerve is the major branch of the lumbar plexus and exits at the L2-4 level. The saphenous nerve is a sensory branch of the femoral nerve and innervates the medial side of the leg and foot. The saphenous nerve can be found between the vastus medialis and sartorius on the medial part of the thigh. Because it is surrounded by these large muscles and not superficial, it is best to use needle electrodes for stimulation. Higher current intensities and pulse durations relative to posterior tibial nerve SSEPs may be required for a good signal to noise ratio. The cortical responses should be shorter in latency compared to posterior tibial nerve SSEPs.
#'''Spinal nerve root'''
#'''Spinal cord'''
#'''Brainstem and thalamus'''
#'''Brain'''

== Systemic and Anesthesia Factors ==
#'''Temperature'''. The temperature of the patient can increase or decrease SSEP latencies but does not necessarily affect amplitudes.
#'''Blood pressure'''. An intraoperative decline in blood pressure is associated with a loss in SSEP signals.
#'''Halogenated inhalational agents'''. Higher levels of gas anesthesia (> Half MAC) can increase SSEP latencies and reduce SSEP amplitudes.
#'''Nitrous oxide'''. Nitrous oxide decreases SSEP amplitudes but has no effect on latencies.
#'''Intravenous analgesic agents'''. Intravenous anesthetics, such as propofol, do significantly influence SSEP latencies or amplitudes.
#'''Muscle relaxants'''. Neuromuscular blockers do not influence SSEP latencies or amplitudes.

==References==
# Brown RH, Nash CL, Berilla JA, Amaddio MD. Cortical evoked potential monitoring. A system for intraoperative monitoring of spinal cord function. Spine 1984; 9: 256-261.
# Celesia GG. Somatosensory evoked potentials recorded directly from human thalamus and Sm I cortical area. Arch Neurol 1979; 36: 399-405.
# Cohen AR, Young W, Ransohoff J. Intraspinal localization of the somatosensory evoked potential. Neurosurgery 1981; 9: 157-62.
# Kelly DL Jr, Goldring S, O’Leary JL. Averaged evoked somatosensory responses from exposed cortex of man. Arch Neurol 1965; 13: 1-9.
# Larson SJ, Sances A. Evoked potentials in man: neurosurgical applications. Am J Surg 1966; 111: 857-861.
# MacDonald DB. Individually optimizing posterior tibial somatosensory evoked potential P37 scalp derivations for intraoperative monitoring. J Clin Neurophysiol. 2001 Jul;18(4):364-71.
# Mauguiere F. Anatomic origin of the cervical N13 potential evoked by upper extremity stimulation. J of Clin Neurophysiology 2000; 17: 236-245.
# McCallum JE, Bennett MH. Electrophysiologic monitoring of spinal cord function during intraspinal surgery. Surg Forum 1975; 26: 469-471.
# Nash CL, Long RA, Schatzinger LA, Brown RH. Spinal cord monitoring during operative treatment of the spine. Clin Orthop 1977; 126: 100-105.
# Nuwer MR, Dawson EG, Carlson LG, Kanim LEA, Sherman JE. Somatosensory evoked potential spinal cord monitoring reduces neurologic deficits after scoliosis surgery: results of a large multicenter survey. Electroencepholog and Clin Neurophysiol 1995; 96: 6-11.
# Nuwer MR, et al. IFCN recommended standards for short latency somatosensory evoked potentials. Report of an IFCN committee. Electroencephalography and Clinical Neurophysiology 1994; 91: 6-11.
# Powers SK, Bolger CA, Edwards MSB. Spinal cord pathways mediating somatosensory evoked potentials. J Neurosurg 1982; 57: 472-82.
# Simpson RK JR, Blackburn JG, Martin HF III, Katz S. Peripheral nerve fibers and spinal cord pathway contribution to the somatosensory evoked potentials. Exp Neurol 1981; 73: 700-15.
# Sloan TB. Anesthetic Effects on Electrophysiologic Recordings. Journal of Clinical Neurophysiology 1998; 15: 217-226.
# Toleikis JR. Intraoperative monitoring using somatosensory evoked potentials: A position statement by the American Society of Neurophysiological Monitoring. J Clin Monit Comput. 2005; 19(3):241-58.
# Wang BP, Turner LA, Kamp AM, Venier LH. CPz-CP4 and CPz-CP3 are superior to CPz-FPz for recording left and right tibial nerve somatosensory evoked potentials for intraoperative monitoring. A review study in 264 patients. 2006 The 17th Annual Meeting of American Society of Neurophysiological Monitoring.
# Yanni DS, Ulkatan S, Deletis V, Barrenechea IJ, Sen C, Perin NI. Utility of neurophysiological monitoring using dorsal column mapping in intramedullary spinal cord surgery. J Neurosurg Spine. 2010;12:623-628.
# York DH, Chabot RJ, Gaines RW. Response variability of somatosensory evoked potentials during scoliosis surgery. Spine 1987; 12: 864-876.

Somatosensory Evoked Potentials (SSEP)

2024-09-16T16:42:36Z

Wdoyon: /* Intraoperative Monitoring */

Somatosensory evoked potentials (SSEP) are recorded from the central nervous system following stimulation of peripheral nerves.

== Introduction==

Somatosensory Evoked Potentials (SSEPs) are electric signals recorded from the scalp or spine following stimulation to the peripheral nerves. They are time-locked responses, representing the function of the ascending sensory pathways. Early in the 1960s Larson et al introduced the use of somatosensory evoked potentials to monitor neural structure during neurosurgical procedures. It was utilized as a supplement to the wake-up test during correctional spinal surgeries for spinal deformities such as scoliosis to provide warning of compromised spinal cord function to the spine surgeons, as reported by McCallum et al and Nash et al in the 1970s. Since then SSEP has become one of the earliest and primary tools for intraoperative neurophysiological monitoring.

== Somatosensory Pathways ==
[[Image:Ascending Pathways.jpg|right|100px|200px]]

Distal peripheral nerves are stimulated for SSEP recordings; typically the median or ulnar nerve at the wrist for upper extremity SSEPs, or posterior tibial nerve at the ankle for lower extremity SSEPs. The ascending sensory volley enters the spinal cord through dorsal nerve roots. Multiple dorsal or posterior column spinal pathways such as the gracile fasciculus (legs and trunk) and the cuneate fasciculus (arms and trunk) mediate the SSEP responses. They arrive at the medulla and make synaptic connection at the medullary nuclei (nucleus cuneatus and nucleus gracilis). From there they cross and ascend in the medial lemniscal pathways to thalamic nuclei, which in turn project up to the somatosenory cortex.

There is no synapses between the peripheral stimulation sites and the medullary nuclei. Synapses are the sites of action for inhalational anesthetic agents. Therefore any SSEP responses recorded below the level of medullary nuclei are affected only minimally by general anesthesia. The cortical SSEP responses, however, are greatly affected by inhalational agents; whichever recorded anesthetic management is very much required.

== Stimulation ==
#'''Stimulation Sites''': SSEPs may be elicited by electrical stimulation to major nerve trunks or dermatomes. Upper extremity mixed or major nerve SSEPs are typically obtained by stimulating the median nerve or ulnar nerve near the wrist; or sometimes the ulnar nerve at the elbow. Lower extremity SSEPs are normally recorded to stimulation of the posterior tibial nerve at the ankle, or peroneal nerve near the head of the fibula at the knee. Usually the anode electrode should be placed 2-4 cm distal to the cathode electrode to avoid anodal block.
#'''Electrodes''': Subdermal needle electrodes or adhesive surface electrodes are most commonly used for intraoperative SSEP recordings. Some other types of electrodes such as bar electrodes and EEG metal disc electrodes are more suitable for diagnostic SSEP recordings in the lab. Needle electrodes can be placed closer to the underlying nerves than surface electrodes; however they are associated with additional risks of infection, bleeding, and inadvertent needle sticks.
#'''Constant Current vs Constant Voltage''': Constant current stimulation is recommended for consistent and reliable stimulation for optimal SSEP recording in the OR, especially for long surgical procedures. Constant current stimuli may compensate for any changes in electrical conductivity, or electrode contact resistance.
#'''Stimulation Parameters''': a series of rectangular pulses with certain pulse width and frequency are typically used as electrical stimuli for SSEP recording.
##'''Pulse width (or pulse duration)''': 200-300 microsecond is suggested
##'''Frequency (or rep rate)''': Higher stimulus rate is desired for quick acquisition of SSEP responses in the operating room. However increasing the rate too high may result in degradation of the responses; also it is limited when interleaving stimulation of 2 or 4 limbs. In general a frequency between 2 and 5 Hz are recommended. To avoid synchronization between the responses and the underlying electrical noise, most commonly 60Hz or 50Hz in different countries, the stimulus rate should not be a submultiple of the noise frequency. Sometimes a slight change of stimulus rate, for example from 4.80 to 4.13, may improve the quality of the evoked responses.
##'''Intensity''': Supramaximal stimulation should be used to produce repeatable responses. Some factors, such as pathology of the peripheral nerves, large or edematous extremities, distance of the electrodes to the underlying nerves, types of stimulating electrodes, may limit the effectiveness of stimulation. A stimulus of 50mA or greater is required sometimes. There is concern of tissue damage from high current densities at the stimulation site which is rare.

== Recording Techniques ==

#'''Electrodes''': Metal surface cup electrodes are non-invasive electrodes, which can be applied with conductive gel after skin preparation. Subdermal needles are also commonly used in the operating room; some are available with angled or hooked tips. A corkscrew version can be screwed into the scalp for prolonged recording, however excessive bleeding can be a concern. For direct cortical mapping and recording, a strip or grid electrode array can be used for cranial surgeries.
#'''Recording Sites'''
##Cortical recording of upper extremity SSEP: It is the post-central gyrus of somatosensory cortex, contralateral to the stimulated limb. The locations are called CP3 and CP4 which are 2 cm posterior to the C3 and C4 positions of the 10-20 International System of EEG electrode placement. The recording montage can be CP3-Fz or CP3-CP4 for right arm stimulation; CP4-Fz or CP4-CP3 for left arm stimulation.
##Cortical recording of lower extremity SSEP: CPz is the active electrode site, which is 2 cm posterior to Cz. The traditional derivation of recording is CPz-Fz; however it should not be the only standard recording derivation, according to studies for optimized lower extremity SSEP recording by MacDonald DB et al and many others. Often time CPz-CPc (which is CPz-CP4 for left leg stimulation, and CPz-CP3 for right) may produce higher amplitude and more reliable signals.
##Subcortical: The recording can be made at posterior cervical spine, one or linked earlobes, or mastoid. Subcortical responses are less affected by inhalational agents. However, the subcortical SSEP response is normally very well defined for upper extremity stimulation, it is often poorly defined for lower extremity stimulation.
##Peripheral nerve: Usually it is the ipsilateral Erb's point for upper extremity stimulation, and ipsilateral popliteal fossa for lower extremity stimulation. This is done to verify the status of the peripheral stimulation.
#'''Averaging:''' The SSEP amplitudes tend to be low; as high as only several microvolts, or as low as less than a microvolts especially with pathological subjects. Averaging is required to record the signal against biological and ambient noise. SSEP signal is time-locked to the stimulus and most of the noise occurs randomly, allowing the noise to be averaged out with averaging of repeated responses. Some earlier guidelines such as that from American EEG Society suggested acquiring 500-2000 trails per averaged response. However for surgical monitoring well defined evoked potentials should be acquired as quickly as possible. With good preparation of the recording sites to reduce impedance, and optimized recording montages, clean signals could be recorded as few as 50-200 trials.
#'''Recording Parameters'''
##Filters: More environmental noise in the operating room and necessity of quick acquisition make it important to choose optimal filter settings, different from laboratory SSEP diagnostic studies. The majority of the energy contained in cortical SSEP is present in the frequency bandpass above 30Hz and below 500Hz; filters can be set to (10-30) Hz to (250-1000) Hz. The relative frequency content of the subcortical or peripheral responses is much higher, thus the filters can be set to (30-100)Hz to (500-2000) Hz.
##Timebase: The timebase is usually set at 50 milliseconds for upper extremity SSEPs, and 100 milliseconds for upper extremity SSEPs. It is based on the normal conduction time between the stimulation and recording sites. It may need adjustment depending on the age and size of the individual, and any pathological conditions.
##Sensitivity: The median amplitude of SSEP is about 1 microvolt. The recording sensitivity could range from 0.1 to 5 microvolts/unit. With direct cortical recording during cranial surgeries, the amplitude can be high and the sensitivity my be set to 20-50 microvolts/unit.
##Sweep delay: Normally it should be avoided to use sweep delay. However it may be necessary when large stimulus artifact is present.
##Interleaving recording: Alternating recording of SSEP responses to stimulation of 2 or 4 limbs at the same time is available with modern recording devices from different manufacturers.

== Waveforms ==
'''Upper Extremity SSEPs'''
#N9: Recorded from ipsilateral Erb's point, and sometimes it's called EP. It is a compound action potential from the axons to the median nerve or ulnar nerve stimulation.
#N13-P14: Cervical potentials, recorded near C5 spinal process. The origin is thought to be dorsal horn neurons, and the caudal medial lemniscus.
#N20-P23: Scalp potentials, generated from the thalamocortical radiations.

'''Lower Extremity SSEPs'''
#LP: Recorded from the T12 spinal process, arise from the afferent nerve volley in the dorsal roots and dorsal root entry zone.
#N34: Recorded at the cervical spine; the amplitude can be very low and difficult to identify. The origin is thought to be brainstem or perhaps thalamus.
#P37-N45: Scalp potentials, reflect activation of the primary somatosensory cortex.

[[Image:SSEP Waveforms.jpg|center|2000px|750px]]

== Intraoperative Monitoring ==
The basic principle of intraoperative monitoring with mixed nerve SSEP is to stimulate nerves distal to the surgical site, and to record responses proximal to the surgical site. In most cases, these recording sites should include one cortical and one subcortical recording site.

#'''Data acquisition''': Baseline SSEP responses should be established, ideally after induction and before patient positioning and surgical incision. Data should be collected throughout the surgical procedure, with corresponding documentation of surgical events such as incision, exposure, decompression, instrumentation, closure, etc. In addition, relevant physiological variables such as blood pressure and temperature changes, anesthetic agents being used and the levels, significant signal changes during the surgery and any communication and interventions should be documented.
#'''Alert criteria''': Significant signal changes, with 50% decrease of peak to peak amplitude and 10% increase of latency, may be indication of nervous system functional changes and should be reported to the operating surgeon, and call for possible intervention.

Depending on the specific surgery, planning is required to monitor the part of the nervous system at risk.
#'''Peripheral nerve and plexus''' For example, saphenous nerve SSEPs are useful for monitoring the femoral nerve during transpsoas (lateral) lumbar fusions. The femoral nerve and lumbar plexus are at risk for injury with the transpsoas approach, which is due to the approach itself as well as retractor related nerve damage. The femoral nerve is the major branch of the lumbar plexus and exits at the L2-4 level. The saphenous nerve is a sensory branch of the femoral nerve and innervates the medial side of the leg and foot. The saphenous nerve can be found between the vastus medialis and sartorius on the medial part of the thigh. Because it is surrounded by these large muscles and not superficial, it is best to use needle electrodes for stimulation. Higher current intensities and pulse durations relative to posterior tibial nerve SSEPs may be required for a good signal to noise ratio. The cortical responses should be shorter in latency compared to posterior tibial nerve SSEPs.
#'''Spinal nerve root'''
#'''Spinal cord'''
#'''Brainstem and thalamus'''
#'''Brain'''

== Systemic and Anesthesia Factors ==
#'''Temperature'''. The temperature of the patient can increase or decrease SSEP latencies but does not necessarily affect amplitudes.
#'''Blood pressure'''. An intraoperative decline in blood pressure is associated with a loss in SSEP signals.
#'''Halogenated inhalational agents'''. Higher levels of gas anesthesia (> Half MAC) can increase SSEP latencies and reduce SSEP amplitudes.
#'''Nitrous oxide'''. Nitrous oxide decreases SSEP amplitudes but has no effect on latencies.
#'''Intravenous analgesic agents'''. Intravenous anesthetics, such as propofol, do significantly influence SSEP latencies or amplitudes.
#'''Muscle relaxants'''. Neuromuscular blockers do not influence SSEP latencies or amplitudes.

==References==
# Brown RH, Nash CL, Berilla JA, Amaddio MD. Cortical evoked potential monitoring. A system for intraoperative monitoring of spinal cord function. Spine 1984; 9: 256-261.
# Celesia GG. Somatosensory evoked potentials recorded directly from human thalamus and Sm I cortical area. Arch Neurol 1979; 36: 399-405.
# Cohen AR, Young W, Ransohoff J. Intraspinal localization of the somatosensory evoked potential. Neurosurgery 1981; 9: 157-62.
# Kelly DL Jr, Goldring S, O’Leary JL. Averaged evoked somatosensory responses from exposed cortex of man. Arch Neurol 1965; 13: 1-9.
# Larson SJ, Sances A. Evoked potentials in man: neurosurgical applications. Am J Surg 1966; 111: 857-861.
# MacDonald DB. Individually optimizing posterior tibial somatosensory evoked potential P37 scalp derivations for intraoperative monitoring. J Clin Neurophysiol. 2001 Jul;18(4):364-71.
# Mauguiere F. Anatomic origin of the cervical N13 potential evoked by upper extremity stimulation. J of Clin Neurophysiology 2000; 17: 236-245.
# McCallum JE, Bennett MH. Electrophysiologic monitoring of spinal cord function during intraspinal surgery. Surg Forum 1975; 26: 469-471.
# Nash CL, Long RA, Schatzinger LA, Brown RH. Spinal cord monitoring during operative treatment of the spine. Clin Orthop 1977; 126: 100-105.
# Nuwer MR, Dawson EG, Carlson LG, Kanim LEA, Sherman JE. Somatosensory evoked potential spinal cord monitoring reduces neurologic deficits after scoliosis surgery: results of a large multicenter survey. Electroencepholog and Clin Neurophysiol 1995; 96: 6-11.
# Nuwer MR, et al. IFCN recommended standards for short latency somatosensory evoked potentials. Report of an IFCN committee. Electroencephalography and Clinical Neurophysiology 1994; 91: 6-11.
# Powers SK, Bolger CA, Edwards MSB. Spinal cord pathways mediating somatosensory evoked potentials. J Neurosurg 1982; 57: 472-82.
# Simpson RK JR, Blackburn JG, Martin HF III, Katz S. Peripheral nerve fibers and spinal cord pathway contribution to the somatosensory evoked potentials. Exp Neurol 1981; 73: 700-15.
# Sloan TB. Anesthetic Effects on Electrophysiologic Recordings. Journal of Clinical Neurophysiology 1998; 15: 217-226.
# Toleikis JR. Intraoperative monitoring using somatosensory evoked potentials: A position statement by the American Society of Neurophysiological Monitoring. J Clin Monit Comput. 2005; 19(3):241-58.
# Wang BP, Turner LA, Kamp AM, Venier LH. CPz-CP4 and CPz-CP3 are superior to CPz-FPz for recording left and right tibial nerve somatosensory evoked potentials for intraoperative monitoring. A review study in 264 patients. 2006 The 17th Annual Meeting of American Society of Neurophysiological Monitoring.
# Yanni DS, Ulkatan S, Deletis V, Barrenechea IJ, Sen C, Perin NI. Utility of neurophysiological monitoring using dorsal column mapping in intramedullary spinal cord surgery. J Neurosurg Spine. 2010;12:623-628.
# York DH, Chabot RJ, Gaines RW. Response variability of somatosensory evoked potentials during scoliosis surgery. Spine 1987; 12: 864-876.

IONM in Spinal Surgery

2022-01-28T01:51:47Z

Wdoyon: /* Others */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral spine surgery==
'''Posterior lumbosacral interbody fusions'''. Lumbosacral fusions are usually performed to stabilize the spine and relieve pressure on the exiting nerve roots. The lumbosacral spine includes the L1-L5 levels as well as the sacrum (S1-S5 are five fused segments). Compression of the exiting nerve roots, or the spinal cord at the L1-2 level, can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Problems of the lumbar spine that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, lordosis and scoliosis.

The lumbar plexus is at risk for nerve injury during a fusion. At the upper lumbar levels, the nerves of the lumbar plexus are in most cases posterior to the surgical site. But the risk for injury to the lumbar plexus becomes greater if the surgical approach is below the L3 level. IONM of lower lumbar and sacral regions involves the monitoring of spinal nerve root function, primarily with EMG recordings and SSEPs. Ascending and descending spinal cord function is monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw and the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles percutaneously. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

'''Lateral lumbar interbody fusions'''. This procedure is considered minimally invasive. In contrast to posterior lumbar fusion, the incision for a lateral lumbar fusion approaches the lumbar spine from a far lateral approach that passes through the psoas muscles. The main advantages of this technique is that injury to the posterior spinal ligaments is avoided and that larger cages can be inserted for greater stability of the interbody device. However, there is a higher risk of injury to the lumbar plexus using this approach due to direct injury or prolonged use of retractors. While nerve injuries tend to be temporary, there is a risk for permanent motor and sensory deficits due to injury of the femoral nerve. IONM strategies for monitoring of LLIFs include a combination of EMG, transcranial MEPs and SSEPs of the saphenous nerve. The saphenous nerve is the largest terminal cutaneous branch of the femoral nerve.

==Cervical spine surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic spinal surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

For IONM of thoracic spinal surgeries, such as an instrumented fusion or a laminectomy, we monitor spinal cord and nerve roots function, similar to upper lumbar surgeries. For thoracic fusions, the pedicle screws are not always tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach. However, we are most concerned about a medial breach due to the proximity to the spinal cord. Although the thresholds for thoracic pedicle screws are lower in general, it may be necessary to increase the stimulation intensity to see a spinal cord response, using pulse-train protocols that are similar to those used to generate MEPs. One study found that pedicle screw testing with triggered EMG was more effective at detecting medial breaches at the T10-12 level compared to the T2-9 level (Samdani et al., Eur Spine J. 2011 Jun; 20(6): 869–874). Alternatively, another approach to direct pedicle screw testing has shown that pulse-train stimulation inside of the pedicle track with electromyography from lower limb muscles was very effective at detecting medial breaches from misplaced screws (Calancie et al., J Neurosurg Spine 20:675–691, 2014).

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their location relative to the dura mater, one of the protective membranes of the spinal cord and brain. Extradural tumors are located outside the dura, whereas intradural tumors are located underneath the dura but outside of the spinal cord. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness, numbness and paralysis.

IONM of spinal tumor resections is similar to other spinal surgeries and includes recordings of MEPs, SSEPs and EMG activity. Extra lower extremity electrodes are often used to distinguish between different muscle groups for great spatial resolution. For thoracic tumors, the abdominal and intercostal muscles are also recorded depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes. During the tumor resection, the surgeon may ask to stimulate specific nerve roots to ensure that nerve function is intact. Nerve root stimulation is more common for lumbar tumors than for thoracic tumors.

==Others==

'''1. Interbody cages and bone grafts.''' For spinal decompressions involving a discectomy, the surgeon will remove a part or all of the intervertebral disc. If so, it is necessary to fill the empty disc space between the two vertebral bodies with either a bone graft (e.g., autograft, allograft) or an interbody cage to maintain the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-28T00:18:34Z

Wdoyon: /* Others */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral spine surgery==
'''Posterior lumbosacral interbody fusions'''. Lumbosacral fusions are usually performed to stabilize the spine and relieve pressure on the exiting nerve roots. The lumbosacral spine includes the L1-L5 levels as well as the sacrum (S1-S5 are five fused segments). Compression of the exiting nerve roots, or the spinal cord at the L1-2 level, can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Problems of the lumbar spine that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, lordosis and scoliosis.

The lumbar plexus is at risk for nerve injury during a fusion. At the upper lumbar levels, the nerves of the lumbar plexus are in most cases posterior to the surgical site. But the risk for injury to the lumbar plexus becomes greater if the surgical approach is below the L3 level. IONM of lower lumbar and sacral regions involves the monitoring of spinal nerve root function, primarily with EMG recordings and SSEPs. Ascending and descending spinal cord function is monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw and the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles percutaneously. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

'''Lateral lumbar interbody fusions'''. This procedure is considered minimally invasive. In contrast to posterior lumbar fusion, the incision for a lateral lumbar fusion approaches the lumbar spine from a far lateral approach that passes through the psoas muscles. The main advantages of this technique is that injury to the posterior spinal ligaments is avoided and that larger cages can be inserted for greater stability of the interbody device. However, there is a higher risk of injury to the lumbar plexus using this approach due to direct injury or prolonged use of retractors. While nerve injuries tend to be temporary, there is a risk for permanent motor and sensory deficits due to injury of the femoral nerve. IONM strategies for monitoring of LLIFs include a combination of EMG, transcranial MEPs and SSEPs of the saphenous nerve. The saphenous nerve is the largest terminal cutaneous branch of the femoral nerve.

==Cervical spine surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic spinal surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

For IONM of thoracic spinal surgeries, such as an instrumented fusion or a laminectomy, we monitor spinal cord and nerve roots function, similar to upper lumbar surgeries. For thoracic fusions, the pedicle screws are not always tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach. However, we are most concerned about a medial breach due to the proximity to the spinal cord. Although the thresholds for thoracic pedicle screws are lower in general, it may be necessary to increase the stimulation intensity to see a spinal cord response, using pulse-train protocols that are similar to those used to generate MEPs. One study found that pedicle screw testing with triggered EMG was more effective at detecting medial breaches at the T10-12 level compared to the T2-9 level (Samdani et al., Eur Spine J. 2011 Jun; 20(6): 869–874). Alternatively, another approach to direct pedicle screw testing has shown that pulse-train stimulation inside of the pedicle track with electromyography from lower limb muscles was very effective at detecting medial breaches from misplaced screws (Calancie et al., J Neurosurg Spine 20:675–691, 2014).

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their location relative to the dura mater, one of the protective membranes of the spinal cord and brain. Extradural tumors are located outside the dura, whereas intradural tumors are located underneath the dura but outside of the spinal cord. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness, numbness and paralysis.

IONM of spinal tumor resections is similar to other spinal surgeries and includes recordings of MEPs, SSEPs and EMG activity. Extra lower extremity electrodes are often used to distinguish between different muscle groups for great spatial resolution. For thoracic tumors, the abdominal and intercostal muscles are also recorded depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes. During the tumor resection, the surgeon may ask to stimulate specific nerve roots to ensure that nerve function is intact. Nerve root stimulation is more common for lumbar tumors than for thoracic tumors.

==Others==

'''1. Interbody cages and bone grafts.''' For spinal decompressions involving a discectomy, the surgeon will remove a part or all of the intervertebral disc. If so, it is necessary to fill the empty disc space between the vertebral body with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-27T20:01:32Z

Wdoyon: /* Lumbosacral spine surgery */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral spine surgery==
'''Posterior lumbosacral interbody fusions'''. Lumbosacral fusions are usually performed to stabilize the spine and relieve pressure on the exiting nerve roots. The lumbosacral spine includes the L1-L5 levels as well as the sacrum (S1-S5 are five fused segments). Compression of the exiting nerve roots, or the spinal cord at the L1-2 level, can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Problems of the lumbar spine that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, lordosis and scoliosis.

The lumbar plexus is at risk for nerve injury during a fusion. At the upper lumbar levels, the nerves of the lumbar plexus are in most cases posterior to the surgical site. But the risk for injury to the lumbar plexus becomes greater if the surgical approach is below the L3 level. IONM of lower lumbar and sacral regions involves the monitoring of spinal nerve root function, primarily with EMG recordings and SSEPs. Ascending and descending spinal cord function is monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw and the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles percutaneously. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

'''Lateral lumbar interbody fusions'''. This procedure is considered minimally invasive. In contrast to posterior lumbar fusion, the incision for a lateral lumbar fusion approaches the lumbar spine from a far lateral approach that passes through the psoas muscles. The main advantages of this technique is that injury to the posterior spinal ligaments is avoided and that larger cages can be inserted for greater stability of the interbody device. However, there is a higher risk of injury to the lumbar plexus using this approach due to direct injury or prolonged use of retractors. While nerve injuries tend to be temporary, there is a risk for permanent motor and sensory deficits due to injury of the femoral nerve. IONM strategies for monitoring of LLIFs include a combination of EMG, transcranial MEPs and SSEPs of the saphenous nerve. The saphenous nerve is the largest terminal cutaneous branch of the femoral nerve.

==Cervical spine surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic spinal surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

For IONM of thoracic spinal surgeries, such as an instrumented fusion or a laminectomy, we monitor spinal cord and nerve roots function, similar to upper lumbar surgeries. For thoracic fusions, the pedicle screws are not always tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach. However, we are most concerned about a medial breach due to the proximity to the spinal cord. Although the thresholds for thoracic pedicle screws are lower in general, it may be necessary to increase the stimulation intensity to see a spinal cord response, using pulse-train protocols that are similar to those used to generate MEPs. One study found that pedicle screw testing with triggered EMG was more effective at detecting medial breaches at the T10-12 level compared to the T2-9 level (Samdani et al., Eur Spine J. 2011 Jun; 20(6): 869–874). Alternatively, another approach to direct pedicle screw testing has shown that pulse-train stimulation inside of the pedicle track with electromyography from lower limb muscles was very effective at detecting medial breaches from misplaced screws (Calancie et al., J Neurosurg Spine 20:675–691, 2014).

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their location relative to the dura mater, one of the protective membranes of the spinal cord and brain. Extradural tumors are located outside the dura, whereas intradural tumors are located underneath the dura but outside of the spinal cord. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness, numbness and paralysis.

IONM of spinal tumor resections is similar to other spinal surgeries and includes recordings of MEPs, SSEPs and EMG activity. Extra lower extremity electrodes are often used to distinguish between different muscle groups for great spatial resolution. For thoracic tumors, the abdominal and intercostal muscles are also recorded depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes. During the tumor resection, the surgeon may ask to stimulate specific nerve roots to ensure that nerve function is intact. Nerve root stimulation is more common for lumbar tumors than for thoracic tumors.

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part or all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-27T20:00:21Z

Wdoyon: /* Lumbosacral spine surgery */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral spine surgery==
'''Posterior lumbosacral interbody fusions'''. Lumbosacral fusions are usually performed to stabilize the spine and relieve pressure on the exiting nerve roots. The lumbosacral spine includes the L1-L5 levels as well as the sacrum (S1-S5 are five fused segments). Compression of the exiting nerve roots, or the spinal cord at the L1-2 level, can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Problems of the lumbar spine that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, lordosis and scoliosis.

The lumbar plexus is at risk for nerve injury during a fusion. At the upper lumbar levels, the nerves of the lumbar plexus are in most cases posterior to the surgical site. But the risk for injury to the lumbar plexus becomes greater if the surgical approach is below the L3 level. IONM of lower lumbar and sacral regions involves the monitoring of spinal nerve root function, primarily with EMG recordings and SSEPs. Ascending and descending spinal cord function is monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles percutaneously. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

'''Lateral lumbar interbody fusions'''. This procedure is considered minimally invasive. In contrast to posterior lumbar fusion, the incision for a lateral lumbar fusion approaches the lumbar spine from a far lateral approach that passes through the psoas muscles. The main advantages of this technique is that injury to the posterior spinal ligaments is avoided and that larger cages can be inserted for greater stability of the interbody device. However, there is a higher risk of injury to the lumbar plexus using this approach due to direct injury or prolonged use of retractors. While nerve injuries tend to be temporary, there is a risk for permanent motor and sensory deficits due to injury of the femoral nerve. IONM strategies for monitoring of LLIFs include a combination of EMG, transcranial MEPs and SSEPs of the saphenous nerve. The saphenous nerve is the largest terminal cutaneous branch of the femoral nerve.

==Cervical spine surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic spinal surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

For IONM of thoracic spinal surgeries, such as an instrumented fusion or a laminectomy, we monitor spinal cord and nerve roots function, similar to upper lumbar surgeries. For thoracic fusions, the pedicle screws are not always tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach. However, we are most concerned about a medial breach due to the proximity to the spinal cord. Although the thresholds for thoracic pedicle screws are lower in general, it may be necessary to increase the stimulation intensity to see a spinal cord response, using pulse-train protocols that are similar to those used to generate MEPs. One study found that pedicle screw testing with triggered EMG was more effective at detecting medial breaches at the T10-12 level compared to the T2-9 level (Samdani et al., Eur Spine J. 2011 Jun; 20(6): 869–874). Alternatively, another approach to direct pedicle screw testing has shown that pulse-train stimulation inside of the pedicle track with electromyography from lower limb muscles was very effective at detecting medial breaches from misplaced screws (Calancie et al., J Neurosurg Spine 20:675–691, 2014).

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their location relative to the dura mater, one of the protective membranes of the spinal cord and brain. Extradural tumors are located outside the dura, whereas intradural tumors are located underneath the dura but outside of the spinal cord. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness, numbness and paralysis.

IONM of spinal tumor resections is similar to other spinal surgeries and includes recordings of MEPs, SSEPs and EMG activity. Extra lower extremity electrodes are often used to distinguish between different muscle groups for great spatial resolution. For thoracic tumors, the abdominal and intercostal muscles are also recorded depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes. During the tumor resection, the surgeon may ask to stimulate specific nerve roots to ensure that nerve function is intact. Nerve root stimulation is more common for lumbar tumors than for thoracic tumors.

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part or all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-27T19:38:22Z

Wdoyon: /* Lumbosacral spine surgery */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral spine surgery==
'''Posterior lumbosacral interbody fusions'''. Lumbosacral fusions are performed to stabilize the spine and relieve pressure on the exiting nerve roots. The lumbosacral spine includes the L1-L5 levels as well as the sacrum (S1-S5 are five fused segments). Compression of the exiting nerve roots, or the spinal cord at the L1-2 level, can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Problems of the lumbar spine that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, lordosis and scoliosis.

The lumbar plexus is at risk for nerve injury during a fusion. At the upper lumbar levels, the nerves of the lumbar plexus are in most cases posterior to the surgical site. But the risk for injury to the lumbar plexus becomes greater if the surgical site is below the L3 level. IONM of lower lumbar and sacral regions involves the monitoring of spinal nerve root function, primarily with EMG recordings. Ascending and descending spinal cord function is also monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles percutaneously. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

'''Lateral lumbar interbody fusions'''. This procedure is considered minimally invasive. In contrast to posterior lumbar fusion, the incision for a lateral lumbar fusion approaches the lumbar spine from a far lateral approach that passes through the psoas muscles. The main advantages of this technique is that injury to the posterior spinal ligaments is avoided and that larger cages can be inserted for greater stability of the interbody device. However, there is a higher risk of injury to the lumbar plexus using this approach due to direct injury or prolonged use of retractors. While nerve injuries tend to be temporary, there is a risk for permanent motor and sensory deficits due to injury of the femoral nerve. IONM strategies for monitoring of LLIFs include a combination of EMG, transcranial MEPs and SSEPs of the saphenous nerve. The saphenous nerve is the largest terminal cutaneous branch of the femoral nerve.

==Cervical spine surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic spinal surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

For IONM of thoracic spinal surgeries, such as an instrumented fusion or a laminectomy, we monitor spinal cord and nerve roots function, similar to upper lumbar surgeries. For thoracic fusions, the pedicle screws are not always tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach. However, we are most concerned about a medial breach due to the proximity to the spinal cord. Although the thresholds for thoracic pedicle screws are lower in general, it may be necessary to increase the stimulation intensity to see a spinal cord response, using pulse-train protocols that are similar to those used to generate MEPs. One study found that pedicle screw testing with triggered EMG was more effective at detecting medial breaches at the T10-12 level compared to the T2-9 level (Samdani et al., Eur Spine J. 2011 Jun; 20(6): 869–874). Alternatively, another approach to direct pedicle screw testing has shown that pulse-train stimulation inside of the pedicle track with electromyography from lower limb muscles was very effective at detecting medial breaches from misplaced screws (Calancie et al., J Neurosurg Spine 20:675–691, 2014).

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their location relative to the dura mater, one of the protective membranes of the spinal cord and brain. Extradural tumors are located outside the dura, whereas intradural tumors are located underneath the dura but outside of the spinal cord. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness, numbness and paralysis.

IONM of spinal tumor resections is similar to other spinal surgeries and includes recordings of MEPs, SSEPs and EMG activity. Extra lower extremity electrodes are often used to distinguish between different muscle groups for great spatial resolution. For thoracic tumors, the abdominal and intercostal muscles are also recorded depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes. During the tumor resection, the surgeon may ask to stimulate specific nerve roots to ensure that nerve function is intact. Nerve root stimulation is more common for lumbar tumors than for thoracic tumors.

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part or all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-21T03:51:58Z

Wdoyon: /* Thoracic spinal surgery */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral spine surgery==
'''Posterior lumbar-sacral interbody fusions'''. Posterior lumbosacral fusions are common surgeries and can include the L1-5 lumbar levels as well as the S1 sacral level S1 (S1-S5 are five fused segments). Lumbosacral fusions are performed to relieve pressure on the nerve roots or to stabilize the spine, which can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, and scoliosis.

IONM of lower lumbar and sacral regions (L3 and below) involves the monitoring of spinal nerve root function, primarily with EMG recordings. Ascending and descending spinal cord function is also monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles percutaneously. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

'''Lateral lumbar interbody fusions'''. This procedure is considered minimally invasive. In contrast to posterior lumbar fusion, the incision for a lateral lumbar fusion approaches the lumbar spine from a far lateral approach that passes through the psoas muscles. The main advantages of this technique is that injury to the posterior spinal ligaments is avoided and that larger cages can be inserted for greater stability of the interbody device. However, there is a higher risk of injury to the lumbar plexus using this approach. While nerve injuries tend to be temporary, there is a risk for permanent motor and sensory deficits due to injury of the femoral nerve. IONM strategies for monitoring of LLIFs include a combination of EMG, transcranial MEPs and SSEPs of the saphenous nerve. The saphenous nerve is the largest terminal cutaneous branch of the femoral nerve.

==Cervical spine surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic spinal surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

For IONM of thoracic spinal surgeries, such as an instrumented fusion or a laminectomy, we monitor spinal cord and nerve roots function, similar to upper lumbar surgeries. For thoracic fusions, the pedicle screws are not always tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach. However, we are most concerned about a medial breach due to the proximity to the spinal cord. Although the thresholds for thoracic pedicle screws are lower in general, it may be necessary to increase the stimulation intensity to see a spinal cord response, using pulse-train protocols that are similar to those used to generate MEPs. One study found that pedicle screw testing with triggered EMG was more effective at detecting medial breaches at the T10-12 level compared to the T2-9 level (Samdani et al., Eur Spine J. 2011 Jun; 20(6): 869–874). Alternatively, another approach to direct pedicle screw testing has shown that pulse-train stimulation inside of the pedicle track with electromyography from lower limb muscles was very effective at detecting medial breaches from misplaced screws (Calancie et al., J Neurosurg Spine 20:675–691, 2014).

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their location relative to the dura mater, one of the protective membranes of the spinal cord and brain. Extradural tumors are located outside the dura, whereas intradural tumors are located underneath the dura but outside of the spinal cord. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness, numbness and paralysis.

IONM of spinal tumor resections is similar to other spinal surgeries and includes recordings of MEPs, SSEPs and EMG activity. Extra lower extremity electrodes are often used to distinguish between different muscle groups for great spatial resolution. For thoracic tumors, the abdominal and intercostal muscles are also recorded depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes. During the tumor resection, the surgeon may ask to stimulate specific nerve roots to ensure that nerve function is intact. Nerve root stimulation is more common for lumbar tumors than for thoracic tumors.

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part or all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-21T03:37:10Z

Wdoyon: /* Thoracic spinal surgery */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral spine surgery==
'''Posterior lumbar-sacral interbody fusions'''. Posterior lumbosacral fusions are common surgeries and can include the L1-5 lumbar levels as well as the S1 sacral level S1 (S1-S5 are five fused segments). Lumbosacral fusions are performed to relieve pressure on the nerve roots or to stabilize the spine, which can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, and scoliosis.

IONM of lower lumbar and sacral regions (L3 and below) involves the monitoring of spinal nerve root function, primarily with EMG recordings. Ascending and descending spinal cord function is also monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles percutaneously. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

'''Lateral lumbar interbody fusions'''. This procedure is considered minimally invasive. In contrast to posterior lumbar fusion, the incision for a lateral lumbar fusion approaches the lumbar spine from a far lateral approach that passes through the psoas muscles. The main advantages of this technique is that injury to the posterior spinal ligaments is avoided and that larger cages can be inserted for greater stability of the interbody device. However, there is a higher risk of injury to the lumbar plexus using this approach. While nerve injuries tend to be temporary, there is a risk for permanent motor and sensory deficits due to injury of the femoral nerve. IONM strategies for monitoring of LLIFs include a combination of EMG, transcranial MEPs and SSEPs of the saphenous nerve. The saphenous nerve is the largest terminal cutaneous branch of the femoral nerve.

==Cervical spine surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic spinal surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

For IONM of thoracic spinal surgeries, such as an instrumented fusion or a laminectomy, we monitor spinal cord and nerve roots function, similar to upper lumbar surgeries. For thoracic fusions, the pedicle screws are not always tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach. However, we are most concerned about a medial breach due to the proximity to the spinal cord. Although the thresholds for thoracic pedicle screws are lower in general, it may be necessary to increase the stimulation intensity to see a spinal cord response, using protocols that are similar to those used to generate MEPs. One study found that triggered EMG was more effective at detecting medial breaches at the T10-12 level compared to the T2-9 level (Samdani et al., Eur Spine J. 2011 Jun; 20(6): 869–874).

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their location relative to the dura mater, one of the protective membranes of the spinal cord and brain. Extradural tumors are located outside the dura, whereas intradural tumors are located underneath the dura but outside of the spinal cord. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness, numbness and paralysis.

IONM of spinal tumor resections is similar to other spinal surgeries and includes recordings of MEPs, SSEPs and EMG activity. Extra lower extremity electrodes are often used to distinguish between different muscle groups for great spatial resolution. For thoracic tumors, the abdominal and intercostal muscles are also recorded depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes. During the tumor resection, the surgeon may ask to stimulate specific nerve roots to ensure that nerve function is intact. Nerve root stimulation is more common for lumbar tumors than for thoracic tumors.

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part or all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-21T03:28:23Z

Wdoyon: /* Thoracic spinal surgery */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral spine surgery==
'''Posterior lumbar-sacral interbody fusions'''. Posterior lumbosacral fusions are common surgeries and can include the L1-5 lumbar levels as well as the S1 sacral level S1 (S1-S5 are five fused segments). Lumbosacral fusions are performed to relieve pressure on the nerve roots or to stabilize the spine, which can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, and scoliosis.

IONM of lower lumbar and sacral regions (L3 and below) involves the monitoring of spinal nerve root function, primarily with EMG recordings. Ascending and descending spinal cord function is also monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles percutaneously. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

'''Lateral lumbar interbody fusions'''. This procedure is considered minimally invasive. In contrast to posterior lumbar fusion, the incision for a lateral lumbar fusion approaches the lumbar spine from a far lateral approach that passes through the psoas muscles. The main advantages of this technique is that injury to the posterior spinal ligaments is avoided and that larger cages can be inserted for greater stability of the interbody device. However, there is a higher risk of injury to the lumbar plexus using this approach. While nerve injuries tend to be temporary, there is a risk for permanent motor and sensory deficits due to injury of the femoral nerve. IONM strategies for monitoring of LLIFs include a combination of EMG, transcranial MEPs and SSEPs of the saphenous nerve. The saphenous nerve is the largest terminal cutaneous branch of the femoral nerve.

==Cervical spine surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic spinal surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

For IONM of thoracic spinal surgeries, such as an instrumented fusion or a laminectomy, we monitor spinal cord and nerve roots function, similar to upper lumbar surgeries. For thoracic fusions, the pedicle screws are not always tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach. However, we are most concerned about a medial breach due to the proximity to the spinal cord. Although the thresholds for thoracic pedicle screws are lower in general, it may be necessary to increase the stimulation intensity to see a spinal cord response, using protocols that are similar to those used to generate MEPs.

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their location relative to the dura mater, one of the protective membranes of the spinal cord and brain. Extradural tumors are located outside the dura, whereas intradural tumors are located underneath the dura but outside of the spinal cord. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness, numbness and paralysis.

IONM of spinal tumor resections is similar to other spinal surgeries and includes recordings of MEPs, SSEPs and EMG activity. Extra lower extremity electrodes are often used to distinguish between different muscle groups for great spatial resolution. For thoracic tumors, the abdominal and intercostal muscles are also recorded depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes. During the tumor resection, the surgeon may ask to stimulate specific nerve roots to ensure that nerve function is intact. Nerve root stimulation is more common for lumbar tumors than for thoracic tumors.

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part or all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

Somatosensory Evoked Potentials (SSEP)

2022-01-19T03:10:10Z

Wdoyon: /* Systemic and Anesthesia Factors */

Somatosensory evoked potentials (SSEP) are recorded from the central nervous system following stimulation of peripheral nerves.

== Introduction==

Somatosensory Evoked Potentials (SSEPs) are electric signals recorded from the scalp or spine following stimulation to the peripheral nerves. They are time-locked responses, representing the function of the ascending sensory pathways. Early in the 1960s Larson et al introduced the use of somatosensory evoked potentials to monitor neural structure during neurosurgical procedures. It was utilized as a supplement to the wake-up test during correctional spinal surgeries for spinal deformities such as scoliosis to provide warning of compromised spinal cord function to the spine surgeons, as reported by McCallum et al and Nash et al in the 1970s. Since then SSEP has become one of the earliest and primary tools for intraoperative neurophysiological monitoring.

== Somatosensory Pathways ==
[[Image:Ascending Pathways.jpg|right|100px|200px]]

Distal peripheral nerves are stimulated for SSEP recordings; typically the median or ulnar nerve at the wrist for upper extremity SSEPs, or posterior tibial nerve at the ankle for lower extremity SSEPs. The ascending sensory volley enters the spinal cord through dorsal nerve roots. Multiple dorsal or posterior column spinal pathways such as the gracile fasciculus (legs and trunk) and the cuneate fasciculus (arms and trunk) mediate the SSEP responses. They arrive at the medulla and make synaptic connection at the medullary nuclei (nucleus cuneatus and nucleus gracilis). From there they cross and ascend in the medial lemniscal pathways to thalamic nuclei, which in turn project up to the somatosenory cortex.

There is no synapses between the peripheral stimulation sites and the medullary nuclei. Synapses are the sites of action for inhalational anesthetic agents. Therefore any SSEP responses recorded below the level of medullary nuclei are affected only minimally by general anesthesia. The cortical SSEP responses, however, are greatly affected by inhalational agents; whichever recorded anesthetic management is very much required.

== Stimulation ==
#'''Stimulation Sites''': SSEPs may be elicited by electrical stimulation to major nerve trunks or dermatomes. Upper extremity mixed or major nerve SSEPs are typically obtained by stimulating the median nerve or ulnar nerve near the wrist; or sometimes the ulnar nerve at the elbow. Lower extremity SSEPs are normally recorded to stimulation of the posterior tibial nerve at the ankle, or peroneal nerve near the head of the fibula at the knee. Usually the anode electrode should be placed 2-4 cm distal to the cathode electrode to avoid anodal block.
#'''Electrodes''': Subdermal needle electrodes or adhesive surface electrodes are most commonly used for intraoperative SSEP recordings. Some other types of electrodes such as bar electrodes and EEG metal disc electrodes are more suitable for diagnostic SSEP recordings in the lab. Needle electrodes can be placed closer to the underlying nerves than surface electrodes; however they are associated with additional risks of infection, bleeding, and inadvertent needle sticks.
#'''Constant Current vs Constant Voltage''': Constant current stimulation is recommended for consistent and reliable stimulation for optimal SSEP recording in the OR, especially for long surgical procedures. Constant current stimuli may compensate for any changes in electrical conductivity, or electrode contact resistance.
#'''Stimulation Parameters''': a series of rectangular pulses with certain pulse width and frequency are typically used as electrical stimuli for SSEP recording.
##'''Pulse width (or pulse duration)''': 200-300 microsecond is suggested
##'''Frequency (or rep rate)''': Higher stimulus rate is desired for quick acquisition of SSEP responses in the operating room. However increasing the rate too high may result in degradation of the responses; also it is limited when interleaving stimulation of 2 or 4 limbs. In general a frequency between 2 and 5 Hz are recommended. To avoid synchronization between the responses and the underlying electrical noise, most commonly 60Hz or 50Hz in different countries, the stimulus rate should not be a submultiple of the noise frequency. Sometimes a slight change of stimulus rate, for example from 4.80 to 4.13, may improve the quality of the evoked responses.
##'''Intensity''': Supramaximal stimulation should be used to produce repeatable responses. Some factors, such as pathology of the peripheral nerves, large or edematous extremities, distance of the electrodes to the underlying nerves, types of stimulating electrodes, may limit the effectiveness of stimulation. A stimulus of 50mA or greater is required sometimes. There is concern of tissue damage from high current densities at the stimulation site which is rare.

== Recording Techniques ==

#'''Electrodes''': Metal surface cup electrodes are non-invasive electrodes, which can be applied with conductive gel after skin preparation. Subdermal needles are also commonly used in the operating room; some are available with angled or hooked tips. A corkscrew version can be screwed into the scalp for prolonged recording, however excessive bleeding can be a concern. For direct cortical mapping and recording, a strip or grid electrode array can be used for cranial surgeries.
#'''Recording Sites'''
##Cortical recording of upper extremity SSEP: It is the post-central gyrus of somatosensory cortex, contralateral to the stimulated limb. The locations are called CP3 and CP4 which are 2 cm posterior to the C3 and C4 positions of the 10-20 International System of EEG electrode placement. The recording montage can be CP3-Fz or CP3-CP4 for right arm stimulation; CP4-Fz or CP4-CP3 for left arm stimulation.
##Cortical recording of lower extremity SSEP: CPz is the active electrode site, which is 2 cm posterior to Cz. The traditional derivation of recording is CPz-Fz; however it should not be the only standard recording derivation, according to studies for optimized lower extremity SSEP recording by MacDonald DB et al and many others. Often time CPz-CPc (which is CPz-CP4 for left leg stimulation, and CPz-CP3 for right) may produce higher amplitude and more reliable signals.
##Subcortical: The recording can be made at posterior cervical spine, one or linked earlobes, or mastoid. Subcortical responses are less affected by inhalational agents. However, the subcortical SSEP response is normally very well defined for upper extremity stimulation, it is often poorly defined for lower extremity stimulation.
##Peripheral nerve: Usually it is the ipsilateral Erb's point for upper extremity stimulation, and ipsilateral popliteal fossa for lower extremity stimulation. This is done to verify the status of the peripheral stimulation.
#'''Averaging:''' The SSEP amplitudes tend to be low; as high as only several microvolts, or as low as less than a microvolts especially with pathological subjects. Averaging is required to record the signal against biological and ambient noise. SSEP signal is time-locked to the stimulus and most of the noise occurs randomly, allowing the noise to be averaged out with averaging of repeated responses. Some earlier guidelines such as that from American EEG Society suggested acquiring 500-2000 trails per averaged response. However for surgical monitoring well defined evoked potentials should be acquired as quickly as possible. With good preparation of the recording sites to reduce impedance, and optimized recording montages, clean signals could be recorded as few as 50-200 trials.
#'''Recording Parameters'''
##Filters: More environmental noise in the operating room and necessity of quick acquisition make it important to choose optimal filter settings, different from laboratory SSEP diagnostic studies. The majority of the energy contained in cortical SSEP is present in the frequency bandpass above 30Hz and below 500Hz; filters can be set to (10-30) Hz to (250-1000) Hz. The relative frequency content of the subcortical or peripheral responses is much higher, thus the filters can be set to (30-100)Hz to (500-2000) Hz.
##Timebase: The timebase is usually set at 50 milliseconds for upper extremity SSEPs, and 100 milliseconds for upper extremity SSEPs. It is based on the normal conduction time between the stimulation and recording sites. It may need adjustment depending on the age and size of the individual, and any pathological conditions.
##Sensitivity: The median amplitude of SSEP is about 1 microvolt. The recording sensitivity could range from 0.1 to 5 microvolts/unit. With direct cortical recording during cranial surgeries, the amplitude can be high and the sensitivity my be set to 20-50 microvolts/unit.
##Sweep delay: Normally it should be avoided to use sweep delay. However it may be necessary when large stimulus artifact is present.
##Interleaving recording: Alternating recording of SSEP responses to stimulation of 2 or 4 limbs at the same time is available with modern recording devices from different manufacturers.

== Waveforms ==
'''Upper Extremity SSEPs'''
#N9: Recorded from ipsilateral Erb's point, and sometimes it's called EP. It is a compound action potential from the axons to the median nerve or ulnar nerve stimulation.
#N13-P14: Cervical potentials, recorded near C5 spinal process. The origin is thought to be dorsal horn neurons, and the caudal medial lemniscus.
#N20-P23: Scalp potentials, generated from the thalamocortical radiations.

'''Lower Extremity SSEPs'''
#LP: Recorded from the T12 spinal process, arise from the afferent nerve volley in the dorsal roots and dorsal root entry zone.
#N34: Recorded at the cervical spine; the amplitude can be very low and difficult to identify. The origin is thought to be brainstem or perhaps thalamus.
#P37-N45: Scalp potentials, reflect activation of the primary somatosensory cortex.

[[Image:SSEP Waveforms.jpg|center|2000px|750px]]

== Intraoperative Monitoring ==
The basic principle of intraoperative monitoring with mixed nerve SSEP is to stimulate nerves distal to the surgical site, and to record responses proximal to the surgical site. In most cases, these recording sites should include one cortical and one subcortical recording site.

#'''Data acquisition''': Baseline SSEP responses should be established, ideally after induction and before patient positioning and surgical incision. Data should be collected throughout the surgical procedure, with corresponding documentation of surgical events such as incision, exposure, decompression, instrumentation, closure, etc. In addition, relevant physiological variables such as blood pressure and temperature changes, anesthetic agents being used and the levels, significant signal changes during the surgery and any communication and interventions should be documented.
#'''Alert criteria''': Significant signal changes, with 50% decrease of peak to peak amplitude and 10% increase of latency, may be indication of nervous system functional changes and should be reported to the operating surgeon, and call for possible intervention.

Depending on the specific surgery, planning is required to monitor the part of the nervous system at risk.
#'''Peripheral nerve and plexus'''
#'''Spinal nerve root'''
#'''Spinal cord'''
#'''Brainstem and thalamus'''
#'''Brain'''

== Systemic and Anesthesia Factors ==
#'''Temperature'''. The temperature of the patient can increase or decrease SSEP latencies but does not necessarily affect amplitudes.
#'''Blood pressure'''. An intraoperative decline in blood pressure is associated with a loss in SSEP signals.
#'''Halogenated inhalational agents'''. Higher levels of gas anesthesia (> Half MAC) can increase SSEP latencies and reduce SSEP amplitudes.
#'''Nitrous oxide'''. Nitrous oxide decreases SSEP amplitudes but has no effect on latencies.
#'''Intravenous analgesic agents'''. Intravenous anesthetics, such as propofol, do significantly influence SSEP latencies or amplitudes.
#'''Muscle relaxants'''. Neuromuscular blockers do not influence SSEP latencies or amplitudes.

==References==
# Brown RH, Nash CL, Berilla JA, Amaddio MD. Cortical evoked potential monitoring. A system for intraoperative monitoring of spinal cord function. Spine 1984; 9: 256-261.
# Celesia GG. Somatosensory evoked potentials recorded directly from human thalamus and Sm I cortical area. Arch Neurol 1979; 36: 399-405.
# Cohen AR, Young W, Ransohoff J. Intraspinal localization of the somatosensory evoked potential. Neurosurgery 1981; 9: 157-62.
# Kelly DL Jr, Goldring S, O’Leary JL. Averaged evoked somatosensory responses from exposed cortex of man. Arch Neurol 1965; 13: 1-9.
# Larson SJ, Sances A. Evoked potentials in man: neurosurgical applications. Am J Surg 1966; 111: 857-861.
# MacDonald DB. Individually optimizing posterior tibial somatosensory evoked potential P37 scalp derivations for intraoperative monitoring. J Clin Neurophysiol. 2001 Jul;18(4):364-71.
# Mauguiere F. Anatomic origin of the cervical N13 potential evoked by upper extremity stimulation. J of Clin Neurophysiology 2000; 17: 236-245.
# McCallum JE, Bennett MH. Electrophysiologic monitoring of spinal cord function during intraspinal surgery. Surg Forum 1975; 26: 469-471.
# Nash CL, Long RA, Schatzinger LA, Brown RH. Spinal cord monitoring during operative treatment of the spine. Clin Orthop 1977; 126: 100-105.
# Nuwer MR, Dawson EG, Carlson LG, Kanim LEA, Sherman JE. Somatosensory evoked potential spinal cord monitoring reduces neurologic deficits after scoliosis surgery: results of a large multicenter survey. Electroencepholog and Clin Neurophysiol 1995; 96: 6-11.
# Nuwer MR, et al. IFCN recommended standards for short latency somatosensory evoked potentials. Report of an IFCN committee. Electroencephalography and Clinical Neurophysiology 1994; 91: 6-11.
# Powers SK, Bolger CA, Edwards MSB. Spinal cord pathways mediating somatosensory evoked potentials. J Neurosurg 1982; 57: 472-82.
# Simpson RK JR, Blackburn JG, Martin HF III, Katz S. Peripheral nerve fibers and spinal cord pathway contribution to the somatosensory evoked potentials. Exp Neurol 1981; 73: 700-15.
# Sloan TB. Anesthetic Effects on Electrophysiologic Recordings. Journal of Clinical Neurophysiology 1998; 15: 217-226.
# Toleikis JR. Intraoperative monitoring using somatosensory evoked potentials: A position statement by the American Society of Neurophysiological Monitoring. J Clin Monit Comput. 2005; 19(3):241-58.
# Wang BP, Turner LA, Kamp AM, Venier LH. CPz-CP4 and CPz-CP3 are superior to CPz-FPz for recording left and right tibial nerve somatosensory evoked potentials for intraoperative monitoring. A review study in 264 patients. 2006 The 17th Annual Meeting of American Society of Neurophysiological Monitoring.
# Yanni DS, Ulkatan S, Deletis V, Barrenechea IJ, Sen C, Perin NI. Utility of neurophysiological monitoring using dorsal column mapping in intramedullary spinal cord surgery. J Neurosurg Spine. 2010;12:623-628.
# York DH, Chabot RJ, Gaines RW. Response variability of somatosensory evoked potentials during scoliosis surgery. Spine 1987; 12: 864-876.

Somatosensory Evoked Potentials (SSEP)

2022-01-19T03:07:34Z

Wdoyon: /* Systemic and Anesthesia Factors */

Somatosensory evoked potentials (SSEP) are recorded from the central nervous system following stimulation of peripheral nerves.

== Introduction==

Somatosensory Evoked Potentials (SSEPs) are electric signals recorded from the scalp or spine following stimulation to the peripheral nerves. They are time-locked responses, representing the function of the ascending sensory pathways. Early in the 1960s Larson et al introduced the use of somatosensory evoked potentials to monitor neural structure during neurosurgical procedures. It was utilized as a supplement to the wake-up test during correctional spinal surgeries for spinal deformities such as scoliosis to provide warning of compromised spinal cord function to the spine surgeons, as reported by McCallum et al and Nash et al in the 1970s. Since then SSEP has become one of the earliest and primary tools for intraoperative neurophysiological monitoring.

== Somatosensory Pathways ==
[[Image:Ascending Pathways.jpg|right|100px|200px]]

Distal peripheral nerves are stimulated for SSEP recordings; typically the median or ulnar nerve at the wrist for upper extremity SSEPs, or posterior tibial nerve at the ankle for lower extremity SSEPs. The ascending sensory volley enters the spinal cord through dorsal nerve roots. Multiple dorsal or posterior column spinal pathways such as the gracile fasciculus (legs and trunk) and the cuneate fasciculus (arms and trunk) mediate the SSEP responses. They arrive at the medulla and make synaptic connection at the medullary nuclei (nucleus cuneatus and nucleus gracilis). From there they cross and ascend in the medial lemniscal pathways to thalamic nuclei, which in turn project up to the somatosenory cortex.

There is no synapses between the peripheral stimulation sites and the medullary nuclei. Synapses are the sites of action for inhalational anesthetic agents. Therefore any SSEP responses recorded below the level of medullary nuclei are affected only minimally by general anesthesia. The cortical SSEP responses, however, are greatly affected by inhalational agents; whichever recorded anesthetic management is very much required.

== Stimulation ==
#'''Stimulation Sites''': SSEPs may be elicited by electrical stimulation to major nerve trunks or dermatomes. Upper extremity mixed or major nerve SSEPs are typically obtained by stimulating the median nerve or ulnar nerve near the wrist; or sometimes the ulnar nerve at the elbow. Lower extremity SSEPs are normally recorded to stimulation of the posterior tibial nerve at the ankle, or peroneal nerve near the head of the fibula at the knee. Usually the anode electrode should be placed 2-4 cm distal to the cathode electrode to avoid anodal block.
#'''Electrodes''': Subdermal needle electrodes or adhesive surface electrodes are most commonly used for intraoperative SSEP recordings. Some other types of electrodes such as bar electrodes and EEG metal disc electrodes are more suitable for diagnostic SSEP recordings in the lab. Needle electrodes can be placed closer to the underlying nerves than surface electrodes; however they are associated with additional risks of infection, bleeding, and inadvertent needle sticks.
#'''Constant Current vs Constant Voltage''': Constant current stimulation is recommended for consistent and reliable stimulation for optimal SSEP recording in the OR, especially for long surgical procedures. Constant current stimuli may compensate for any changes in electrical conductivity, or electrode contact resistance.
#'''Stimulation Parameters''': a series of rectangular pulses with certain pulse width and frequency are typically used as electrical stimuli for SSEP recording.
##'''Pulse width (or pulse duration)''': 200-300 microsecond is suggested
##'''Frequency (or rep rate)''': Higher stimulus rate is desired for quick acquisition of SSEP responses in the operating room. However increasing the rate too high may result in degradation of the responses; also it is limited when interleaving stimulation of 2 or 4 limbs. In general a frequency between 2 and 5 Hz are recommended. To avoid synchronization between the responses and the underlying electrical noise, most commonly 60Hz or 50Hz in different countries, the stimulus rate should not be a submultiple of the noise frequency. Sometimes a slight change of stimulus rate, for example from 4.80 to 4.13, may improve the quality of the evoked responses.
##'''Intensity''': Supramaximal stimulation should be used to produce repeatable responses. Some factors, such as pathology of the peripheral nerves, large or edematous extremities, distance of the electrodes to the underlying nerves, types of stimulating electrodes, may limit the effectiveness of stimulation. A stimulus of 50mA or greater is required sometimes. There is concern of tissue damage from high current densities at the stimulation site which is rare.

== Recording Techniques ==

#'''Electrodes''': Metal surface cup electrodes are non-invasive electrodes, which can be applied with conductive gel after skin preparation. Subdermal needles are also commonly used in the operating room; some are available with angled or hooked tips. A corkscrew version can be screwed into the scalp for prolonged recording, however excessive bleeding can be a concern. For direct cortical mapping and recording, a strip or grid electrode array can be used for cranial surgeries.
#'''Recording Sites'''
##Cortical recording of upper extremity SSEP: It is the post-central gyrus of somatosensory cortex, contralateral to the stimulated limb. The locations are called CP3 and CP4 which are 2 cm posterior to the C3 and C4 positions of the 10-20 International System of EEG electrode placement. The recording montage can be CP3-Fz or CP3-CP4 for right arm stimulation; CP4-Fz or CP4-CP3 for left arm stimulation.
##Cortical recording of lower extremity SSEP: CPz is the active electrode site, which is 2 cm posterior to Cz. The traditional derivation of recording is CPz-Fz; however it should not be the only standard recording derivation, according to studies for optimized lower extremity SSEP recording by MacDonald DB et al and many others. Often time CPz-CPc (which is CPz-CP4 for left leg stimulation, and CPz-CP3 for right) may produce higher amplitude and more reliable signals.
##Subcortical: The recording can be made at posterior cervical spine, one or linked earlobes, or mastoid. Subcortical responses are less affected by inhalational agents. However, the subcortical SSEP response is normally very well defined for upper extremity stimulation, it is often poorly defined for lower extremity stimulation.
##Peripheral nerve: Usually it is the ipsilateral Erb's point for upper extremity stimulation, and ipsilateral popliteal fossa for lower extremity stimulation. This is done to verify the status of the peripheral stimulation.
#'''Averaging:''' The SSEP amplitudes tend to be low; as high as only several microvolts, or as low as less than a microvolts especially with pathological subjects. Averaging is required to record the signal against biological and ambient noise. SSEP signal is time-locked to the stimulus and most of the noise occurs randomly, allowing the noise to be averaged out with averaging of repeated responses. Some earlier guidelines such as that from American EEG Society suggested acquiring 500-2000 trails per averaged response. However for surgical monitoring well defined evoked potentials should be acquired as quickly as possible. With good preparation of the recording sites to reduce impedance, and optimized recording montages, clean signals could be recorded as few as 50-200 trials.
#'''Recording Parameters'''
##Filters: More environmental noise in the operating room and necessity of quick acquisition make it important to choose optimal filter settings, different from laboratory SSEP diagnostic studies. The majority of the energy contained in cortical SSEP is present in the frequency bandpass above 30Hz and below 500Hz; filters can be set to (10-30) Hz to (250-1000) Hz. The relative frequency content of the subcortical or peripheral responses is much higher, thus the filters can be set to (30-100)Hz to (500-2000) Hz.
##Timebase: The timebase is usually set at 50 milliseconds for upper extremity SSEPs, and 100 milliseconds for upper extremity SSEPs. It is based on the normal conduction time between the stimulation and recording sites. It may need adjustment depending on the age and size of the individual, and any pathological conditions.
##Sensitivity: The median amplitude of SSEP is about 1 microvolt. The recording sensitivity could range from 0.1 to 5 microvolts/unit. With direct cortical recording during cranial surgeries, the amplitude can be high and the sensitivity my be set to 20-50 microvolts/unit.
##Sweep delay: Normally it should be avoided to use sweep delay. However it may be necessary when large stimulus artifact is present.
##Interleaving recording: Alternating recording of SSEP responses to stimulation of 2 or 4 limbs at the same time is available with modern recording devices from different manufacturers.

== Waveforms ==
'''Upper Extremity SSEPs'''
#N9: Recorded from ipsilateral Erb's point, and sometimes it's called EP. It is a compound action potential from the axons to the median nerve or ulnar nerve stimulation.
#N13-P14: Cervical potentials, recorded near C5 spinal process. The origin is thought to be dorsal horn neurons, and the caudal medial lemniscus.
#N20-P23: Scalp potentials, generated from the thalamocortical radiations.

'''Lower Extremity SSEPs'''
#LP: Recorded from the T12 spinal process, arise from the afferent nerve volley in the dorsal roots and dorsal root entry zone.
#N34: Recorded at the cervical spine; the amplitude can be very low and difficult to identify. The origin is thought to be brainstem or perhaps thalamus.
#P37-N45: Scalp potentials, reflect activation of the primary somatosensory cortex.

[[Image:SSEP Waveforms.jpg|center|2000px|750px]]

== Intraoperative Monitoring ==
The basic principle of intraoperative monitoring with mixed nerve SSEP is to stimulate nerves distal to the surgical site, and to record responses proximal to the surgical site. In most cases, these recording sites should include one cortical and one subcortical recording site.

#'''Data acquisition''': Baseline SSEP responses should be established, ideally after induction and before patient positioning and surgical incision. Data should be collected throughout the surgical procedure, with corresponding documentation of surgical events such as incision, exposure, decompression, instrumentation, closure, etc. In addition, relevant physiological variables such as blood pressure and temperature changes, anesthetic agents being used and the levels, significant signal changes during the surgery and any communication and interventions should be documented.
#'''Alert criteria''': Significant signal changes, with 50% decrease of peak to peak amplitude and 10% increase of latency, may be indication of nervous system functional changes and should be reported to the operating surgeon, and call for possible intervention.

Depending on the specific surgery, planning is required to monitor the part of the nervous system at risk.
#'''Peripheral nerve and plexus'''
#'''Spinal nerve root'''
#'''Spinal cord'''
#'''Brainstem and thalamus'''
#'''Brain'''

== Systemic and Anesthesia Factors ==
#'''Temperature'''. The temperature of the patient can increase or decrease the latency of the SSEP but does not necessarily affect amplitudes.
#'''Blood pressure'''. An intraoperative decline in blood pressure is associated with a loss in SSEP signals.
#'''Halogenated inhalational agents'''. Higher levels of gas anesthesia (> Half MAC) can increase SSEP latency and reduce SSEP amplitudes.
#'''Nitrous oxide'''. Nitrous oxide decreases SSEP amplitudes but has no effect on latency.
#'''Intravenous analgesic agents'''
#'''Muscle relaxants'''. Neuromuscular blockers do not influence SSEP latency or amplitudes.

==References==
# Brown RH, Nash CL, Berilla JA, Amaddio MD. Cortical evoked potential monitoring. A system for intraoperative monitoring of spinal cord function. Spine 1984; 9: 256-261.
# Celesia GG. Somatosensory evoked potentials recorded directly from human thalamus and Sm I cortical area. Arch Neurol 1979; 36: 399-405.
# Cohen AR, Young W, Ransohoff J. Intraspinal localization of the somatosensory evoked potential. Neurosurgery 1981; 9: 157-62.
# Kelly DL Jr, Goldring S, O’Leary JL. Averaged evoked somatosensory responses from exposed cortex of man. Arch Neurol 1965; 13: 1-9.
# Larson SJ, Sances A. Evoked potentials in man: neurosurgical applications. Am J Surg 1966; 111: 857-861.
# MacDonald DB. Individually optimizing posterior tibial somatosensory evoked potential P37 scalp derivations for intraoperative monitoring. J Clin Neurophysiol. 2001 Jul;18(4):364-71.
# Mauguiere F. Anatomic origin of the cervical N13 potential evoked by upper extremity stimulation. J of Clin Neurophysiology 2000; 17: 236-245.
# McCallum JE, Bennett MH. Electrophysiologic monitoring of spinal cord function during intraspinal surgery. Surg Forum 1975; 26: 469-471.
# Nash CL, Long RA, Schatzinger LA, Brown RH. Spinal cord monitoring during operative treatment of the spine. Clin Orthop 1977; 126: 100-105.
# Nuwer MR, Dawson EG, Carlson LG, Kanim LEA, Sherman JE. Somatosensory evoked potential spinal cord monitoring reduces neurologic deficits after scoliosis surgery: results of a large multicenter survey. Electroencepholog and Clin Neurophysiol 1995; 96: 6-11.
# Nuwer MR, et al. IFCN recommended standards for short latency somatosensory evoked potentials. Report of an IFCN committee. Electroencephalography and Clinical Neurophysiology 1994; 91: 6-11.
# Powers SK, Bolger CA, Edwards MSB. Spinal cord pathways mediating somatosensory evoked potentials. J Neurosurg 1982; 57: 472-82.
# Simpson RK JR, Blackburn JG, Martin HF III, Katz S. Peripheral nerve fibers and spinal cord pathway contribution to the somatosensory evoked potentials. Exp Neurol 1981; 73: 700-15.
# Sloan TB. Anesthetic Effects on Electrophysiologic Recordings. Journal of Clinical Neurophysiology 1998; 15: 217-226.
# Toleikis JR. Intraoperative monitoring using somatosensory evoked potentials: A position statement by the American Society of Neurophysiological Monitoring. J Clin Monit Comput. 2005; 19(3):241-58.
# Wang BP, Turner LA, Kamp AM, Venier LH. CPz-CP4 and CPz-CP3 are superior to CPz-FPz for recording left and right tibial nerve somatosensory evoked potentials for intraoperative monitoring. A review study in 264 patients. 2006 The 17th Annual Meeting of American Society of Neurophysiological Monitoring.
# Yanni DS, Ulkatan S, Deletis V, Barrenechea IJ, Sen C, Perin NI. Utility of neurophysiological monitoring using dorsal column mapping in intramedullary spinal cord surgery. J Neurosurg Spine. 2010;12:623-628.
# York DH, Chabot RJ, Gaines RW. Response variability of somatosensory evoked potentials during scoliosis surgery. Spine 1987; 12: 864-876.

Motor Evoked Potentials (MEP)

2022-01-19T02:57:22Z

Wdoyon: /* Intraoperative Monitoring */

== Introduction==
Motor evoked potentials (MEPs) are electrical signals recorded from muscle tissue in response to stimulation of the motor cortex. The stimulation may be magnetic or electrical and applied directly to the motor cortex or applied transcranially through the skull.

== Motor Pathways==
MEPs are used to monitor the functional integrity of the descending motor pathways, which mediate voluntary movement of the face, limbs, and torso, respectively. The descending motor pathways can be divided into a lateral system and a medial system. The lateral system includes the corticospinal tract and a smaller rubrospinal tract. The corticospinal pathway originates mainly in the primary motor cortex (Brodmann’s area 4), but fibers from other regions, such as the premotor, the supplemental motor, and somatosensory cortices contribute as well. The descending motor pathways consist of upper and lower motor neurons, as well as interneurons at the level of the spinal cord. The cell bodies of the upper motor neurons lie in the motor cortices, whereas the cell bodies of lower motor neurons lie in the brain stem and spinal cord. Each lower motor neuron gives rise to one axon that exits the spinal cord and passes through a ventral nerve root. The ventral nerve roots form a fiber bundle with the dorsal nerve roots, which together form a peripheral nerve bundle. Each lower motor axon passes through through the peripheral nerve bundle and then arborizes to innervate multiple muscle fibers. Each synaptic connection forms an excitatory synapse. One lower motor neuron and the muscle fibers that it innervates is known as a motor unit. There are hundreds of motor units within a single muscle each of which occupying a space of approximately 5 to 11 mm in diameter (Leppanen et al., 2005).

'''The corticospinal system'''. The corticospinal pathway is composed primarily of a lateral system and a smaller anterior system, and these innervates the distal limbs. The axons of the lateral tract descend through the internal capsule and project to the lower medulla. The upper motor neurons of the lateral tract cross the midline to the contralateral side and descend to the lateral column of the dorsal horn. After reaching the ventral horn of the spinal cord, these motor neuron axons form synaptic connections with lower motor neurons, often via local interneurons. The axons of the anterior tract do not appear to cross the midline.

'''The medial system'''. The medial system innervates the trunk and proximal limb muscles. Fibers in the motor cortices descend bilaterally and do not cross the midline.

'''The corticobulbar system'''. The upper motor neurons of the corticobulbar tract descend to the brain stem and innervate several nuclei for cranial nerves that control the facial muscles, including cranial nerves V, VII, IX, X, and XII. In most cases, the corticobulbar tract innervates the facial motor nuclei, bilaterally, including those nuclei involved in swallowing, speech, chewing and tongue movement. In contrast, upper motor neurons innervate the nuclei the lower facial nuclei and the hypoglossal nerves on the contralateral side of the body.

'''The basal ganglia'''. The basal ganglia is a set of subcortical structures that help to control movement. Many now view the basal ganglia as part of the upper motor systems because the basal ganglia modulates these systems. The structures of the basal ganglia typically include the caudate/putamen (striatum), globus pallidus internal (GPi) and external (GPe) segments, subthalamic nucleus, substantia nigra pars compacta (SNc) and reticulata (SNr). The cortical motor areas and SNc provide input to the striatum. The projection neurons of the striatum are connected to the GPi and the SNr via the direct and indirect pathways. The projection neurons of the direct pathway predominantly express dopamine D1 receptors, whereas those of the indirect pathway predominantly expresses dopamine D2 receptors. The indirect pathway includes the GPe and the subthalamic nucleus. The GPi and the SNr provide tonic inhibitory feedback to the motor cortex via the thalamocortical pathway.

==Stimulation==
'''1.''' '''Transcranial electrical stimulation (tES)'''. tES is a commonly used, non-invasive technique for generating MEPs. Stimulating electrodes for tES are placed on the scalp or subcutaneously above the primary motor cortex. Electrical current is then applied to the head to alter neuronal activity in the brain. However, due to the thickness and high resistance of the skull bone, only a small percentage of current reaches the brain tissue. Therefore, to record MEPs from muscle tissue, the stimulus strength must be set high enough to overcome that resistance and activate the underlying motor pathways. We call this type of stimulation supra-maximal, as the stimulus is higher than that required for the recruitment of all muscle fibers around the recording electrode. Train stimulation is required to elicit reliable MEPs under anesthesia, requiring the use of a multi-pulse stimulator.

The stimulating electrodes for MEPs are placed at C1 (for right extremities) and C2 (for left extremities) using the 10-20 system. Unlike the cathodal stimulation used for SSEPs, clinicians use anodal (+) stimulation to elicit MEPs. Electrical current flow from the anode to the cathode is more effective at depolarizing the pyramidal neurons of the primary motor cortex due to the more vertical organization of these cells.[1] Typical tES parameters used to elicit myogenic MEPs include intensities ranging from ~400-600 V, a train of 4-9 pulses, an inter-pulse interval ranging from 2-4 ms, and a pulse width between 50-75 μs (but higher values are used in other countries).

'''2. Transcranial magnetic stimulation (TMS)'''. TMS is another non-invasive technique for generating MEPs. By applying a magnetic field over the scalp, TMS can induce electrical currents in brain tissue in awake patients. One disadvantage of TMS, however, is that TMS-induced MEPs are suppressed under anesthesia.

'''3. Direct cortical stimulation'''. For some brain surgeries, such as tumor resections, the surgeon will directly stimulate the motor cortex or adjacent subcortical tissue to elicit a MEP. Direct cortical stimulation requires a much lower stimulation intensity because of the absence of the skull. In most cases, it is necessary to find the lowest stimulation threshold for MEPs, which provides important information on the proximity of the neural probe to the motor pathways. The lower the stimulation intensity, the closer the probe is to the motor pathways. The position of the neural probe relative to the homunculus will normally dictate which muscle groups are activated.

'''4. Spinal cord stimulation'''. Also called neurogenic motor evoked potentials, these signals are evoked in the rostral spinal cord and often recorded from electrodes on peripheral nerves in the legs. However, this technique is not widely used for monitoring the motor pathways because the recordings are not easy to interpret. Neurogenic MEPs likely arise from retrograde activation of the sensory pathways [2], in addition to the corticospinal tract. Other forms of spinal cord stimulation are used to test the efficacy of spinal cord stimulators during implantation. Spinal cord stimulators are typically placed in the thoracic spine and used for the treatment of neuropathic pain and other chronic pain disorders.

==Recording Techniques==
'''1. Recording Sites and Parameters'''.
MEPs are typically recorded from muscles (myogenic MEPs), as compound muscle action potentials, following tES of the primary motor cortex. Myogenic MEPs are recorded on different muscle groups associated with the myotome. The myotome is a set of muscle groups innervated by a single motor neuron, which together is known as a motor unit. Myogenic MEPs are recorded with needle or surface electrodes on muscle groups of the upper and lower extremities. The placement of the electrodes on specific muscles depends on the site of the surgical procedure and which nerve roots are potentially at risk. For an anterior cervical discectomy at C5-6, for example, electrodes should be placed on the deltoid and bicep, as these muscle groups are innervated by motor neurons that exit the spinal cord at C5-6. Likewise, for an ACDF at C6-7, electrodes should be placed on the bicep and tricep.

MEPs are large amplitude signals that do not require averaging, as SSEPs do. The signals are filtered at the high and low frequency range to eliminate electrical artifacts (e.g., 10 Hz for the low-cut filter and 2000-3000 Hz for the high cut filter). The resistance of the recording electrodes in the muscle tissue should be low (< 5 kOhms). Myogenic MEPs can be repeated at a rate of 0.5-2 Hz. The latency of the MEP will depend on how far the signal has to travel to reach the recording site on the muscle. Lower extremity MEPs can be approximately 25-32 ms, depending on the height of the patient. And upper extremity MEPs can be approximately 17-25 ms.

'''2. Surgical and medical considerations'''. Some considerations for recording MEPs include the use of anesthetics and neuromuscular blocking agents. For example, myogenic MEPS are very sensitive to inhalation anesthetics, which limits the use of these anesthetics during IONM. Gas anesthesia strongly influences synaptic transmission. Inhalation anesthetics reduce the amplitude and prolong the latency of MEPs, and this affect becomes more pronounced when multiple synapses are involved, as is the case for myogenic MEPs, which involve the upper and lower motor neurons as well as interneurons in the spinal column. Intravenous anesthetics have a similar effect on MEPs but to a lesser degree. Total intravenous anesthesia (TIVA) - e.g., propofol and narcotic cocktails - is preferred in most cases, particularly those involving the spinal cord at L2 vertebrae and above. Propofol acts as a positive allosteric modulator of the GABA-A receptor, and it may also act directly as an agonist. For cases involving L3 and below, corresponding to the cauda equina, it is acceptable to supplement the TIVA with gas anesthesia, also known as half MAC (Monitored Anesthesia Care).

Muscle relaxants are normally needed for the intubation and for exposure of the surgical site. As expected, neuromuscular blockers, such as Rocuronium and Succinylcholine, strongly suppress myogenic MEPs. Rocuronium (ROC) is a commonly used, non‐depolarizing neuromuscular blocker. ROC is a nicotinic receptor antagonist that has a duration of ~30-60 min at standard doses. In contrast, succinylcholine is a depolarizing neuromuscular blocker with a rapid onset and elimination, which can be used as an alternative to ROC.

MEPs are also influenced by neurological disorders, such as myasthenia gravis, botulinum toxin treatments for dystonia, and muscular dystrophy. Therefore, it is important to understand the patient's medical history when preparing for cases that involve MEPs.

==Waveform==
The MEP waveform arises from direct and indirect activation of the pyramidal cells in the motor cortex.[3] Electrical and magnetic stimulation directly activate the pyramidal cells. MEPs from these upper motor neurons can be recorded directly in the spinal cord as near-field potentials called ‘D-waves,’ as there are no synaptic connections between the activated neurons in the cortex and the recording site, or far-field potentials from muscles of the upper and lower extremities. Indirect synaptic activation of the pyramidal neurons from activated local interneurons also contributes to the MEP. These components of the MEP are called ‘I-waves.’ The number of I-waves in the MEP waveform, and the interval between each I-wave, depend on the number of synapses or synaptic delays in the circuit between the cortical interneurons and the activated pyramidal cell.

MEPs recorded from muscle tissue also involve another level of synaptic integration at the ventral horn, where upper motor neurons form synaptic connections with lower motor neurons. As a result, pulse-train stimulation tends to produce a multi-phasic waveform, which reflects greater complexity of the far-field potentials recorded from muscles.

==Intraoperative Monitoring==
'''1. Data acquisition:''' Baseline MEP responses should be recorded shortly after the patient is sedated, but the exact timing depends on the surgical procedure. For posterior cervical spinal surgeries, as an example, it is ideal to record baseline responses prior to neck and shoulder positioning because these manipulations could adversely affect spinal cord function and negatively impact the patient's condition. For posterior lumbar decompressions, the baseline responses can be recorded after the patient has been flipped to the prone position. In this case, patient positioning is less likely to cause neurological damage because the nerve roots are mainly at risk, not the spinal cord. MEP tests should be run throughout the surgical procedure and should correspond to surgical events such as the incision, insertion of hardware, decompression, closure, etc.

'''2. Alarm criteria:''' Similar to the criterion used for SSEPs, a 50% decrease in MEP amplitude is cause for concern. The amplitude of the myogenic MEP is measured from the most positive peak to the most negative peak in the waveform. The MEP amplitude and waveform shape can vary over time, even in the absence of changes in corticospinal tract function. Therefore, some clinicians have argued that the criterion of 50% decrease in amplitude is not always reliable and can lead to false alarms.[4] However, an 80-100% drop in the MEP signal would certainly constitute an alarm and should be reported to the surgeon.

==References==
1. Vogel RW (2017). Understanding Anodal and Cathodal Stimulation. The ASNM Monitor, The American Society of Neurophysiological Monitoring. https://www.asnm.org/blogpost/1635804/290597/Understanding-Anodal-and-Cathodal-Stimulation

2. MacDonald DB (2006). Intraoperative motor evoked potential monitoring: overview and update. J Clin Monit Comput 20:347–377.

3. Legatt AD, Emerson RG, Epstein CM, MacDonald DB, Deletis V, Bravo RJ, López JR (2016). ACNS Guideline: Transcranial Electrical Stimulation Motor Evoked Potential Monitoring.
J Clin Neurophysiol 33(1):42-50.

4. Langeloo DD, Journée HL, de Kleuver M, et al. (2007). Criteria for transcranial electrical motor evoked potential monitoring during spinal deformity surgery: A review and discussion of the literature. Neurophysiol Clin 37:431–439.

Motor Evoked Potentials (MEP)

2022-01-18T21:25:46Z

Wdoyon: /* Recording Techniques */

== Introduction==
Motor evoked potentials (MEPs) are electrical signals recorded from muscle tissue in response to stimulation of the motor cortex. The stimulation may be magnetic or electrical and applied directly to the motor cortex or applied transcranially through the skull.

== Motor Pathways==
MEPs are used to monitor the functional integrity of the descending motor pathways, which mediate voluntary movement of the face, limbs, and torso, respectively. The descending motor pathways can be divided into a lateral system and a medial system. The lateral system includes the corticospinal tract and a smaller rubrospinal tract. The corticospinal pathway originates mainly in the primary motor cortex (Brodmann’s area 4), but fibers from other regions, such as the premotor, the supplemental motor, and somatosensory cortices contribute as well. The descending motor pathways consist of upper and lower motor neurons, as well as interneurons at the level of the spinal cord. The cell bodies of the upper motor neurons lie in the motor cortices, whereas the cell bodies of lower motor neurons lie in the brain stem and spinal cord. Each lower motor neuron gives rise to one axon that exits the spinal cord and passes through a ventral nerve root. The ventral nerve roots form a fiber bundle with the dorsal nerve roots, which together form a peripheral nerve bundle. Each lower motor axon passes through through the peripheral nerve bundle and then arborizes to innervate multiple muscle fibers. Each synaptic connection forms an excitatory synapse. One lower motor neuron and the muscle fibers that it innervates is known as a motor unit. There are hundreds of motor units within a single muscle each of which occupying a space of approximately 5 to 11 mm in diameter (Leppanen et al., 2005).

'''The corticospinal system'''. The corticospinal pathway is composed primarily of a lateral system and a smaller anterior system, and these innervates the distal limbs. The axons of the lateral tract descend through the internal capsule and project to the lower medulla. The upper motor neurons of the lateral tract cross the midline to the contralateral side and descend to the lateral column of the dorsal horn. After reaching the ventral horn of the spinal cord, these motor neuron axons form synaptic connections with lower motor neurons, often via local interneurons. The axons of the anterior tract do not appear to cross the midline.

'''The medial system'''. The medial system innervates the trunk and proximal limb muscles. Fibers in the motor cortices descend bilaterally and do not cross the midline.

'''The corticobulbar system'''. The upper motor neurons of the corticobulbar tract descend to the brain stem and innervate several nuclei for cranial nerves that control the facial muscles, including cranial nerves V, VII, IX, X, and XII. In most cases, the corticobulbar tract innervates the facial motor nuclei, bilaterally, including those nuclei involved in swallowing, speech, chewing and tongue movement. In contrast, upper motor neurons innervate the nuclei the lower facial nuclei and the hypoglossal nerves on the contralateral side of the body.

'''The basal ganglia'''. The basal ganglia is a set of subcortical structures that help to control movement. Many now view the basal ganglia as part of the upper motor systems because the basal ganglia modulates these systems. The structures of the basal ganglia typically include the caudate/putamen (striatum), globus pallidus internal (GPi) and external (GPe) segments, subthalamic nucleus, substantia nigra pars compacta (SNc) and reticulata (SNr). The cortical motor areas and SNc provide input to the striatum. The projection neurons of the striatum are connected to the GPi and the SNr via the direct and indirect pathways. The projection neurons of the direct pathway predominantly express dopamine D1 receptors, whereas those of the indirect pathway predominantly expresses dopamine D2 receptors. The indirect pathway includes the GPe and the subthalamic nucleus. The GPi and the SNr provide tonic inhibitory feedback to the motor cortex via the thalamocortical pathway.

==Stimulation==
'''1.''' '''Transcranial electrical stimulation (tES)'''. tES is a commonly used, non-invasive technique for generating MEPs. Stimulating electrodes for tES are placed on the scalp or subcutaneously above the primary motor cortex. Electrical current is then applied to the head to alter neuronal activity in the brain. However, due to the thickness and high resistance of the skull bone, only a small percentage of current reaches the brain tissue. Therefore, to record MEPs from muscle tissue, the stimulus strength must be set high enough to overcome that resistance and activate the underlying motor pathways. We call this type of stimulation supra-maximal, as the stimulus is higher than that required for the recruitment of all muscle fibers around the recording electrode. Train stimulation is required to elicit reliable MEPs under anesthesia, requiring the use of a multi-pulse stimulator.

The stimulating electrodes for MEPs are placed at C1 (for right extremities) and C2 (for left extremities) using the 10-20 system. Unlike the cathodal stimulation used for SSEPs, clinicians use anodal (+) stimulation to elicit MEPs. Electrical current flow from the anode to the cathode is more effective at depolarizing the pyramidal neurons of the primary motor cortex due to the more vertical organization of these cells.[1] Typical tES parameters used to elicit myogenic MEPs include intensities ranging from ~400-600 V, a train of 4-9 pulses, an inter-pulse interval ranging from 2-4 ms, and a pulse width between 50-75 μs (but higher values are used in other countries).

'''2. Transcranial magnetic stimulation (TMS)'''. TMS is another non-invasive technique for generating MEPs. By applying a magnetic field over the scalp, TMS can induce electrical currents in brain tissue in awake patients. One disadvantage of TMS, however, is that TMS-induced MEPs are suppressed under anesthesia.

'''3. Direct cortical stimulation'''. For some brain surgeries, such as tumor resections, the surgeon will directly stimulate the motor cortex or adjacent subcortical tissue to elicit a MEP. Direct cortical stimulation requires a much lower stimulation intensity because of the absence of the skull. In most cases, it is necessary to find the lowest stimulation threshold for MEPs, which provides important information on the proximity of the neural probe to the motor pathways. The lower the stimulation intensity, the closer the probe is to the motor pathways. The position of the neural probe relative to the homunculus will normally dictate which muscle groups are activated.

'''4. Spinal cord stimulation'''. Also called neurogenic motor evoked potentials, these signals are evoked in the rostral spinal cord and often recorded from electrodes on peripheral nerves in the legs. However, this technique is not widely used for monitoring the motor pathways because the recordings are not easy to interpret. Neurogenic MEPs likely arise from retrograde activation of the sensory pathways [2], in addition to the corticospinal tract. Other forms of spinal cord stimulation are used to test the efficacy of spinal cord stimulators during implantation. Spinal cord stimulators are typically placed in the thoracic spine and used for the treatment of neuropathic pain and other chronic pain disorders.

==Recording Techniques==
'''1. Recording Sites and Parameters'''.
MEPs are typically recorded from muscles (myogenic MEPs), as compound muscle action potentials, following tES of the primary motor cortex. Myogenic MEPs are recorded on different muscle groups associated with the myotome. The myotome is a set of muscle groups innervated by a single motor neuron, which together is known as a motor unit. Myogenic MEPs are recorded with needle or surface electrodes on muscle groups of the upper and lower extremities. The placement of the electrodes on specific muscles depends on the site of the surgical procedure and which nerve roots are potentially at risk. For an anterior cervical discectomy at C5-6, for example, electrodes should be placed on the deltoid and bicep, as these muscle groups are innervated by motor neurons that exit the spinal cord at C5-6. Likewise, for an ACDF at C6-7, electrodes should be placed on the bicep and tricep.

MEPs are large amplitude signals that do not require averaging, as SSEPs do. The signals are filtered at the high and low frequency range to eliminate electrical artifacts (e.g., 10 Hz for the low-cut filter and 2000-3000 Hz for the high cut filter). The resistance of the recording electrodes in the muscle tissue should be low (< 5 kOhms). Myogenic MEPs can be repeated at a rate of 0.5-2 Hz. The latency of the MEP will depend on how far the signal has to travel to reach the recording site on the muscle. Lower extremity MEPs can be approximately 25-32 ms, depending on the height of the patient. And upper extremity MEPs can be approximately 17-25 ms.

'''2. Surgical and medical considerations'''. Some considerations for recording MEPs include the use of anesthetics and neuromuscular blocking agents. For example, myogenic MEPS are very sensitive to inhalation anesthetics, which limits the use of these anesthetics during IONM. Gas anesthesia strongly influences synaptic transmission. Inhalation anesthetics reduce the amplitude and prolong the latency of MEPs, and this affect becomes more pronounced when multiple synapses are involved, as is the case for myogenic MEPs, which involve the upper and lower motor neurons as well as interneurons in the spinal column. Intravenous anesthetics have a similar effect on MEPs but to a lesser degree. Total intravenous anesthesia (TIVA) - e.g., propofol and narcotic cocktails - is preferred in most cases, particularly those involving the spinal cord at L2 vertebrae and above. Propofol acts as a positive allosteric modulator of the GABA-A receptor, and it may also act directly as an agonist. For cases involving L3 and below, corresponding to the cauda equina, it is acceptable to supplement the TIVA with gas anesthesia, also known as half MAC (Monitored Anesthesia Care).

Muscle relaxants are normally needed for the intubation and for exposure of the surgical site. As expected, neuromuscular blockers, such as Rocuronium and Succinylcholine, strongly suppress myogenic MEPs. Rocuronium (ROC) is a commonly used, non‐depolarizing neuromuscular blocker. ROC is a nicotinic receptor antagonist that has a duration of ~30-60 min at standard doses. In contrast, succinylcholine is a depolarizing neuromuscular blocker with a rapid onset and elimination, which can be used as an alternative to ROC.

MEPs are also influenced by neurological disorders, such as myasthenia gravis, botulinum toxin treatments for dystonia, and muscular dystrophy. Therefore, it is important to understand the patient's medical history when preparing for cases that involve MEPs.

==Waveform==
The MEP waveform arises from direct and indirect activation of the pyramidal cells in the motor cortex.[3] Electrical and magnetic stimulation directly activate the pyramidal cells. MEPs from these upper motor neurons can be recorded directly in the spinal cord as near-field potentials called ‘D-waves,’ as there are no synaptic connections between the activated neurons in the cortex and the recording site, or far-field potentials from muscles of the upper and lower extremities. Indirect synaptic activation of the pyramidal neurons from activated local interneurons also contributes to the MEP. These components of the MEP are called ‘I-waves.’ The number of I-waves in the MEP waveform, and the interval between each I-wave, depend on the number of synapses or synaptic delays in the circuit between the cortical interneurons and the activated pyramidal cell.

MEPs recorded from muscle tissue also involve another level of synaptic integration at the ventral horn, where upper motor neurons form synaptic connections with lower motor neurons. As a result, pulse-train stimulation tends to produce a multi-phasic waveform, which reflects greater complexity of the far-field potentials recorded from muscles.

==Intraoperative Monitoring==
'''1. Data acquisition:''' Baseline MEP responses should be established shortly after the patient is sedated, but the exact timing depends on the type of surgery being monitored. For example, for anterior cervical spinal surgeries, it is ideal to record baseline responses prior to neck and shoulder positioning because these manipulations could adversely affect spinal cord function and exacerbate the patient's condition. For posterior lumbar spinal surgeries, the baseline responses can be recorded after the patient has been flipped to the prone position. In this case, patient positioning is less likely to cause neurological damage because the nerve roots are mainly at risk, not the spinal cord. MEP tests should be run throughout the surgical procedure and should correspond to surgical events such as the incision, insertion of hardware, decompression, closure, etc.

'''2. Alarm criteria:''' Similar to the criterion used for SSEPs, a 50% decrease in MEP amplitude is cause for concern. The amplitude of the myogenic MEP is measured from the most positive peak to the most negative peak in the waveform. The MEP amplitude and waveform shape can vary over time, even in the absence of changes in corticospinal tract function. Therefore, some clinicians have argued that the criterion of 50% decrease in amplitude is not always reliable and can lead to false alarms.[4] However, an 80-100% drop in the MEP signal would certainly constitute an alarm and should be reported to the surgeon.

==References==
1. Vogel RW (2017). Understanding Anodal and Cathodal Stimulation. The ASNM Monitor, The American Society of Neurophysiological Monitoring. https://www.asnm.org/blogpost/1635804/290597/Understanding-Anodal-and-Cathodal-Stimulation

2. MacDonald DB (2006). Intraoperative motor evoked potential monitoring: overview and update. J Clin Monit Comput 20:347–377.

3. Legatt AD, Emerson RG, Epstein CM, MacDonald DB, Deletis V, Bravo RJ, López JR (2016). ACNS Guideline: Transcranial Electrical Stimulation Motor Evoked Potential Monitoring.
J Clin Neurophysiol 33(1):42-50.

4. Langeloo DD, Journée HL, de Kleuver M, et al. (2007). Criteria for transcranial electrical motor evoked potential monitoring during spinal deformity surgery: A review and discussion of the literature. Neurophysiol Clin 37:431–439.

Motor Evoked Potentials (MEP)

2022-01-18T21:10:13Z

Wdoyon: /* Stimulation */

== Introduction==
Motor evoked potentials (MEPs) are electrical signals recorded from muscle tissue in response to stimulation of the motor cortex. The stimulation may be magnetic or electrical and applied directly to the motor cortex or applied transcranially through the skull.

== Motor Pathways==
MEPs are used to monitor the functional integrity of the descending motor pathways, which mediate voluntary movement of the face, limbs, and torso, respectively. The descending motor pathways can be divided into a lateral system and a medial system. The lateral system includes the corticospinal tract and a smaller rubrospinal tract. The corticospinal pathway originates mainly in the primary motor cortex (Brodmann’s area 4), but fibers from other regions, such as the premotor, the supplemental motor, and somatosensory cortices contribute as well. The descending motor pathways consist of upper and lower motor neurons, as well as interneurons at the level of the spinal cord. The cell bodies of the upper motor neurons lie in the motor cortices, whereas the cell bodies of lower motor neurons lie in the brain stem and spinal cord. Each lower motor neuron gives rise to one axon that exits the spinal cord and passes through a ventral nerve root. The ventral nerve roots form a fiber bundle with the dorsal nerve roots, which together form a peripheral nerve bundle. Each lower motor axon passes through through the peripheral nerve bundle and then arborizes to innervate multiple muscle fibers. Each synaptic connection forms an excitatory synapse. One lower motor neuron and the muscle fibers that it innervates is known as a motor unit. There are hundreds of motor units within a single muscle each of which occupying a space of approximately 5 to 11 mm in diameter (Leppanen et al., 2005).

'''The corticospinal system'''. The corticospinal pathway is composed primarily of a lateral system and a smaller anterior system, and these innervates the distal limbs. The axons of the lateral tract descend through the internal capsule and project to the lower medulla. The upper motor neurons of the lateral tract cross the midline to the contralateral side and descend to the lateral column of the dorsal horn. After reaching the ventral horn of the spinal cord, these motor neuron axons form synaptic connections with lower motor neurons, often via local interneurons. The axons of the anterior tract do not appear to cross the midline.

'''The medial system'''. The medial system innervates the trunk and proximal limb muscles. Fibers in the motor cortices descend bilaterally and do not cross the midline.

'''The corticobulbar system'''. The upper motor neurons of the corticobulbar tract descend to the brain stem and innervate several nuclei for cranial nerves that control the facial muscles, including cranial nerves V, VII, IX, X, and XII. In most cases, the corticobulbar tract innervates the facial motor nuclei, bilaterally, including those nuclei involved in swallowing, speech, chewing and tongue movement. In contrast, upper motor neurons innervate the nuclei the lower facial nuclei and the hypoglossal nerves on the contralateral side of the body.

'''The basal ganglia'''. The basal ganglia is a set of subcortical structures that help to control movement. Many now view the basal ganglia as part of the upper motor systems because the basal ganglia modulates these systems. The structures of the basal ganglia typically include the caudate/putamen (striatum), globus pallidus internal (GPi) and external (GPe) segments, subthalamic nucleus, substantia nigra pars compacta (SNc) and reticulata (SNr). The cortical motor areas and SNc provide input to the striatum. The projection neurons of the striatum are connected to the GPi and the SNr via the direct and indirect pathways. The projection neurons of the direct pathway predominantly express dopamine D1 receptors, whereas those of the indirect pathway predominantly expresses dopamine D2 receptors. The indirect pathway includes the GPe and the subthalamic nucleus. The GPi and the SNr provide tonic inhibitory feedback to the motor cortex via the thalamocortical pathway.

==Stimulation==
'''1.''' '''Transcranial electrical stimulation (tES)'''. tES is a commonly used, non-invasive technique for generating MEPs. Stimulating electrodes for tES are placed on the scalp or subcutaneously above the primary motor cortex. Electrical current is then applied to the head to alter neuronal activity in the brain. However, due to the thickness and high resistance of the skull bone, only a small percentage of current reaches the brain tissue. Therefore, to record MEPs from muscle tissue, the stimulus strength must be set high enough to overcome that resistance and activate the underlying motor pathways. We call this type of stimulation supra-maximal, as the stimulus is higher than that required for the recruitment of all muscle fibers around the recording electrode. Train stimulation is required to elicit reliable MEPs under anesthesia, requiring the use of a multi-pulse stimulator.

The stimulating electrodes for MEPs are placed at C1 (for right extremities) and C2 (for left extremities) using the 10-20 system. Unlike the cathodal stimulation used for SSEPs, clinicians use anodal (+) stimulation to elicit MEPs. Electrical current flow from the anode to the cathode is more effective at depolarizing the pyramidal neurons of the primary motor cortex due to the more vertical organization of these cells.[1] Typical tES parameters used to elicit myogenic MEPs include intensities ranging from ~400-600 V, a train of 4-9 pulses, an inter-pulse interval ranging from 2-4 ms, and a pulse width between 50-75 μs (but higher values are used in other countries).

'''2. Transcranial magnetic stimulation (TMS)'''. TMS is another non-invasive technique for generating MEPs. By applying a magnetic field over the scalp, TMS can induce electrical currents in brain tissue in awake patients. One disadvantage of TMS, however, is that TMS-induced MEPs are suppressed under anesthesia.

'''3. Direct cortical stimulation'''. For some brain surgeries, such as tumor resections, the surgeon will directly stimulate the motor cortex or adjacent subcortical tissue to elicit a MEP. Direct cortical stimulation requires a much lower stimulation intensity because of the absence of the skull. In most cases, it is necessary to find the lowest stimulation threshold for MEPs, which provides important information on the proximity of the neural probe to the motor pathways. The lower the stimulation intensity, the closer the probe is to the motor pathways. The position of the neural probe relative to the homunculus will normally dictate which muscle groups are activated.

'''4. Spinal cord stimulation'''. Also called neurogenic motor evoked potentials, these signals are evoked in the rostral spinal cord and often recorded from electrodes on peripheral nerves in the legs. However, this technique is not widely used for monitoring the motor pathways because the recordings are not easy to interpret. Neurogenic MEPs likely arise from retrograde activation of the sensory pathways [2], in addition to the corticospinal tract. Other forms of spinal cord stimulation are used to test the efficacy of spinal cord stimulators during implantation. Spinal cord stimulators are typically placed in the thoracic spine and used for the treatment of neuropathic pain and other chronic pain disorders.

==Recording Techniques==
'''1. Recording Sites and Parameters'''.
MEPs are typically recorded from muscles (myogenic MEPs), as compound muscle action potentials, following tES of the primary motor cortex. Myogenic MEPs are recorded on different muscle groups associated with the myotome. The myotome is a set of muscle groups innervated by a single motor neuron, which together is known as a motor unit. Myogenic MEPs are recorded with needle or surface electrodes on bilateral muscle groups of the upper and lower extremities. The placement of the electrodes on specific muscles depends on the surgical procedure and which motor unit is potentially at risk. For an anterior cervical discectomy involving C6-7, for example, electrodes would be placed on the biceps and triceps, bilaterally, as these muscle groups are innervated by lower motor neurons that exit the spinal cord at C6-7.

MEPs are large amplitude signals that do not require averaging, as SSEPs do. The signals are filtered at the high and low frequency range to eliminate electrical artifacts (e.g., 10 Hz for the low-cut filter and 2000-3000 Hz for the high cut filter). The resistance of the recording electrodes in the muscle tissue should be low (< 5 kOhms). Myogenic MEPs can be repeated at a rate of 0.5-2 Hz.

'''2. Surgical and medical considerations'''. Some considerations for recording MEPs include the use of anesthetics and neuromuscular blocking agents. For example, myogenic MEPS are very sensitive to inhalation anesthetics, which limits the use of these anesthetics during IONM. Gas anesthesia strongly influences synaptic transmission. Inhalation anesthetics reduce the amplitude and prolong the latency of MEPs, and this affect becomes more pronounced when multiple synapses are involved, as is the case for myogenic MEPs, which involve the upper and lower motor neurons as well as interneurons in the spinal column. Intravenous anesthetics have a similar effect on MEPs but to a lesser degree. Total intravenous anesthesia (TIVA) - e.g., propofol and narcotic cocktails - is preferred in most cases, particularly those involving the spinal cord at L2 vertebrae and above. Propofol acts as a positive allosteric modulator of the GABA-A receptor, and it may also act directly as an agonist. For cases involving L3 and below, corresponding to the cauda equina, it is acceptable to supplement the TIVA with gas anesthesia, also known as half MAC (Monitored Anesthesia Care).

Muscle relaxants are normally needed for the intubation and for exposure of the surgical site. As expected, neuromuscular blockers, such as Rocuronium and Succinylcholine, strongly suppress myogenic MEPs. Rocuronium (ROC) is a commonly used, non‐depolarizing neuromuscular blocker. ROC is a nicotinic receptor antagonist that has a duration of ~30-60 min at standard doses. In contrast, succinylcholine is a depolarizing neuromuscular blocker with a rapid onset and elimination, which can be used as an alternative to ROC.

MEPs are also influenced by neurological disorders, such as myasthenia gravis, botulinum toxin treatments for dystonia, and muscular dystrophy. Therefore, it is important to understand the patient's medical history when preparing for cases that involve MEPs.

==Waveform==
The MEP waveform arises from direct and indirect activation of the pyramidal cells in the motor cortex.[3] Electrical and magnetic stimulation directly activate the pyramidal cells. MEPs from these upper motor neurons can be recorded directly in the spinal cord as near-field potentials called ‘D-waves,’ as there are no synaptic connections between the activated neurons in the cortex and the recording site, or far-field potentials from muscles of the upper and lower extremities. Indirect synaptic activation of the pyramidal neurons from activated local interneurons also contributes to the MEP. These components of the MEP are called ‘I-waves.’ The number of I-waves in the MEP waveform, and the interval between each I-wave, depend on the number of synapses or synaptic delays in the circuit between the cortical interneurons and the activated pyramidal cell.

MEPs recorded from muscle tissue also involve another level of synaptic integration at the ventral horn, where upper motor neurons form synaptic connections with lower motor neurons. As a result, pulse-train stimulation tends to produce a multi-phasic waveform, which reflects greater complexity of the far-field potentials recorded from muscles.

==Intraoperative Monitoring==
'''1. Data acquisition:''' Baseline MEP responses should be established shortly after the patient is sedated, but the exact timing depends on the type of surgery being monitored. For example, for anterior cervical spinal surgeries, it is ideal to record baseline responses prior to neck and shoulder positioning because these manipulations could adversely affect spinal cord function and exacerbate the patient's condition. For posterior lumbar spinal surgeries, the baseline responses can be recorded after the patient has been flipped to the prone position. In this case, patient positioning is less likely to cause neurological damage because the nerve roots are mainly at risk, not the spinal cord. MEP tests should be run throughout the surgical procedure and should correspond to surgical events such as the incision, insertion of hardware, decompression, closure, etc.

'''2. Alarm criteria:''' Similar to the criterion used for SSEPs, a 50% decrease in MEP amplitude is cause for concern. The amplitude of the myogenic MEP is measured from the most positive peak to the most negative peak in the waveform. The MEP amplitude and waveform shape can vary over time, even in the absence of changes in corticospinal tract function. Therefore, some clinicians have argued that the criterion of 50% decrease in amplitude is not always reliable and can lead to false alarms.[4] However, an 80-100% drop in the MEP signal would certainly constitute an alarm and should be reported to the surgeon.

==References==
1. Vogel RW (2017). Understanding Anodal and Cathodal Stimulation. The ASNM Monitor, The American Society of Neurophysiological Monitoring. https://www.asnm.org/blogpost/1635804/290597/Understanding-Anodal-and-Cathodal-Stimulation

2. MacDonald DB (2006). Intraoperative motor evoked potential monitoring: overview and update. J Clin Monit Comput 20:347–377.

3. Legatt AD, Emerson RG, Epstein CM, MacDonald DB, Deletis V, Bravo RJ, López JR (2016). ACNS Guideline: Transcranial Electrical Stimulation Motor Evoked Potential Monitoring.
J Clin Neurophysiol 33(1):42-50.

4. Langeloo DD, Journée HL, de Kleuver M, et al. (2007). Criteria for transcranial electrical motor evoked potential monitoring during spinal deformity surgery: A review and discussion of the literature. Neurophysiol Clin 37:431–439.

Motor Evoked Potentials (MEP)

2022-01-18T20:53:33Z

Wdoyon: /* Motor Pathways */

== Introduction==
Motor evoked potentials (MEPs) are electrical signals recorded from muscle tissue in response to stimulation of the motor cortex. The stimulation may be magnetic or electrical and applied directly to the motor cortex or applied transcranially through the skull.

== Motor Pathways==
MEPs are used to monitor the functional integrity of the descending motor pathways, which mediate voluntary movement of the face, limbs, and torso, respectively. The descending motor pathways can be divided into a lateral system and a medial system. The lateral system includes the corticospinal tract and a smaller rubrospinal tract. The corticospinal pathway originates mainly in the primary motor cortex (Brodmann’s area 4), but fibers from other regions, such as the premotor, the supplemental motor, and somatosensory cortices contribute as well. The descending motor pathways consist of upper and lower motor neurons, as well as interneurons at the level of the spinal cord. The cell bodies of the upper motor neurons lie in the motor cortices, whereas the cell bodies of lower motor neurons lie in the brain stem and spinal cord. Each lower motor neuron gives rise to one axon that exits the spinal cord and passes through a ventral nerve root. The ventral nerve roots form a fiber bundle with the dorsal nerve roots, which together form a peripheral nerve bundle. Each lower motor axon passes through through the peripheral nerve bundle and then arborizes to innervate multiple muscle fibers. Each synaptic connection forms an excitatory synapse. One lower motor neuron and the muscle fibers that it innervates is known as a motor unit. There are hundreds of motor units within a single muscle each of which occupying a space of approximately 5 to 11 mm in diameter (Leppanen et al., 2005).

'''The corticospinal system'''. The corticospinal pathway is composed primarily of a lateral system and a smaller anterior system, and these innervates the distal limbs. The axons of the lateral tract descend through the internal capsule and project to the lower medulla. The upper motor neurons of the lateral tract cross the midline to the contralateral side and descend to the lateral column of the dorsal horn. After reaching the ventral horn of the spinal cord, these motor neuron axons form synaptic connections with lower motor neurons, often via local interneurons. The axons of the anterior tract do not appear to cross the midline.

'''The medial system'''. The medial system innervates the trunk and proximal limb muscles. Fibers in the motor cortices descend bilaterally and do not cross the midline.

'''The corticobulbar system'''. The upper motor neurons of the corticobulbar tract descend to the brain stem and innervate several nuclei for cranial nerves that control the facial muscles, including cranial nerves V, VII, IX, X, and XII. In most cases, the corticobulbar tract innervates the facial motor nuclei, bilaterally, including those nuclei involved in swallowing, speech, chewing and tongue movement. In contrast, upper motor neurons innervate the nuclei the lower facial nuclei and the hypoglossal nerves on the contralateral side of the body.

'''The basal ganglia'''. The basal ganglia is a set of subcortical structures that help to control movement. Many now view the basal ganglia as part of the upper motor systems because the basal ganglia modulates these systems. The structures of the basal ganglia typically include the caudate/putamen (striatum), globus pallidus internal (GPi) and external (GPe) segments, subthalamic nucleus, substantia nigra pars compacta (SNc) and reticulata (SNr). The cortical motor areas and SNc provide input to the striatum. The projection neurons of the striatum are connected to the GPi and the SNr via the direct and indirect pathways. The projection neurons of the direct pathway predominantly express dopamine D1 receptors, whereas those of the indirect pathway predominantly expresses dopamine D2 receptors. The indirect pathway includes the GPe and the subthalamic nucleus. The GPi and the SNr provide tonic inhibitory feedback to the motor cortex via the thalamocortical pathway.

==Stimulation==
'''1.''' '''Transcranial electrical stimulation (tES)'''. tES is a commonly used, non-invasive technique for generating MEPs. Stimulating electrodes for tES are placed on the scalp or subcutaneously above the primary motor cortex. Electrical current is then applied to the head to alter neuronal activity in the brain. However, due to the thickness and high resistance of the skull bone, only a small percentage of current reaches the brain tissue. Therefore, to record MEPs from muscle tissue, the stimulus strength must be set high enough to overcome that resistance and activate the underlying motor pathways. We call this type of stimulation supra-maximal, as the stimulus is higher than that required for the recruitment of all muscle fibers around the recording electrode. Train stimulation is required to elicit reliable MEPs under anesthesia, requiring the use of a multi-pulse stimulator.

The stimulating electrodes for MEPs are placed at C1 (for right extremities) and C2 (for left extremities) using the 10-20 system. Unlike the cathodal stimulation used for SSEPs, clinicians use anodal (+) stimulation to elicit MEPs. Electrical current flow from the anode to the cathode is more effective at depolarizing the pyramidal neurons of the primary motor cortex due to the more vertical organization of these cells.[1] Typical tES parameters used to elicit myogenic MEPs include intensities ranging from ~400-600 V, a train of 4-9 pulses, an inter-pulse interval ranging from 2-4 ms, and a pulse width between 50-75 μs (but higher values are used in other countries).

'''2. Transcranial magnetic stimulation (TMS)'''. TMS is another non-invasive technique for generating MEPs. By applying a magnetic field over the scalp, TMS can induce electrical currents in brain tissue in awake patients. One disadvantage of TMS, however, is that TMS-induced MEPs are suppressed under anesthesia.

'''3. Direct cortical stimulation'''. For some craniotomies, MEPs can also be induced by direct stimulation of the primary motor cortex. Without the resistance of the skull, direct cortical stimulation involves the use of much lower stimulation levels.

'''4. Spinal cord stimulation'''. Also called neurogenic motor evoked potentials, these signals are evoked in the rostral spinal cord and often recorded from electrodes on peripheral nerves in the legs. However, this technique is not widely used for monitoring the motor pathways because the recordings are not easy to interpret. Neurogenic MEPs likely arise from retrograde activation of the sensory pathways [2], in addition to the corticospinal tract. Other forms of spinal cord stimulation are used to test the efficacy of spinal cord stimulators during implantation. Spinal cord stimulators are typically placed in the thoracic spine and used for the treatment of neuropathic pain and other chronic pain disorders.

==Recording Techniques==
'''1. Recording Sites and Parameters'''.
MEPs are typically recorded from muscles (myogenic MEPs), as compound muscle action potentials, following tES of the primary motor cortex. Myogenic MEPs are recorded on different muscle groups associated with the myotome. The myotome is a set of muscle groups innervated by a single motor neuron, which together is known as a motor unit. Myogenic MEPs are recorded with needle or surface electrodes on bilateral muscle groups of the upper and lower extremities. The placement of the electrodes on specific muscles depends on the surgical procedure and which motor unit is potentially at risk. For an anterior cervical discectomy involving C6-7, for example, electrodes would be placed on the biceps and triceps, bilaterally, as these muscle groups are innervated by lower motor neurons that exit the spinal cord at C6-7.

MEPs are large amplitude signals that do not require averaging, as SSEPs do. The signals are filtered at the high and low frequency range to eliminate electrical artifacts (e.g., 10 Hz for the low-cut filter and 2000-3000 Hz for the high cut filter). The resistance of the recording electrodes in the muscle tissue should be low (< 5 kOhms). Myogenic MEPs can be repeated at a rate of 0.5-2 Hz.

'''2. Surgical and medical considerations'''. Some considerations for recording MEPs include the use of anesthetics and neuromuscular blocking agents. For example, myogenic MEPS are very sensitive to inhalation anesthetics, which limits the use of these anesthetics during IONM. Gas anesthesia strongly influences synaptic transmission. Inhalation anesthetics reduce the amplitude and prolong the latency of MEPs, and this affect becomes more pronounced when multiple synapses are involved, as is the case for myogenic MEPs, which involve the upper and lower motor neurons as well as interneurons in the spinal column. Intravenous anesthetics have a similar effect on MEPs but to a lesser degree. Total intravenous anesthesia (TIVA) - e.g., propofol and narcotic cocktails - is preferred in most cases, particularly those involving the spinal cord at L2 vertebrae and above. Propofol acts as a positive allosteric modulator of the GABA-A receptor, and it may also act directly as an agonist. For cases involving L3 and below, corresponding to the cauda equina, it is acceptable to supplement the TIVA with gas anesthesia, also known as half MAC (Monitored Anesthesia Care).

Muscle relaxants are normally needed for the intubation and for exposure of the surgical site. As expected, neuromuscular blockers, such as Rocuronium and Succinylcholine, strongly suppress myogenic MEPs. Rocuronium (ROC) is a commonly used, non‐depolarizing neuromuscular blocker. ROC is a nicotinic receptor antagonist that has a duration of ~30-60 min at standard doses. In contrast, succinylcholine is a depolarizing neuromuscular blocker with a rapid onset and elimination, which can be used as an alternative to ROC.

MEPs are also influenced by neurological disorders, such as myasthenia gravis, botulinum toxin treatments for dystonia, and muscular dystrophy. Therefore, it is important to understand the patient's medical history when preparing for cases that involve MEPs.

==Waveform==
The MEP waveform arises from direct and indirect activation of the pyramidal cells in the motor cortex.[3] Electrical and magnetic stimulation directly activate the pyramidal cells. MEPs from these upper motor neurons can be recorded directly in the spinal cord as near-field potentials called ‘D-waves,’ as there are no synaptic connections between the activated neurons in the cortex and the recording site, or far-field potentials from muscles of the upper and lower extremities. Indirect synaptic activation of the pyramidal neurons from activated local interneurons also contributes to the MEP. These components of the MEP are called ‘I-waves.’ The number of I-waves in the MEP waveform, and the interval between each I-wave, depend on the number of synapses or synaptic delays in the circuit between the cortical interneurons and the activated pyramidal cell.

MEPs recorded from muscle tissue also involve another level of synaptic integration at the ventral horn, where upper motor neurons form synaptic connections with lower motor neurons. As a result, pulse-train stimulation tends to produce a multi-phasic waveform, which reflects greater complexity of the far-field potentials recorded from muscles.

==Intraoperative Monitoring==
'''1. Data acquisition:''' Baseline MEP responses should be established shortly after the patient is sedated, but the exact timing depends on the type of surgery being monitored. For example, for anterior cervical spinal surgeries, it is ideal to record baseline responses prior to neck and shoulder positioning because these manipulations could adversely affect spinal cord function and exacerbate the patient's condition. For posterior lumbar spinal surgeries, the baseline responses can be recorded after the patient has been flipped to the prone position. In this case, patient positioning is less likely to cause neurological damage because the nerve roots are mainly at risk, not the spinal cord. MEP tests should be run throughout the surgical procedure and should correspond to surgical events such as the incision, insertion of hardware, decompression, closure, etc.

'''2. Alarm criteria:''' Similar to the criterion used for SSEPs, a 50% decrease in MEP amplitude is cause for concern. The amplitude of the myogenic MEP is measured from the most positive peak to the most negative peak in the waveform. The MEP amplitude and waveform shape can vary over time, even in the absence of changes in corticospinal tract function. Therefore, some clinicians have argued that the criterion of 50% decrease in amplitude is not always reliable and can lead to false alarms.[4] However, an 80-100% drop in the MEP signal would certainly constitute an alarm and should be reported to the surgeon.

==References==
1. Vogel RW (2017). Understanding Anodal and Cathodal Stimulation. The ASNM Monitor, The American Society of Neurophysiological Monitoring. https://www.asnm.org/blogpost/1635804/290597/Understanding-Anodal-and-Cathodal-Stimulation

2. MacDonald DB (2006). Intraoperative motor evoked potential monitoring: overview and update. J Clin Monit Comput 20:347–377.

3. Legatt AD, Emerson RG, Epstein CM, MacDonald DB, Deletis V, Bravo RJ, López JR (2016). ACNS Guideline: Transcranial Electrical Stimulation Motor Evoked Potential Monitoring.
J Clin Neurophysiol 33(1):42-50.

4. Langeloo DD, Journée HL, de Kleuver M, et al. (2007). Criteria for transcranial electrical motor evoked potential monitoring during spinal deformity surgery: A review and discussion of the literature. Neurophysiol Clin 37:431–439.

Motor Evoked Potentials (MEP)

2022-01-18T20:01:52Z

Wdoyon: /* Introduction */

== Introduction==
Motor evoked potentials (MEPs) are electrical signals recorded from muscle tissue in response to stimulation of the motor cortex. The stimulation may be magnetic or electrical and applied directly to the motor cortex or applied transcranially through the skull.

== Motor Pathways==
MEPs are used to monitor the functional integrity of the descending motor pathways, which mediate voluntary movement of the face, limbs, and torso, respectively. The descending motor pathways can be divided into a lateral system and a medial system. The lateral system includes the corticospinal tract and a smaller rubrospinal tract. The corticospinal pathway originates mainly in the primary motor cortex (Brodmann’s area 4), but fibers from other regions, such as the premotor, the supplemental motor, and somatosensory cortices contribute as well. The descending motor pathways consist of upper and lower motor neurons, as well as interneurons at the level of the spinal cord. The cell bodies of the upper motor neurons lie in the motor cortices, and the cell bodies of lower motor neurons lie in the brain stem and spinal cord. The axons of the lower motor neurons exit the spinal cord, forming the ventral nerve roots. The ventral nerve roots form a fiber bundle with the dorsal nerve roots of the somatosensory pathway, which together form a peripheral nerve bundle. The ventral nerve roots then innervate the upper and lower extremity musculature.

'''The corticospinal system'''. The corticospinal pathway is composed primarily of a lateral system and a smaller anterior system, and these innervates the distal limbs. The axons of the lateral tract descend through the internal capsule and project to the lower medulla. The upper motor neurons of the lateral tract cross the midline to the contralateral side and descend to the lateral column of the dorsal horn. After reaching the ventral horn of the spinal cord, these motor neuron axons form synaptic connections with lower motor neurons, often via local interneurons. The axons of the anterior tract do not appear to cross the midline.

'''The medial system'''. The medial system innervates the trunk and proximal limb muscles. Fibers in the motor cortices descend bilaterally and do not cross the midline.

'''The corticobulbar system'''. The upper motor neurons of the corticobulbar tract descend to the brain stem and innervate several nuclei for cranial nerves that control the facial muscles, including cranial nerves V, VII, IX, X, and XII. In most cases, the corticobulbar tract innervates the facial motor nuclei, bilaterally, including those nuclei involved in swallowing, speech, chewing and tongue movement. In contrast, upper motor neurons innervate the nuclei the lower facial nuclei and the hypoglossal nerves on the contralateral side of the body.

'''The basal ganglia'''. The basal ganglia is a set of subcortical structures that help to control movement. Many now view the basal ganglia as part of the upper motor systems because the basal ganglia modulates these systems. The structures of the basal ganglia typically include the caudate/putamen (striatum), globus pallidus internal (GPi) and external (GPe) segments, subthalamic nucleus, substantia nigra pars compacta (SNc) and reticulata (SNr). The cortical motor areas and SNc provide input to the striatum. The projection neurons of the striatum are connected to the GPi and the SNr via the direct and indirect pathways. The projection neurons of the direct pathway predominantly express dopamine D1 receptors, whereas those of the indirect pathway predominantly expresses dopamine D2 receptors. The indirect pathway includes the GPe and the subthalamic nucleus. The GPi and the SNr provide tonic inhibitory feedback to the motor cortex via the thalamocortical pathway.

==Stimulation==
'''1.''' '''Transcranial electrical stimulation (tES)'''. tES is a commonly used, non-invasive technique for generating MEPs. Stimulating electrodes for tES are placed on the scalp or subcutaneously above the primary motor cortex. Electrical current is then applied to the head to alter neuronal activity in the brain. However, due to the thickness and high resistance of the skull bone, only a small percentage of current reaches the brain tissue. Therefore, to record MEPs from muscle tissue, the stimulus strength must be set high enough to overcome that resistance and activate the underlying motor pathways. We call this type of stimulation supra-maximal, as the stimulus is higher than that required for the recruitment of all muscle fibers around the recording electrode. Train stimulation is required to elicit reliable MEPs under anesthesia, requiring the use of a multi-pulse stimulator.

The stimulating electrodes for MEPs are placed at C1 (for right extremities) and C2 (for left extremities) using the 10-20 system. Unlike the cathodal stimulation used for SSEPs, clinicians use anodal (+) stimulation to elicit MEPs. Electrical current flow from the anode to the cathode is more effective at depolarizing the pyramidal neurons of the primary motor cortex due to the more vertical organization of these cells.[1] Typical tES parameters used to elicit myogenic MEPs include intensities ranging from ~400-600 V, a train of 4-9 pulses, an inter-pulse interval ranging from 2-4 ms, and a pulse width between 50-75 μs (but higher values are used in other countries).

'''2. Transcranial magnetic stimulation (TMS)'''. TMS is another non-invasive technique for generating MEPs. By applying a magnetic field over the scalp, TMS can induce electrical currents in brain tissue in awake patients. One disadvantage of TMS, however, is that TMS-induced MEPs are suppressed under anesthesia.

'''3. Direct cortical stimulation'''. For some craniotomies, MEPs can also be induced by direct stimulation of the primary motor cortex. Without the resistance of the skull, direct cortical stimulation involves the use of much lower stimulation levels.

'''4. Spinal cord stimulation'''. Also called neurogenic motor evoked potentials, these signals are evoked in the rostral spinal cord and often recorded from electrodes on peripheral nerves in the legs. However, this technique is not widely used for monitoring the motor pathways because the recordings are not easy to interpret. Neurogenic MEPs likely arise from retrograde activation of the sensory pathways [2], in addition to the corticospinal tract. Other forms of spinal cord stimulation are used to test the efficacy of spinal cord stimulators during implantation. Spinal cord stimulators are typically placed in the thoracic spine and used for the treatment of neuropathic pain and other chronic pain disorders.

==Recording Techniques==
'''1. Recording Sites and Parameters'''.
MEPs are typically recorded from muscles (myogenic MEPs), as compound muscle action potentials, following tES of the primary motor cortex. Myogenic MEPs are recorded on different muscle groups associated with the myotome. The myotome is a set of muscle groups innervated by a single motor neuron, which together is known as a motor unit. Myogenic MEPs are recorded with needle or surface electrodes on bilateral muscle groups of the upper and lower extremities. The placement of the electrodes on specific muscles depends on the surgical procedure and which motor unit is potentially at risk. For an anterior cervical discectomy involving C6-7, for example, electrodes would be placed on the biceps and triceps, bilaterally, as these muscle groups are innervated by lower motor neurons that exit the spinal cord at C6-7.

MEPs are large amplitude signals that do not require averaging, as SSEPs do. The signals are filtered at the high and low frequency range to eliminate electrical artifacts (e.g., 10 Hz for the low-cut filter and 2000-3000 Hz for the high cut filter). The resistance of the recording electrodes in the muscle tissue should be low (< 5 kOhms). Myogenic MEPs can be repeated at a rate of 0.5-2 Hz.

'''2. Surgical and medical considerations'''. Some considerations for recording MEPs include the use of anesthetics and neuromuscular blocking agents. For example, myogenic MEPS are very sensitive to inhalation anesthetics, which limits the use of these anesthetics during IONM. Gas anesthesia strongly influences synaptic transmission. Inhalation anesthetics reduce the amplitude and prolong the latency of MEPs, and this affect becomes more pronounced when multiple synapses are involved, as is the case for myogenic MEPs, which involve the upper and lower motor neurons as well as interneurons in the spinal column. Intravenous anesthetics have a similar effect on MEPs but to a lesser degree. Total intravenous anesthesia (TIVA) - e.g., propofol and narcotic cocktails - is preferred in most cases, particularly those involving the spinal cord at L2 vertebrae and above. Propofol acts as a positive allosteric modulator of the GABA-A receptor, and it may also act directly as an agonist. For cases involving L3 and below, corresponding to the cauda equina, it is acceptable to supplement the TIVA with gas anesthesia, also known as half MAC (Monitored Anesthesia Care).

Muscle relaxants are normally needed for the intubation and for exposure of the surgical site. As expected, neuromuscular blockers, such as Rocuronium and Succinylcholine, strongly suppress myogenic MEPs. Rocuronium (ROC) is a commonly used, non‐depolarizing neuromuscular blocker. ROC is a nicotinic receptor antagonist that has a duration of ~30-60 min at standard doses. In contrast, succinylcholine is a depolarizing neuromuscular blocker with a rapid onset and elimination, which can be used as an alternative to ROC.

MEPs are also influenced by neurological disorders, such as myasthenia gravis, botulinum toxin treatments for dystonia, and muscular dystrophy. Therefore, it is important to understand the patient's medical history when preparing for cases that involve MEPs.

==Waveform==
The MEP waveform arises from direct and indirect activation of the pyramidal cells in the motor cortex.[3] Electrical and magnetic stimulation directly activate the pyramidal cells. MEPs from these upper motor neurons can be recorded directly in the spinal cord as near-field potentials called ‘D-waves,’ as there are no synaptic connections between the activated neurons in the cortex and the recording site, or far-field potentials from muscles of the upper and lower extremities. Indirect synaptic activation of the pyramidal neurons from activated local interneurons also contributes to the MEP. These components of the MEP are called ‘I-waves.’ The number of I-waves in the MEP waveform, and the interval between each I-wave, depend on the number of synapses or synaptic delays in the circuit between the cortical interneurons and the activated pyramidal cell.

MEPs recorded from muscle tissue also involve another level of synaptic integration at the ventral horn, where upper motor neurons form synaptic connections with lower motor neurons. As a result, pulse-train stimulation tends to produce a multi-phasic waveform, which reflects greater complexity of the far-field potentials recorded from muscles.

==Intraoperative Monitoring==
'''1. Data acquisition:''' Baseline MEP responses should be established shortly after the patient is sedated, but the exact timing depends on the type of surgery being monitored. For example, for anterior cervical spinal surgeries, it is ideal to record baseline responses prior to neck and shoulder positioning because these manipulations could adversely affect spinal cord function and exacerbate the patient's condition. For posterior lumbar spinal surgeries, the baseline responses can be recorded after the patient has been flipped to the prone position. In this case, patient positioning is less likely to cause neurological damage because the nerve roots are mainly at risk, not the spinal cord. MEP tests should be run throughout the surgical procedure and should correspond to surgical events such as the incision, insertion of hardware, decompression, closure, etc.

'''2. Alarm criteria:''' Similar to the criterion used for SSEPs, a 50% decrease in MEP amplitude is cause for concern. The amplitude of the myogenic MEP is measured from the most positive peak to the most negative peak in the waveform. The MEP amplitude and waveform shape can vary over time, even in the absence of changes in corticospinal tract function. Therefore, some clinicians have argued that the criterion of 50% decrease in amplitude is not always reliable and can lead to false alarms.[4] However, an 80-100% drop in the MEP signal would certainly constitute an alarm and should be reported to the surgeon.

==References==
1. Vogel RW (2017). Understanding Anodal and Cathodal Stimulation. The ASNM Monitor, The American Society of Neurophysiological Monitoring. https://www.asnm.org/blogpost/1635804/290597/Understanding-Anodal-and-Cathodal-Stimulation

2. MacDonald DB (2006). Intraoperative motor evoked potential monitoring: overview and update. J Clin Monit Comput 20:347–377.

3. Legatt AD, Emerson RG, Epstein CM, MacDonald DB, Deletis V, Bravo RJ, López JR (2016). ACNS Guideline: Transcranial Electrical Stimulation Motor Evoked Potential Monitoring.
J Clin Neurophysiol 33(1):42-50.

4. Langeloo DD, Journée HL, de Kleuver M, et al. (2007). Criteria for transcranial electrical motor evoked potential monitoring during spinal deformity surgery: A review and discussion of the literature. Neurophysiol Clin 37:431–439.

IONM in Cranial Surgery

2022-01-18T17:17:24Z

Wdoyon: /* Cranial nerve monitoring */

==Introduction==

==Cortical mapping==
The resection of tumors located within or near the eloquent cortex can result in postoperative sensory, motor and language deficits. Cortical brain mapping techniques have been used for many years to help the neurosurgeon identify and avoid these important brain structures. For tumors that impact the primary motor cortex, the main concern includes loss of motor function in the contralateral face and upper and lower extremities. Two techniques have been developed for intraoperative cortical and subcortical mapping of the corticospinal tracts - the Penfield and Taniguchi techniques. Sufficient stimulation of the motor cortex along the homunculus elicits compound muscle action potentials in lower motor neurons that innervate contralateral muscle tissue. Electromyography is used to record these electrical responses. The Penfield technique uses low frequency/long duration stimulation. The current is delivered between the two points of the probe with trains of monophasic cathodal pulses. In contrast, the Taniguchi technique uses high frequency/short duration stimulation. The current is delivered between the probe and an electrode placed on the skull close to the motor strip with trains of monophasic anodal pulses. In either case, it is essential to determine the motor response threshold - the lowest intensity required to elicit a motor response in muscle tissue. Determination of the motor response threshold provides information on (1) the level of excitation of the motor cortex and (2) the distance between the stimulation site and the motor cortex.

The following are stimulation parameters and information for the Penfield and Taniguchi techniques:

Penfield technique
(1) Stimulator type: Bipolar
(2) Pulse type: Bi or Monophasic cathodal
(3) Frequency: 50 Hz
(4) Pulse width: 300-1000 microsec
(5) Pulse number: 5
(6) Intensity: 2-20 mA
(7) Duration: 2-5 sec with a 10-20 sec interval

The Penfield technique is particularly effective at eliciting motor responses in tumors that do not infiltrate the motor cortex extensively and have sharp margins (reference).

The Penfield technique may induce intraoperative seizures or result in false negative mapping (reference).

Taniguchi technique
(1) Stimulator type: Monopolar
(2) Pulse type: Monophasic anodal
(3) Frequency: 250-500 Hz
(4) Pulse width: 500 microsec
(5) Pulse number 5
(6) Intensity: 2-20 mA
(7) Duration: 20 microsec

Some difficult cases may require adjustment of these parameters, such as increasing the number of pulses to 7-9 or increasing the pulse width up to 800 microsec).

Alternatively, some have suggested using a bipolar probe with the Taniguchi technique to increase the spatial resolution of stimulation. This method may be useful for cases involving the primary motor cortex or supplementary motor cortex (Rossi et al., 2021).

==Subcortical mapping==
Subcortical mapping is used for tumors that are located within or near the descending subcortical motor pathways. A monopolar stimulating electrode is used to determine the motor threshold, indicating the relative distance from the electrode tip to the motor pathways. Every 1 mA change is approximately 1 mm distance change from the motor pathways. It is recommended that the surgeon not proceed further if a motor threshold of 7 mA is reached. Continued resection may result in postoperative motor deficits.

The following are stimulation parameters and information for subcortical mapping:
(1) Intensity: 2-20 mA (2) Stimulation type: monopolar cathodal stimulation (3) pulse duration: 0.5-1.0 ms (4) Frequency: 250-500 Hz (5) Pulse number: 4-5 (6) Interstimulus interval: 3-4 ms.

Monopolar cathodal stimulation has been shown to be more effective than bipolar stimulation for eliciting subcortical MEPs (Szelenyi et al., 2011).

==Skull base or CP angle tumors==

==Deep brain stimulation==

==Intracranial vascular procedures==

==Neurovascular information==
The brain is very sensitive to changes in blood flow. Cerebral blood flow is maintained and regulated by a homeostatic process called autoregulation. Cerebral blood flow is maintained at approximately 40-70 ml per minute for every 100 g of brain tissue, which occurs over a wide range of arterial blood pressures (~60 - 160 mm Hg) in a healthy brain. To maintain constant cerebral blood flow, several homeostatic mechanisms converge to maintain a balance between vasoconstriction and vasodilation, which includes myogenic, neurogenic, metabolic, and endothelial components (Armstead, Anesthesiol Clin. 2016; 34(3): 465–477).

==Cranial nerve monitoring==
A variety of surgical procedures require cranial nerve IONM. Cranial nerve monitoring typically includes recordings of spontaneous and triggered EMG activity and motor evoked potentials.

1. The recurrent laryngeal nerve, a branch of the vagus nerve (CN X), is monitored for thyroidectomies. The recurrent laryngeal nerve, one of approximately 11 branches of CN X, innervates the intrinsic muscles of the larynx. This nerve is monitored using an endotracheal tube that is lined with recording electrodes.

2. The facial nerve (CN VII) is monitored for parotidectomies, tympanoplasties, mastoidectomies, microvascular decompressions, etc. Major branches of CN VII include the temporal, zygomatic, buccal, marginal mandibular, and cervical. The temporal branch innervates the frontalis, orbicularis oculi and corrugator supercilii muscles. The zygomatic branch innervates the orbicularis oculi muscle. The buccal branch innervates the orbicularis oris, buccinator and zygomaticus muscles. The marginal mandibular branch innervates the mentalis muscle. The cervical branch innervates the platysma, a sheet of muscle fibers extending from the collarbone to the jaw. Pairs of recording electrodes are typically placed at the muscles of each branch.

3. The glossopharyngeal (CN IX) can be monitored for microvascular decompressions and acoustic neuroma resections. For acoustic neuroma resections, multiple cranial nerves could be at risk of injury depending on the size of the tumor, including the vestibulocochlear (CN VIII), facial (CN VII) nerve, vagus nerve (CN X), hypoglossal nerve (CN XII), and accessory nerve (CN XI).

==References==

Electromyography (EMG)

2022-01-17T23:17:28Z

Wdoyon:

Electromyography involves the recording of electrical activity from muscle fibers. Lower motor neurons that exit the spinal cord and brainstem form nerve roots. These nerve roots are at risk for injury during different types of surgeries, such as a spinal decompression. Lower motor neurons generate action potentials during voluntary movement, when electrically stimulated, and in response to disease or injury. Electromyography can detect these changes in electric potential. EMG serves as an important diagnostic tool to monitor nerve root function and motor neuron activity.
==The Neuromuscular Junction==
Lower motor neurons from the anterior horn of the spinal cord and from the motor nuclei in the brainstem send axonal projections to the periphery where they form synaptic connections with multiple muscle fibers. This interface between an axon terminal of the motor neuron and a single muscle fiber is known as the neuromuscular junction or motor end plate. The lower motor neurons release acetylcholine as a neurotransmitter. Acetylcholine binds to nicotinic acetylcholine receptors located on the cell membrane (or sarcolemma) of the muscle fiber, a process that can lead to depolarization and muscle contraction.

==Electromyography Recording==
EMG activity can be recorded using different types of electrodes, including monopolar needles, concentric needles, bipolar needles, and single-fiber needles. Without the use of high and low frequency filters, EMG signals would be very noisy and difficult to interpret. The low frequency filter should be set to 10-30 Hz and the high frequency filter to 10-20 kHz, for example.
#Spontaneous EMG. After the recording electrodes are inserted into the muscle tissue, the background EMG activity is stable and quiet under healthy conditions. However, the presence of an injury or a pathology can generate spontaneous EMG activity in the motor neuron or post-synaptically at the level of the muscle fiber. Examples of spontaneous activity arising from the muscle fiber include fibrillation potentials, positive sharp waves, myotonic discharges, and complex repetitive discharges. Examples of spontaneous activity arising from the motor neuron include neuromyotonic tonic discharges, myokymic discharges, and tremors.
#Stimulated EMG. Electrically stimulated EMG activity is used in IONM to monitor motor neuron or cranial nerve excitation by determining the stimulus threshold required to elicit a compound muscle action potential (CMAP). A CMAP represents the simultaneous activation of multiple muscle fibers and is often characterized by a large amplitude, polyphasic waveform. To determine the stimulus threshold for a CMAP, we slowly ramp up the electrical current until the CMAP is present. The stimulus threshold is the minimum current needed to observe this response above the background noise.
#Single-Fiber EMG. Developed in the 1960's by Stalberg and Eskedt (citation), single-fiber EMG involves the use of small-surface recording electrodes to record the electrical activity of individual muscle fibers. The technique has proven to be useful in the diagnosis of myasthenia gravis and other neuromuscular disorders (Baruca et al., 2016). Single-fiber EMG measures the interval between action potentials from different fibers of the same motor unit, also known as 'jitter'. The measurements are usually performed in awake patients, during which time the patient is asked to contract a muscle while the clinician records EMG activity.

==Peripheral Nerves==
EMG recordings from upper and lower extremity musculature are used to monitor the peripheral motor nerves that arise from the spinal cord. A spinal nerve exits above the vertebra to which it corresponds. For example, cervical spinal nerve 7 (C7) exits between C6-7. There is no C8 vertebra; therefore, the C8 spinal nerve, which exits between C7-T1, is an exception. The motor neurons form a ventral nerve root as they exit the spinal cord. Together with the dorsal (sensory) root, this bundle of fibers is called a spinal nerve root. There are 31 spinal nerve roots.

==Cranial Nerves==
Both spontaneous and electrically stimulated EMG recordings are used to monitor the cranial nerve function in a variety of surgical procedures. Electrically stimulated EMG activity is helpful for identifying the location of the nerves in the tissue. With this information, the surgeon can avoid that area and the possibility of injuring the nerve.
#Cranial Nerve VII. The facial nerve (CN VII) is at risk during surgeries such as a parotidectomy and middle ear cases (e.g., tympanoplasty, stapedectomy, etc.). The facial nerve has four main branches that are most often monitored for EMG activity. The Temporal Branches innervate the muscles above the eyes, including the frontalis, orbicularis oculi and corrugator supercilii. The Zygomatic Branches innervate the orbicularis oculi, underneath the eyes. The Buccal branches innervate the muscles of the upper mouth and cheeks. The Marginal Mandibular Branch innervates the mentalis around the chin. The Cervical Branch is a fifth branch of the facial nerve, which innervates the platysma near the throat, but this branch is not normally monitored for EMG activity.
#Cranial Nerve III, IV and VI. CN III, VI and VI innervate the extraocular muscles, which control the movements of the eyes and upper eyelids. The levator palpebrae superioris controls the upper eyelids. The superior rectus, inferior rectus, medial rectus, lateral rectus, inferior oblique and superior oblique control the movement of the eyes from side to side and up and down. These muscles would be monitored for EMG activity in cases involving the resection of a brain tumor, for example.
#Other Cranial Nerves

==Intraoperative Monitoring==
Spontaneous EMG activity is used to monitor spinal nerve root function for surgeries in which the spinal and cranial nerve roots are at risk for injury.
#Pedicle screws. Electrically triggered EMG recordings are an important technique for evaluating pedicle screw placement in lumbar and sacral surgeries. The technique is sometimes used for thoracic pedicle screws, but there tends to be lower threshold values because the pedicle bone is smaller. Screw testing is not as reliable in the thoracic region.
#Peripheral Nerves. Peripheral nerves, such as the recurrent laryngeal nerve (RLN), which is a branch of the Cranial Nerve X, are monitored to identify if the surgeon violating on the nerve during dissection. In addition to spontaneous EMG recordings, the surgeon will use triggered stimulation to probe the tissue at the surgical site. If the tip of the stimulator is near the nerve, the triggered stimulus will elicit a cMAP.
#Brachial Plexus
#Dorsal Rhizotomy
#Skull Base Tumor
#Others

==Anesthesia and Other Factors==
#Anesthesia Methods. EMG activity is not affected by gas anesthesia.
#Muscle Relaxants
#Temperature
#Tourniquet
#Others

==References==

Electromyography (EMG)

2022-01-17T23:02:20Z

Wdoyon:

Electromyography involves the recording of electrical activity from muscle fibers. EMG serves as an important diagnostic tool to monitor nerve root function and motor neuron activity. The lower motor neurons that exit the spinal cord form nerve roots. These nerve roots are at risk for injury during spinal surgeries. Lower motor neurons generate action potentials during voluntary movement, when electrically stimulated, and in response to disease or injury. Electromyography can detect these changes in electric potential.
==The Neuromuscular Junction==
Lower motor neurons from the anterior horn of the spinal cord and from the motor nuclei in the brainstem send axonal projections to the periphery where they form synaptic connections with multiple muscle fibers. This interface between an axon terminal of the motor neuron and a single muscle fiber is known as the neuromuscular junction or motor end plate. The lower motor neurons release acetylcholine as a neurotransmitter. Acetylcholine binds to nicotinic acetylcholine receptors located on the cell membrane (or sarcolemma) of the muscle fiber, a process that can lead to depolarization and muscle contraction.

==Electromyography Recording==
EMG activity can be recorded using different types of electrodes, including monopolar needles, concentric needles, bipolar needles, and single-fiber needles. Without the use of high and low frequency filters, EMG signals would be very noisy and difficult to interpret. The low frequency filter should be set to 10-30 Hz and the high frequency filter to 10-20 kHz, for example.
#Spontaneous EMG. After the recording electrodes are inserted into the muscle tissue, the background EMG activity is stable and quiet under healthy conditions. However, the presence of an injury or a pathology can generate spontaneous EMG activity in the motor neuron or post-synaptically at the level of the muscle fiber. Examples of spontaneous activity arising from the muscle fiber include fibrillation potentials, positive sharp waves, myotonic discharges, and complex repetitive discharges. Examples of spontaneous activity arising from the motor neuron include neuromyotonic tonic discharges, myokymic discharges, and tremors.
#Stimulated EMG. Electrically stimulated EMG activity is used in IONM to monitor motor neuron or cranial nerve excitation by determining the stimulus threshold required to elicit a compound muscle action potential (CMAP). A CMAP represents the simultaneous activation of multiple muscle fibers and is often characterized by a large amplitude, polyphasic waveform. To determine the stimulus threshold for a CMAP, we slowly ramp up the electrical current until the CMAP is present. The stimulus threshold is the minimum current needed to observe this response above the background noise.
#Single-Fiber EMG. Developed in the 1960's by Stalberg and Eskedt (citation), single-fiber EMG involves the use of small-surface recording electrodes to record the electrical activity of individual muscle fibers. The technique has proven to be useful in the diagnosis of myasthenia gravis and other neuromuscular disorders (Baruca et al., 2016). Single-fiber EMG measures the interval between action potentials from different fibers of the same motor unit, also known as 'jitter'. The measurements are usually performed in awake patients, during which time the patient is asked to contract a muscle while the clinician records EMG activity.

==Peripheral Nerves==
EMG recordings from upper and lower extremity musculature are used to monitor the peripheral motor nerves that arise from the spinal cord. A spinal nerve exits above the vertebra to which it corresponds. For example, cervical spinal nerve 7 (C7) exits between C6-7. There is no C8 vertebra; therefore, the C8 spinal nerve, which exits between C7-T1, is an exception. The motor neurons form a ventral nerve root as they exit the spinal cord. Together with the dorsal (sensory) root, this bundle of fibers is called a spinal nerve root. There are 31 spinal nerve roots.

==Cranial Nerves==
Both spontaneous and electrically stimulated EMG recordings are used to monitor the cranial nerve function in a variety of surgical procedures. Electrically stimulated EMG activity is helpful for identifying the location of the nerves in the tissue. With this information, the surgeon can avoid that area and the possibility of injuring the nerve.
#Cranial Nerve VII. The facial nerve (CN VII) is at risk during surgeries such as a parotidectomy and middle ear cases (e.g., tympanoplasty, stapedectomy, etc.). The facial nerve has four main branches that are most often monitored for EMG activity. The Temporal Branches innervate the muscles above the eyes, including the frontalis, orbicularis oculi and corrugator supercilii. The Zygomatic Branches innervate the orbicularis oculi, underneath the eyes. The Buccal branches innervate the muscles of the upper mouth and cheeks. The Marginal Mandibular Branch innervates the mentalis around the chin. The Cervical Branch is a fifth branch of the facial nerve, which innervates the platysma near the throat, but this branch is not normally monitored for EMG activity.
#Cranial Nerve III, IV and VI. CN III, VI and VI innervate the extraocular muscles, which control the movements of the eyes and upper eyelids. The levator palpebrae superioris controls the upper eyelids. The superior rectus, inferior rectus, medial rectus, lateral rectus, inferior oblique and superior oblique control the movement of the eyes from side to side and up and down. These muscles would be monitored for EMG activity in cases involving the resection of a brain tumor, for example.
#Other Cranial Nerves

==Intraoperative Monitoring==
Spontaneous EMG activity is used to monitor spinal nerve root function for surgeries in which the spinal and cranial nerve roots are at risk for injury.
#Pedicle screws. Electrically triggered EMG recordings are an important technique for evaluating pedicle screw placement in lumbar and sacral surgeries. The technique is sometimes used for thoracic pedicle screws, but there tends to be lower threshold values because the pedicle bone is smaller. Screw testing is not as reliable in the thoracic region.
#Peripheral Nerves. Peripheral nerves, such as the recurrent laryngeal nerve (RLN), which is a branch of the Cranial Nerve X, are monitored to identify if the surgeon violating on the nerve during dissection. In addition to spontaneous EMG recordings, the surgeon will use triggered stimulation to probe the tissue at the surgical site. If the tip of the stimulator is near the nerve, the triggered stimulus will elicit a cMAP.
#Brachial Plexus
#Dorsal Rhizotomy
#Skull Base Tumor
#Others

==Anesthesia and Other Factors==
#Anesthesia Methods. EMG activity is not affected by gas anesthesia.
#Muscle Relaxants
#Temperature
#Tourniquet
#Others

==References==

Electromyography (EMG)

2022-01-17T22:44:20Z

Wdoyon: /* Intraoperative Monitoring */

Electromyography is the recording of electrical activity from muscle fibers and serves as an important diagnostic tool to monitor nerve root function and motor neuron activity. Motor neurons generate action potentials during voluntary movement, when electrically stimulated, and in response to disease or injury. An electromyograph can detect these changes in electric potential.
==The Neuromuscular Junction==
Lower motor neurons from the anterior horn of the spinal cord and from the motor nuclei in the brainstem send axonal projections to the periphery where they form synaptic connections with multiple muscle fibers. This interface between an axon terminal of the motor neuron and a single muscle fiber is known as the neuromuscular junction or motor end plate. The lower motor neurons release acetylcholine as a neurotransmitter. Acetylcholine binds to nicotinic acetylcholine receptors located on the cell membrane (or sarcolemma) of the muscle fiber, a process that can lead to depolarization and muscle contraction.

==Electromyography Recording==
EMG activity can be recorded using different types of electrodes, including monopolar needles, concentric needles, bipolar needles, and single-fiber needles. Without the use of high and low frequency filters, EMG signals would be very noisy and difficult to interpret. The low frequency filter should be set to 10-30 Hz and the high frequency filter to 10-20 kHz, for example.
#Spontaneous EMG. After the recording electrodes are inserted into the muscle tissue, the background EMG activity is stable and quiet under healthy conditions. However, the presence of an injury or a pathology can generate spontaneous EMG activity in the motor neuron or post-synaptically at the level of the muscle fiber. Examples of spontaneous activity arising from the muscle fiber include fibrillation potentials, positive sharp waves, myotonic discharges, and complex repetitive discharges. Examples of spontaneous activity arising from the motor neuron include neuromyotonic tonic discharges, myokymic discharges, and tremors.
#Stimulated EMG. Electrically stimulated EMG activity is used in IONM to monitor motor neuron or cranial nerve excitation by determining the stimulus threshold required to elicit a compound muscle action potential (CMAP). A CMAP represents the simultaneous activation of multiple muscle fibers and is often characterized by a large amplitude, polyphasic waveform. To determine the stimulus threshold for a CMAP, we slowly ramp up the electrical current until the CMAP is present. The stimulus threshold is the minimum current needed to observe this response above the background noise.
#Single-Fiber EMG. Developed in the 1960's by Stalberg and Eskedt (citation), single-fiber EMG involves the use of small-surface recording electrodes to record the electrical activity of individual muscle fibers. The technique has proven to be useful in the diagnosis of myasthenia gravis and other neuromuscular disorders (Baruca et al., 2016). Single-fiber EMG measures the interval between action potentials from different fibers of the same motor unit, also known as 'jitter'. The measurements are usually performed in awake patients, during which time the patient is asked to contract a muscle while the clinician records EMG activity.

==Peripheral Nerves==
EMG recordings from upper and lower extremity musculature are used to monitor the peripheral motor nerves that arise from the spinal cord. A spinal nerve exits above the vertebra to which it corresponds. For example, cervical spinal nerve 7 (C7) exits between C6-7. There is no C8 vertebra; therefore, the C8 spinal nerve, which exits between C7-T1, is an exception. The motor neurons form a ventral nerve root as they exit the spinal cord. Together with the dorsal (sensory) root, this bundle of fibers is called a spinal nerve root. There are 31 spinal nerve roots.

==Cranial Nerves==
Both spontaneous and electrically stimulated EMG recordings are used to monitor the cranial nerve function in a variety of surgical procedures. Electrically stimulated EMG activity is helpful for identifying the location of the nerves in the tissue. With this information, the surgeon can avoid that area and the possibility of injuring the nerve.
#Cranial Nerve VII. The facial nerve (CN VII) is at risk during surgeries such as a parotidectomy and middle ear cases (e.g., tympanoplasty, stapedectomy, etc.). The facial nerve has four main branches that are most often monitored for EMG activity. The Temporal Branches innervate the muscles above the eyes, including the frontalis, orbicularis oculi and corrugator supercilii. The Zygomatic Branches innervate the orbicularis oculi, underneath the eyes. The Buccal branches innervate the muscles of the upper mouth and cheeks. The Marginal Mandibular Branch innervates the mentalis around the chin. The Cervical Branch is a fifth branch of the facial nerve, which innervates the platysma near the throat, but this branch is not normally monitored for EMG activity.
#Cranial Nerve III, IV and VI. CN III, VI and VI innervate the extraocular muscles, which control the movements of the eyes and upper eyelids. The levator palpebrae superioris controls the upper eyelids. The superior rectus, inferior rectus, medial rectus, lateral rectus, inferior oblique and superior oblique control the movement of the eyes from side to side and up and down. These muscles would be monitored for EMG activity in cases involving the resection of a brain tumor, for example.
#Other Cranial Nerves

==Intraoperative Monitoring==
Spontaneous EMG activity is used to monitor spinal nerve root function for surgeries in which the spinal and cranial nerve roots are at risk for injury.
#Pedicle screws. Electrically triggered EMG recordings are an important technique for evaluating pedicle screw placement in lumbar and sacral surgeries. The technique is sometimes used for thoracic pedicle screws, but there tends to be lower threshold values because the pedicle bone is smaller. Screw testing is not as reliable in the thoracic region.
#Peripheral Nerves. Peripheral nerves, such as the recurrent laryngeal nerve (RLN), which is a branch of the Cranial Nerve X, are monitored to identify if the surgeon violating on the nerve during dissection. In addition to spontaneous EMG recordings, the surgeon will use triggered stimulation to probe the tissue at the surgical site. If the tip of the stimulator is near the nerve, the triggered stimulus will elicit a cMAP.
#Brachial Plexus
#Dorsal Rhizotomy
#Skull Base Tumor
#Others

==Anesthesia and Other Factors==
#Anesthesia Methods. EMG activity is not affected by gas anesthesia.
#Muscle Relaxants
#Temperature
#Tourniquet
#Others

==References==

IONM in Spinal Surgery

2022-01-14T03:52:56Z

Wdoyon: /* Lumbosacral spine surgery */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral spine surgery==
'''Posterior lumbar-sacral interbody fusions'''. Posterior lumbosacral fusions are common surgeries and can include the L1-5 lumbar levels as well as the S1 sacral level S1 (S1-S5 are five fused segments). Lumbosacral fusions are performed to relieve pressure on the nerve roots or to stabilize the spine, which can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, and scoliosis.

IONM of lower lumbar and sacral regions (L3 and below) involves the monitoring of spinal nerve root function, primarily with EMG recordings. Ascending and descending spinal cord function is also monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles percutaneously. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

'''Lateral lumbar interbody fusions'''. This procedure is considered minimally invasive. In contrast to posterior lumbar fusion, the incision for a lateral lumbar fusion approaches the lumbar spine from a far lateral approach that passes through the psoas muscles. The main advantages of this technique is that injury to the posterior spinal ligaments is avoided and that larger cages can be inserted for greater stability of the interbody device. However, there is a higher risk of injury to the lumbar plexus using this approach. While nerve injuries tend to be temporary, there is a risk for permanent motor and sensory deficits due to injury of the femoral nerve. IONM strategies for monitoring of LLIFs include a combination of EMG, transcranial MEPs and SSEPs of the saphenous nerve. The saphenous nerve is the largest terminal cutaneous branch of the femoral nerve.

==Cervical spine surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic spinal surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

IONM of the thoracic procedures, such as fusions and laminectomies for placement of spinal cord stimulators, is similar to upper lumbar surgeries with regard to monitoring of spinal cord and nerve root function. For thoracic fusions, the pedicle screws are not always tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be much lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach.

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their location relative to the dura mater, one of the protective membranes of the spinal cord and brain. Extradural tumors are located outside the dura, whereas intradural tumors are located underneath the dura but outside of the spinal cord. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness, numbness and paralysis.

IONM of spinal tumor resections is similar to other spinal surgeries and includes recordings of MEPs, SSEPs and EMG activity. Extra lower extremity electrodes are often used to distinguish between different muscle groups for great spatial resolution. For thoracic tumors, the abdominal and intercostal muscles are also recorded depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes. During the tumor resection, the surgeon may ask to stimulate specific nerve roots to ensure that nerve function is intact. Nerve root stimulation is more common for lumbar tumors than for thoracic tumors.

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part or all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-14T03:51:56Z

Wdoyon: /* Lumbosacral spine surgery */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral spine surgery==
""Posterior lumbar-sacral interbody fusions"". Posterior lumbosacral fusions are common surgeries and can include the L1-5 lumbar levels as well as the S1 sacral level S1 (S1-S5 are five fused segments). Lumbosacral fusions are performed to relieve pressure on the nerve roots or to stabilize the spine, which can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, and scoliosis.

IONM of lower lumbar and sacral regions (L3 and below) involves the monitoring of spinal nerve root function, primarily with EMG recordings. Ascending and descending spinal cord function is also monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles percutaneously. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

""Lateral lumbar interbody fusions"". This procedure is considered minimally invasive. In contrast to posterior lumbar fusion, the incision for a lateral lumbar fusion approaches the lumbar spine from a far lateral approach that passes through the psoas muscles. The main advantages of this technique is that injury to the posterior spinal ligaments is avoided and that larger cages can be inserted for greater stability of the interbody device. However, there is a higher risk of injury to the lumbar plexus using this approach. While nerve injuries tend to be temporary, there is a risk for permanent motor and sensory deficits due to injury of the femoral nerve. IONM strategies for monitoring of LLIFs include a combination of EMG, transcranial MEPs and SSEPs of the saphenous nerve. The saphenous nerve is the largest terminal cutaneous branch of the femoral nerve.

==Cervical spine surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic spinal surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

IONM of the thoracic procedures, such as fusions and laminectomies for placement of spinal cord stimulators, is similar to upper lumbar surgeries with regard to monitoring of spinal cord and nerve root function. For thoracic fusions, the pedicle screws are not always tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be much lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach.

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their location relative to the dura mater, one of the protective membranes of the spinal cord and brain. Extradural tumors are located outside the dura, whereas intradural tumors are located underneath the dura but outside of the spinal cord. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness, numbness and paralysis.

IONM of spinal tumor resections is similar to other spinal surgeries and includes recordings of MEPs, SSEPs and EMG activity. Extra lower extremity electrodes are often used to distinguish between different muscle groups for great spatial resolution. For thoracic tumors, the abdominal and intercostal muscles are also recorded depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes. During the tumor resection, the surgeon may ask to stimulate specific nerve roots to ensure that nerve function is intact. Nerve root stimulation is more common for lumbar tumors than for thoracic tumors.

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part or all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-14T02:51:40Z

Wdoyon: /* Others */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral spine surgery==
Lumbosacral spinal fusions are common surgeries and can include L1-5 lumbar levels as well as the S1 sacral level S1 (S1-S5 are five fused segments). Lumbosacral fusions are performed to relieve pressure on the nerve roots or to stabilize the spine, which can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, and scoliosis.

IONM of lower lumbar and sacral regions (L3 and below) involves the monitoring of spinal nerve root function, primarily with EMG recordings. Ascending and descending spinal cord function is also monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles by a percutaneous approach. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

==Cervical spine surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic spinal surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

IONM of the thoracic procedures, such as fusions and laminectomies for placement of spinal cord stimulators, is similar to upper lumbar surgeries with regard to monitoring of spinal cord and nerve root function. For thoracic fusions, the pedicle screws are not always tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be much lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach.

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their location relative to the dura mater, one of the protective membranes of the spinal cord and brain. Extradural tumors are located outside the dura, whereas intradural tumors are located underneath the dura but outside of the spinal cord. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness, numbness and paralysis.

IONM of spinal tumor resections is similar to other spinal surgeries and includes recordings of MEPs, SSEPs and EMG activity. Extra lower extremity electrodes are often used to distinguish between different muscle groups for great spatial resolution. For thoracic tumors, the abdominal and intercostal muscles are also recorded depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes. During the tumor resection, the surgeon may ask to stimulate specific nerve roots to ensure that nerve function is intact. Nerve root stimulation is more common for lumbar tumors than for thoracic tumors.

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part or all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-14T02:51:05Z

Wdoyon: /* Spinal tumor surgery */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral spine surgery==
Lumbosacral spinal fusions are common surgeries and can include L1-5 lumbar levels as well as the S1 sacral level S1 (S1-S5 are five fused segments). Lumbosacral fusions are performed to relieve pressure on the nerve roots or to stabilize the spine, which can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, and scoliosis.

IONM of lower lumbar and sacral regions (L3 and below) involves the monitoring of spinal nerve root function, primarily with EMG recordings. Ascending and descending spinal cord function is also monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles by a percutaneous approach. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

==Cervical spine surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic spinal surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

IONM of the thoracic procedures, such as fusions and laminectomies for placement of spinal cord stimulators, is similar to upper lumbar surgeries with regard to monitoring of spinal cord and nerve root function. For thoracic fusions, the pedicle screws are not always tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be much lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach.

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their location relative to the dura mater, one of the protective membranes of the spinal cord and brain. Extradural tumors are located outside the dura, whereas intradural tumors are located underneath the dura but outside of the spinal cord. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness, numbness and paralysis.

IONM of spinal tumor resections is similar to other spinal surgeries and includes recordings of MEPs, SSEPs and EMG activity. Extra lower extremity electrodes are often used to distinguish between different muscle groups for great spatial resolution. For thoracic tumors, the abdominal and intercostal muscles are also recorded depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes. During the tumor resection, the surgeon may ask to stimulate specific nerve roots to ensure that nerve function is intact. Nerve root stimulation is more common for lumbar tumors than for thoracic tumors.

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part of all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-14T02:45:47Z

Wdoyon: /* Spinal tumor surgery */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral spine surgery==
Lumbosacral spinal fusions are common surgeries and can include L1-5 lumbar levels as well as the S1 sacral level S1 (S1-S5 are five fused segments). Lumbosacral fusions are performed to relieve pressure on the nerve roots or to stabilize the spine, which can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, and scoliosis.

IONM of lower lumbar and sacral regions (L3 and below) involves the monitoring of spinal nerve root function, primarily with EMG recordings. Ascending and descending spinal cord function is also monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles by a percutaneous approach. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

==Cervical spine surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic spinal surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

IONM of the thoracic procedures, such as fusions and laminectomies for placement of spinal cord stimulators, is similar to upper lumbar surgeries with regard to monitoring of spinal cord and nerve root function. For thoracic fusions, the pedicle screws are not always tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be much lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach.

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their location relative to the dura mater, one of the protective membranes of the spinal cord and brain. Extradural tumors are located outside the dura, whereas intradural tumors are located underneath the dura but outside of the spinal cord. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness, numbness and paralysis.

IONM of spinal tumor resections is similar to other spinal surgeries and includes recordings of MEPs, SSEPs and EMG activity. Extra lower extremity electrodes are often used to distinguish between different muscle groups for great spatial resolution. For thoracic tumors, the abdominal and intercostal muscles are also recorded depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes. The surgeon may ask to stimulate specific nerve roots, but this is more common for lumbar tumors than for thoracic tumors.

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part of all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-13T20:24:50Z

Wdoyon: /* Thoracic fusion and laminectomy surgery */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral spine surgery==
Lumbosacral spinal fusions are common surgeries and can include L1-5 lumbar levels as well as the S1 sacral level S1 (S1-S5 are five fused segments). Lumbosacral fusions are performed to relieve pressure on the nerve roots or to stabilize the spine, which can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, and scoliosis.

IONM of lower lumbar and sacral regions (L3 and below) involves the monitoring of spinal nerve root function, primarily with EMG recordings. Ascending and descending spinal cord function is also monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles by a percutaneous approach. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

==Cervical spine surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic spinal surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

IONM of the thoracic procedures, such as fusions and laminectomies for placement of spinal cord stimulators, is similar to upper lumbar surgeries with regard to monitoring of spinal cord and nerve root function. For thoracic fusions, the pedicle screws are not always tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be much lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach.

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their position to the dura mater, one of the protective membranes of the spinal cord. Intradural tumors are located inside the dura, whereas extradural tumors are located outside the dura mater. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness and paralysis.

IONM of spinal tumors is similar to other spinal surgeries, and includes recordings of MEPs, SSEPs and EMG activity. We use extra lower extremity electrodes to distinguish between different muscle groups for great spatial resolution. For thoracic tumors we also record the abdominal and intercostal muscles depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes.

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part of all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-13T20:23:23Z

Wdoyon: /* Lumbosacral spinal fusions */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral spine surgery==
Lumbosacral spinal fusions are common surgeries and can include L1-5 lumbar levels as well as the S1 sacral level S1 (S1-S5 are five fused segments). Lumbosacral fusions are performed to relieve pressure on the nerve roots or to stabilize the spine, which can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, and scoliosis.

IONM of lower lumbar and sacral regions (L3 and below) involves the monitoring of spinal nerve root function, primarily with EMG recordings. Ascending and descending spinal cord function is also monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles by a percutaneous approach. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

==Cervical spine surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic fusion and laminectomy surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

IONM of the thoracic procedures, such as fusions and laminectomies for placement of spinal cord stimulators, is similar to upper lumbar surgeries with regard to monitoring of spinal cord and nerve root function. For thoracic fusions, the pedicle screws are not usually tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be much lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach.

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their position to the dura mater, one of the protective membranes of the spinal cord. Intradural tumors are located inside the dura, whereas extradural tumors are located outside the dura mater. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness and paralysis.

IONM of spinal tumors is similar to other spinal surgeries, and includes recordings of MEPs, SSEPs and EMG activity. We use extra lower extremity electrodes to distinguish between different muscle groups for great spatial resolution. For thoracic tumors we also record the abdominal and intercostal muscles depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes.

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part of all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-13T20:21:24Z

Wdoyon: /* Cervical disc surgery */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral spinal fusions==
Lumbar spine fusions are common and can include sacral level S1 (S1-S5 are five fused segments). Lumbosacral fusions are performed to relieve pressure on the nerve roots or to stabilize the spine, which can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, and scoliosis.

IONM of lower lumbar and sacral regions (L3 and below) involves the monitoring of spinal nerve root function, primarily with EMG recordings. Ascending and descending spinal cord function is also monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles by a percutaneous approach. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

==Cervical spine surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic fusion and laminectomy surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

IONM of the thoracic procedures, such as fusions and laminectomies for placement of spinal cord stimulators, is similar to upper lumbar surgeries with regard to monitoring of spinal cord and nerve root function. For thoracic fusions, the pedicle screws are not usually tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be much lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach.

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their position to the dura mater, one of the protective membranes of the spinal cord. Intradural tumors are located inside the dura, whereas extradural tumors are located outside the dura mater. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness and paralysis.

IONM of spinal tumors is similar to other spinal surgeries, and includes recordings of MEPs, SSEPs and EMG activity. We use extra lower extremity electrodes to distinguish between different muscle groups for great spatial resolution. For thoracic tumors we also record the abdominal and intercostal muscles depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes.

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part of all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-13T20:20:43Z

Wdoyon: /* Lumbosacral fusion surgery */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral spinal fusions==
Lumbar spine fusions are common and can include sacral level S1 (S1-S5 are five fused segments). Lumbosacral fusions are performed to relieve pressure on the nerve roots or to stabilize the spine, which can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, and scoliosis.

IONM of lower lumbar and sacral regions (L3 and below) involves the monitoring of spinal nerve root function, primarily with EMG recordings. Ascending and descending spinal cord function is also monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles by a percutaneous approach. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

==Cervical disc surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic fusion and laminectomy surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

IONM of the thoracic procedures, such as fusions and laminectomies for placement of spinal cord stimulators, is similar to upper lumbar surgeries with regard to monitoring of spinal cord and nerve root function. For thoracic fusions, the pedicle screws are not usually tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be much lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach.

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their position to the dura mater, one of the protective membranes of the spinal cord. Intradural tumors are located inside the dura, whereas extradural tumors are located outside the dura mater. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness and paralysis.

IONM of spinal tumors is similar to other spinal surgeries, and includes recordings of MEPs, SSEPs and EMG activity. We use extra lower extremity electrodes to distinguish between different muscle groups for great spatial resolution. For thoracic tumors we also record the abdominal and intercostal muscles depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes.

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part of all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-13T20:18:56Z

Wdoyon: /* Spinal tumor surgery */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral fusion surgery==
Lumbar spine fusions are common and can include sacral level S1 (S1-S5 are five fused segments). Lumbosacral fusions are performed to relieve pressure on the nerve roots or to stabilize the spine, which can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, and scoliosis.

IONM of lower lumbar and sacral regions (L3 and below) involves the monitoring of spinal nerve root function, primarily with EMG recordings. Ascending and descending spinal cord function is also monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles by a percutaneous approach. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

==Cervical disc surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic fusion and laminectomy surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

IONM of the thoracic procedures, such as fusions and laminectomies for placement of spinal cord stimulators, is similar to upper lumbar surgeries with regard to monitoring of spinal cord and nerve root function. For thoracic fusions, the pedicle screws are not usually tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be much lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach.

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==
Tumors of the spinal cord are classified into two main categories: extramedullary and intramedullary. Extramedullary tumors are subdivided into intradural and extradural, depending on their position to the dura mater, one of the protective membranes of the spinal cord. Intradural tumors are located inside the dura, whereas extradural tumors are located outside the dura mater. Examples of extramedullary tumors include meningiomas, neurofibromas, schwannomas and nerve sheath tumors. Intramedullary tumors occur within the substance of the spinal cord. Examples include gliomas, astrocytomas or ependymomas. These different types of tumors can affect spinal cord function by causing spinal cord compression, leading to symptoms such as pain, weakness and paralysis.

IONM of spinal tumors is similar to other spinal surgeries, and includes recordings of MEPs, SSEPs and EMG activity. We use extra lower extremity electrodes to distinguish between different muscle groups for great spatial resolution. For thoracic tumors we also record the abdominal and intercostal muscles depending on the location of the spinal tumor. The closer the surgeon is to the tumor, the more critical it is to monitor and report any signal changes.

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part of all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-13T19:52:16Z

Wdoyon: /* Lumbosacral fusion surgery */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral fusion surgery==
Lumbar spine fusions are common and can include sacral level S1 (S1-S5 are five fused segments). Lumbosacral fusions are performed to relieve pressure on the nerve roots or to stabilize the spine, which can cause symptoms like pain, numbness and weakness in the legs. A posterior lumbar fusion involves the connection of two or more vertebrae by inserting screws into the pedicle bones and fastening them together with rod instrumentation. Surgeons will use different size screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. The fixation of screws into the lumbar spinal column requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column and tightened into place. Therefore, there are two sets of rods, one for each side of the spine.

Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, and scoliosis.

IONM of lower lumbar and sacral regions (L3 and below) involves the monitoring of spinal nerve root function, primarily with EMG recordings. Ascending and descending spinal cord function is also monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots. Current shunting is also an issue for minimally invasive surgeries in which the screws are inserted into the pedicles by a percutaneous approach. Here, the screws are inserted through the skin and soft tissue with the aid of fluoroscopy. When the pedicle screws are tested, the current is shunted to the surrounding tissue, which typically results in higher stimulus thresholds.

==Cervical disc surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic fusion and laminectomy surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

IONM of the thoracic procedures, such as fusions and laminectomies for placement of spinal cord stimulators, is similar to upper lumbar surgeries with regard to monitoring of spinal cord and nerve root function. For thoracic fusions, the pedicle screws are not usually tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be much lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach.

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part of all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-13T19:21:52Z

Wdoyon: /* Lumbosacral fusion surgery */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral fusion surgery==
A fusion can be performed at any level of the lumbar spine and can include the sacrum level S1 (S1-S5 are five fused segments). Lumbosacral fusions are performed to relieve pressure on the nerve roots or to stabilize the spine, which can cause symptoms like pain, numbness and weakness in the legs. A lumbar fusion involves the connection (or fusion) of two or more vertebrae by inserting screws into the pedicle bones, bilaterally, and connecting them with rod instrumentation. Surgeons will use different sized screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. For posterior spinal surgeries, the fixation of screws into the spinal column always requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, and scoliosis.

IONM of lower lumbar and sacral regions (L3 and below) involves the monitoring of spinal nerve root function, primarily with EMG recordings. Ascending and descending spinal cord function is also monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run through the spinal nerves between the screw to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue and have high stimulus thresholds. If the screw penetrates the bone, the applied current may activate the nearby nerve roots, resulting in compound muscle action potentials from muscles innervated by motor nerves at the level of the screw. CMAPs that are triggered by stimulus thresholds less than 10 mA in intensity are worrisome and may indicate a pedicle breach, which may cause irritation or injury to the nerve tissue. However, stimulus thresholds can vary from person to person. Individuals with osteoporosis may have lower stimulus thresholds because of the poor bone density. Also, the current can take different paths through the tissue that depend on the surgical environment. A wet surgical field may result in current shunting, which could increase the threshold needed to activate the nerve roots.

==Cervical disc surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic fusion and laminectomy surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

IONM of the thoracic procedures, such as fusions and laminectomies for placement of spinal cord stimulators, is similar to upper lumbar surgeries with regard to monitoring of spinal cord and nerve root function. For thoracic fusions, the pedicle screws are not usually tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be much lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach.

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part of all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Spinal Surgery

2022-01-13T18:30:00Z

Wdoyon: /* Lumbosacral fusion surgery */

==Introduction==
IONM is used in a variety of spinal surgeries to assess spinal cord, spinal nerve root, and brachial plexus function. IONM is used to monitor neurophysiological function of the ascending and descending spinal pathways, which could be affected by the surgical procedure.

==Symptoms and Diagnostics==
'''''Relevant clinical symptoms'''''

'''1. Myelopathies.''' Damage to the spinal cord caused by injury, disease, and disc deterioration can result in symptoms of myelopathy. Initial symptoms may including clumsiness, difficulty with fine motor skills, poor balance and coordination. As the symptoms progress they can become more severe, including pain, weakness, and numbness in the upper and lower extremities and bladder and bowel incontinence.

'''2. Radiculopathies.''' Compression or irritation of the exiting nerve roots along the spine can result in symptoms of radiculopathy. These symptoms vary depending on the individual and on the level of the spine where the compression occurred. Generalized symptoms include sharp pain in the shoulders or back that radiates into the extremities, often with weakness, numbness, and tingling. Cervical radiculopathy includes symptoms such pain in the neck, shoulders, upper back, often with arm weakness, numbness or pins and needles experienced on one side of the body. Thoracic radiculopathy is an uncommon condition but symptoms may include burning or shooting pain in the ribs, sides, or abdomen, as well as numbness and tingling. Lumbar radiculopathy, also known as sciatica, includes symptoms such as pain and numbness in the low back, hips, buttock, leg, or foot. These symptoms can be exacerbated by long periods of sitting or walking.

'''3. Foot drop.''' Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

'''4. Scoliosis.''' Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

'''5. Kyphosis.'''Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

'''6. Lordosis.''' Lordosis is an abnormal inward curvature of the lower spine.

'''''Diagnostic Tests'''''

'''1. Muscle strength exams.''' Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

'''2. Nerve conduction studies.''' Nerve conduction studying are used to determine if nerve damage is present on motor and sensory neurons. A stimulating and recording electrode are placed over a nerve (e.g. the Ulner or Median nerve). The time it takes for the impulse to reach the recording electrode is termed the latency. Latencies are on the order of milliseconds. The conduction velocity is calculated by dividing the distance between the electrodes by the latency, which equals the conduction velocity.

'''3. Imaging studies.''' CT, MRI and x-ray scans enable a doctor to see the structures in the neck or back that are contributing to the clinical symptoms.

==Lumbosacral fusion surgery==
A fusion can be performed at any level of the lumbar spine and can include the sacrum level S1 (S1-S5 are five fused segments). Lumbosacral fusions are performed to relieve pressure on the nerve roots or to stabilize the spine, which can cause symptoms like pain, numbness and weakness in the legs. A lumbar fusion involves the connection (or fusion) of two or more vertebrae by inserting screws into the pedicle bones, bilaterally, and connecting them with rod instrumentation. Surgeons will use different sized screws depending on the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. For posterior spinal surgeries, the fixation of screws into the spinal column always requires the use of rods to join them together. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, and scoliosis.

IONM of lower lumbar and sacral regions (L3 and below) involves the monitoring of spinal nerve root function, primarily with EMG recordings. Ascending and descending spinal cord function is also monitored with SSEPs and MEPs, respectively. SSEPs are particularly important because the surgical approach for most lower lumbar surgeries is posterior, which has a greater potential to damage spinal nerve roots that enter the dorsal horn of the spinal column. For upper lumbar cases (L1-2), MEPs along with SSEPs are essential for monitoring spinal cord function.

'''Pedicle screw testing'''. After insertion of the pedicle screws, triggered EMG is used to determine whether there is a medial breach of the pedicle bone. Pedicle screws are tested by applying direct electrical current (0-50 mA cathodal stimulation) to each screw. The current return is placed in tissue on the contralateral side of the spine, which makes the current run from the screw through the spinal nerves and to the anode. A screw that is well positioned in the pedicle bone will be well insulated by the bone tissue, and therefore the screw will require a higher current to. If there is a breach in the pedicle bone, the screw will not be well insulated. A screw that is not well-insulated by bone will pass current more easily to any nerve fibers or roots that are in close proximity, leading to a decrease in the electrical threshold that is needed to elicit a CMAP on a specific motor unit recorded in the muscle tissue. CMAPs that are triggered by thresholds less than 10 mA in intensity are a cause for concern and may indicate a breach, which may also cause irritation or damage to the nerve tissue.

==Cervical disc surgery==
In the modern era, lateral mass screws are used almost universally for posterior cervical level procedures. As indicated by their name, these screws are inserted into the lateral mass, the bony junction between the superior and inferior articular processes. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior surface of the vertebral body is exposed, which is more flat in morphology and can be fused to an adjacent vertebral body by a simple plate with screws. For posterior spinal procedures, rods are used. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spinal column. Therefore, there are two sets of rods, one for each side of the spine.

IONM of the cervical spine involves the monitoring of the spinal cord and spinal nerve root function. For these cases, MEPs along with SSEPs are essential for monitoring spinal cord function. If there is a change in either SSEPs or MEPs during the surgery, this may or may not be a cause for concern; however, if there is a simultaneous change in both the SSEPs and MEPs, the probability of spinal cord injury is higher. Spinal nerve roots are monitored by EMG primarily. During decompression of the spinal cord, it is not uncommon to see spontaneous neurotonic EMG activity from upper extremity musculature, including the deltoids, biceps, triceps, first dorsal interossei muscles. This EMG activity normally subsides after cessation of nerve root manipulation.

==Thoracic fusion and laminectomy surgery==
Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone compared to the lumbar pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

IONM of the thoracic procedures, such as fusions and laminectomies for placement of spinal cord stimulators, is similar to upper lumbar surgeries with regard to monitoring of spinal cord and nerve root function. For thoracic fusions, the pedicle screws are not usually tested with electrical stimulation because of the smaller size of the pedicle bone in the thoracic spine. The stimulus thresholds tend to be much lower compared to those in the lumbar spine, making it difficult to determine a criterion for a pedicle breach.

==Scoliosis surgery==
The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

==Spinal tumor surgery==

==Others==

'''1. Interbody cages and bone grafts.''' For different reasons, spinal surgeries may require the removal of part of all of the intervertebral disc. If so, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

'''2. Ondontoid (dens) fracture'''

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. C2 has a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the C1 vertebrae, which allows the head to rotate. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joint differs from the others vertebral joints because it does not have an intervertebral disc. This allows for a greater range of motion than the other vertebrae.

There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common.

'''Type I''': avulsion fracture of the apex, in which a fragment tears away from the odontoid bone.
'''Type II''': fracture through the base of the dens, at the junction of the odontoid base and the body of C2.
'''Type III''': fracture extends into the body of the axis.

==References==

IONM in Cranial Surgery

2021-12-16T03:05:11Z

Wdoyon: /* Cortical mapping */

==Introduction==

==Cortical mapping==
The resection of tumors located within or near the eloquent cortex can result in postoperative sensory, motor and language deficits. Cortical brain mapping techniques have been used for many years to help the neurosurgeon identify and avoid these important brain structures. For tumors that impact the primary motor cortex, the main concern includes loss of motor function in the contralateral face and upper and lower extremities. Two techniques have been developed for intraoperative cortical and subcortical mapping of the corticospinal tracts - the Penfield and Taniguchi techniques. Sufficient stimulation of the motor cortex along the homunculus elicits compound muscle action potentials in lower motor neurons that innervate contralateral muscle tissue. Electromyography is used to record these electrical responses. The Penfield technique uses low frequency/long duration stimulation. The current is delivered between the two points of the probe with trains of monophasic cathodal pulses. In contrast, the Taniguchi technique uses high frequency/short duration stimulation. The current is delivered between the probe and an electrode placed on the skull close to the motor strip with trains of monophasic anodal pulses. In either case, it is essential to determine the motor response threshold - the lowest intensity required to elicit a motor response in muscle tissue. Determination of the motor response threshold provides information on (1) the level of excitation of the motor cortex and (2) the distance between the stimulation site and the motor cortex.

The following are stimulation parameters and information for the Penfield and Taniguchi techniques:

Penfield technique
(1) Stimulator type: Bipolar
(2) Pulse type: Bi or Monophasic cathodal
(3) Frequency: 50 Hz
(4) Pulse width: 300-1000 microsec
(5) Pulse number: 5
(6) Intensity: 2-20 mA
(7) Duration: 2-5 sec with a 10-20 sec interval

The Penfield technique is particularly effective at eliciting motor responses in tumors that do not infiltrate the motor cortex extensively and have sharp margins (reference).

The Penfield technique may induce intraoperative seizures or result in false negative mapping (reference).

Taniguchi technique
(1) Stimulator type: Monopolar
(2) Pulse type: Monophasic anodal
(3) Frequency: 250-500 Hz
(4) Pulse width: 500 microsec
(5) Pulse number 5
(6) Intensity: 2-20 mA
(7) Duration: 20 microsec

Some difficult cases may require adjustment of these parameters, such as increasing the number of pulses to 7-9 or increasing the pulse width up to 800 microsec).

Alternatively, some have suggested using a bipolar probe with the Taniguchi technique to increase the spatial resolution of stimulation. This method may be useful for cases involving the primary motor cortex or supplementary motor cortex (Rossi et al., 2021).

==Subcortical mapping==
Subcortical mapping is used for tumors that are located within or near the descending subcortical motor pathways. A monopolar stimulating electrode is used to determine the motor threshold, indicating the relative distance from the electrode tip to the motor pathways. Every 1 mA change is approximately 1 mm distance change from the motor pathways. It is recommended that the surgeon not proceed further if a motor threshold of 7 mA is reached. Continued resection may result in postoperative motor deficits.

The following are stimulation parameters and information for subcortical mapping:
(1) Intensity: 2-20 mA (2) Stimulation type: monopolar cathodal stimulation (3) pulse duration: 0.5-1.0 ms (4) Frequency: 250-500 Hz (5) Pulse number: 4-5 (6) Interstimulus interval: 3-4 ms.

Monopolar cathodal stimulation has been shown to be more effective than bipolar stimulation for eliciting subcortical MEPs (Szelenyi et al., 2011).

==Skull base or CP angle tumors==

==Deep brain stimulation==

==Intracranial vascular procedures==

==Neurovascular information==
The brain is very sensitive to changes in blood flow. Cerebral blood flow is maintained and regulated by a homeostatic process called autoregulation. Cerebral blood flow is maintained at approximately 40-70 ml per minute for every 100 g of brain tissue, which occurs over a wide range of arterial blood pressures (~60 - 160 mm Hg) in a healthy brain. To maintain constant cerebral blood flow, several homeostatic mechanisms converge to maintain a balance between vasoconstriction and vasodilation, which includes myogenic, neurogenic, metabolic, and endothelial components (Armstead, Anesthesiol Clin. 2016; 34(3): 465–477).

==Cranial nerve monitoring==
The cranial nerves are monitored with for a variety of surgical procedures. The modalities that are monitored typically include spontaneous and triggered EMG recordings and motor evoked potentials.

1. The recurrent laryngeal nerve, a branch of the vagus nerve (CN X), is monitored for thyroidectomies. The recurrent laryngeal nerve, one of approximately 11 branches of CN X, innervates the intrinsic muscles of the larynx. This nerve is monitored using an endotracheal tube that is lined with recording electrodes.

2. The facial nerve (CN VII) is monitored for parotidectomies, tympanoplasties, mastoidectomies, microvascular decompressions, etc. Major branches of CN VII include the temporal, zygomatic, buccal, marginal mandibular, and cervical. The temporal branch innervates the frontalis, orbicularis oculi and corrugator supercilii muscles. The zygomatic branch innervates the orbicularis oculi muscle. The buccal branch innervates the orbicularis oris, buccinator and zygomaticus muscles. The marginal mandibular branch innervates the mentalis muscle. The cervical branch innervates the platysma, a sheet of muscle fibers extending from the collarbone to the jaw. Pairs of recording electrodes are typically placed at the muscles of each branch.

3. The glossopharyngeal (CN IX) can be monitored for microvascular decompressions and acoustic neuroma resections. For acoustic neuroma resections, multiple cranial nerves could be at risk of injury depending on the size of the tumor, including the vestibulocochlear (CN VIII), facial (CN VII) nerve, vagus nerve (CN X), hypoglossal nerve (CN XII), and accessory nerve (CN XI).

==References==

IONM in Cranial Surgery

2021-09-06T03:42:02Z

Wdoyon: /* Subcortical mapping */

==Introduction==

==Cortical mapping==
The resection of tumors located within or near the eloquent cortex can result in postoperative sensory, motor and language deficits. Cortical brain mapping techniques have been used for many years to help the neurosurgeon identify and avoid these important brain structures. For tumors that impact the primary motor cortex, the main concern includes loss of motor functions in the contralateral face and upper and lower extremities. Two techniques have been developed for intraoperative cortical and subcortical mapping of the corticospinal tracts - the Penfield and Taniguchi techniques. Sufficient stimulation of the motor cortex along the homunculus elicits compound muscle action potentials in lower motor neurons that innervate contralateral muscle tissue. Electromyography is used to record these electrical responses. The Penfield technique uses low frequency stimulation. The current is delivered between the two points of the probe with trains of monophasic cathodal pulses. In contrast, the Taniguchi technique uses high frequency stimulation. The current is delivered between the probe and a reference electrode placed on the skull close to the motor strip with trains of monophasic anodal pulses. In either case, it is essential to determine the motor response threshold - the lowest intensity required to elicit a motor response in muscle tissue. Determination of the motor response threshold provides information on (1) the level of excitation of the motor cortex and (2) the distance between the stimulation site and the motor cortex.

The following are stimulation parameters and information for the Penfield and Taniguchi techniques:

Penfield technique
(1) Stimulator type: Bipolar
(2) Pulse type: Bi or Monophasic cathodal
(3) Frequency: 50 Hz
(4) Pulse width: 300-1000 microsec
(5) Pulse number: 5
(6) Intensity: 2-20 mA
(7) Duration: 2-5 sec with a 10-20 sec interval

The Penfield technique is particularly effective at eliciting motor responses in tumors that do not infiltrate the motor cortex extensively and have sharp margins (reference).

The Penfield technique may induce intraoperative seizures or result in false negative mapping (reference).

Taniguchi technique
(1) Stimulator type: Monopolar
(2) Pulse type: Monophasic anodal
(3) Frequency: 250-500 Hz
(4) Pulse width: 500 microsec
(5) Pulse number 5
(6) Intensity: 2-20 mA
(7) Duration: 20 microsec

Some difficult cases may require adjustment of these parameters, such as increasing the number of pulses to 7-9 or increasing the pulse width up to 800 microsec).

Alternatively, some have suggested using a bipolar probe with the Taniguchi technique to increase the spatial resolution of stimulation. This method may be useful for cases involving the primary motor cortex or supplementary motor cortex (Rossi et al., 2021).

==Subcortical mapping==
Subcortical mapping is used for tumors that are located within or near the descending subcortical motor pathways. A monopolar stimulating electrode is used to determine the motor threshold, indicating the relative distance from the electrode tip to the motor pathways. Every 1 mA change is approximately 1 mm distance change from the motor pathways. It is recommended that the surgeon not proceed further if a motor threshold of 7 mA is reached. Continued resection may result in postoperative motor deficits.

The following are stimulation parameters and information for subcortical mapping:
(1) Intensity: 2-20 mA (2) Stimulation type: monopolar cathodal stimulation (3) pulse duration: 0.5-1.0 ms (4) Frequency: 250-500 Hz (5) Pulse number: 4-5 (6) Interstimulus interval: 3-4 ms.

Monopolar cathodal stimulation has been shown to be more effective than bipolar stimulation for eliciting subcortical MEPs (Szelenyi et al., 2011).

==Skull base or CP angle tumors==

==Deep brain stimulation==

==Intracranial vascular procedures==

==Neurovascular information==
The brain is very sensitive to changes in blood flow. Cerebral blood flow is maintained and regulated by a homeostatic process called autoregulation. Cerebral blood flow is maintained at approximately 40-70 ml per minute for every 100 g of brain tissue, which occurs over a wide range of arterial blood pressures (~60 - 160 mm Hg) in a healthy brain. To maintain constant cerebral blood flow, several homeostatic mechanisms converge to maintain a balance between vasoconstriction and vasodilation, which includes myogenic, neurogenic, metabolic, and endothelial components (Armstead, Anesthesiol Clin. 2016; 34(3): 465–477).

==Cranial nerve monitoring==
The cranial nerves are monitored with for a variety of surgical procedures. The modalities that are monitored typically include spontaneous and triggered EMG recordings and motor evoked potentials.

1. The recurrent laryngeal nerve, a branch of the vagus nerve (CN X), is monitored for thyroidectomies. The recurrent laryngeal nerve, one of approximately 11 branches of CN X, innervates the intrinsic muscles of the larynx. This nerve is monitored using an endotracheal tube that is lined with recording electrodes.

2. The facial nerve (CN VII) is monitored for parotidectomies, tympanoplasties, mastoidectomies, microvascular decompressions, etc. Major branches of CN VII include the temporal, zygomatic, buccal, marginal mandibular, and cervical. The temporal branch innervates the frontalis, orbicularis oculi and corrugator supercilii muscles. The zygomatic branch innervates the orbicularis oculi muscle. The buccal branch innervates the orbicularis oris, buccinator and zygomaticus muscles. The marginal mandibular branch innervates the mentalis muscle. The cervical branch innervates the platysma, a sheet of muscle fibers extending from the collarbone to the jaw. Pairs of recording electrodes are typically placed at the muscles of each branch.

3. The glossopharyngeal (CN IX) can be monitored for microvascular decompressions and acoustic neuroma resections. For acoustic neuroma resections, multiple cranial nerves could be at risk of injury depending on the size of the tumor, including the vestibulocochlear (CN VIII), facial (CN VII) nerve, vagus nerve (CN X), hypoglossal nerve (CN XII), and accessory nerve (CN XI).

==References==

IONM in Cranial Surgery

2021-09-06T03:40:36Z

Wdoyon: /* Subcortical mapping */

==Introduction==

==Cortical mapping==
The resection of tumors located within or near the eloquent cortex can result in postoperative sensory, motor and language deficits. Cortical brain mapping techniques have been used for many years to help the neurosurgeon identify and avoid these important brain structures. For tumors that impact the primary motor cortex, the main concern includes loss of motor functions in the contralateral face and upper and lower extremities. Two techniques have been developed for intraoperative cortical and subcortical mapping of the corticospinal tracts - the Penfield and Taniguchi techniques. Sufficient stimulation of the motor cortex along the homunculus elicits compound muscle action potentials in lower motor neurons that innervate contralateral muscle tissue. Electromyography is used to record these electrical responses. The Penfield technique uses low frequency stimulation. The current is delivered between the two points of the probe with trains of monophasic cathodal pulses. In contrast, the Taniguchi technique uses high frequency stimulation. The current is delivered between the probe and a reference electrode placed on the skull close to the motor strip with trains of monophasic anodal pulses. In either case, it is essential to determine the motor response threshold - the lowest intensity required to elicit a motor response in muscle tissue. Determination of the motor response threshold provides information on (1) the level of excitation of the motor cortex and (2) the distance between the stimulation site and the motor cortex.

The following are stimulation parameters and information for the Penfield and Taniguchi techniques:

Penfield technique
(1) Stimulator type: Bipolar
(2) Pulse type: Bi or Monophasic cathodal
(3) Frequency: 50 Hz
(4) Pulse width: 300-1000 microsec
(5) Pulse number: 5
(6) Intensity: 2-20 mA
(7) Duration: 2-5 sec with a 10-20 sec interval

The Penfield technique is particularly effective at eliciting motor responses in tumors that do not infiltrate the motor cortex extensively and have sharp margins (reference).

The Penfield technique may induce intraoperative seizures or result in false negative mapping (reference).

Taniguchi technique
(1) Stimulator type: Monopolar
(2) Pulse type: Monophasic anodal
(3) Frequency: 250-500 Hz
(4) Pulse width: 500 microsec
(5) Pulse number 5
(6) Intensity: 2-20 mA
(7) Duration: 20 microsec

Some difficult cases may require adjustment of these parameters, such as increasing the number of pulses to 7-9 or increasing the pulse width up to 800 microsec).

Alternatively, some have suggested using a bipolar probe with the Taniguchi technique to increase the spatial resolution of stimulation. This method may be useful for cases involving the primary motor cortex or supplementary motor cortex (Rossi et al., 2021).

==Subcortical mapping==
Subcortical mapping is used for tumors that are located within or near the descending subcortical motor pathways. A monopolar stimulating electrode is used to determine the motor threshold, indicating the relative distance from the electrode tip to the motor pathways. Every 1 mA change is approximately 1 mm distance change from the motor pathways. It is recommended that the surgeon not proceed further if a motor threshold of 7 mA is reached. Resection past this point may result in postoperative motor deficits.

The following are stimulation parameters and information for subcortical mapping:
(1) Intensity: 2-20 mA (2) Stimulation type: monopolar cathodal stimulation (3) pulse duration: 0.5-1.0 ms (4) Frequency: 250-500 Hz (5) Pulse number: 4-5 (6) Interstimulus interval: 3-4 ms.

Monopolar cathodal stimulation has been shown to be more effective than bipolar stimulation for eliciting subcortical MEPs (Szelenyi et al., 2011).

==Skull base or CP angle tumors==

==Deep brain stimulation==

==Intracranial vascular procedures==

==Neurovascular information==
The brain is very sensitive to changes in blood flow. Cerebral blood flow is maintained and regulated by a homeostatic process called autoregulation. Cerebral blood flow is maintained at approximately 40-70 ml per minute for every 100 g of brain tissue, which occurs over a wide range of arterial blood pressures (~60 - 160 mm Hg) in a healthy brain. To maintain constant cerebral blood flow, several homeostatic mechanisms converge to maintain a balance between vasoconstriction and vasodilation, which includes myogenic, neurogenic, metabolic, and endothelial components (Armstead, Anesthesiol Clin. 2016; 34(3): 465–477).

==Cranial nerve monitoring==
The cranial nerves are monitored with for a variety of surgical procedures. The modalities that are monitored typically include spontaneous and triggered EMG recordings and motor evoked potentials.

1. The recurrent laryngeal nerve, a branch of the vagus nerve (CN X), is monitored for thyroidectomies. The recurrent laryngeal nerve, one of approximately 11 branches of CN X, innervates the intrinsic muscles of the larynx. This nerve is monitored using an endotracheal tube that is lined with recording electrodes.

2. The facial nerve (CN VII) is monitored for parotidectomies, tympanoplasties, mastoidectomies, microvascular decompressions, etc. Major branches of CN VII include the temporal, zygomatic, buccal, marginal mandibular, and cervical. The temporal branch innervates the frontalis, orbicularis oculi and corrugator supercilii muscles. The zygomatic branch innervates the orbicularis oculi muscle. The buccal branch innervates the orbicularis oris, buccinator and zygomaticus muscles. The marginal mandibular branch innervates the mentalis muscle. The cervical branch innervates the platysma, a sheet of muscle fibers extending from the collarbone to the jaw. Pairs of recording electrodes are typically placed at the muscles of each branch.

3. The glossopharyngeal (CN IX) can be monitored for microvascular decompressions and acoustic neuroma resections. For acoustic neuroma resections, multiple cranial nerves could be at risk of injury depending on the size of the tumor, including the vestibulocochlear (CN VIII), facial (CN VII) nerve, vagus nerve (CN X), hypoglossal nerve (CN XII), and accessory nerve (CN XI).

==References==

IONM in Cranial Surgery

2021-09-06T03:39:30Z

Wdoyon: /* Subcortical mapping */

==Introduction==

==Cortical mapping==
The resection of tumors located within or near the eloquent cortex can result in postoperative sensory, motor and language deficits. Cortical brain mapping techniques have been used for many years to help the neurosurgeon identify and avoid these important brain structures. For tumors that impact the primary motor cortex, the main concern includes loss of motor functions in the contralateral face and upper and lower extremities. Two techniques have been developed for intraoperative cortical and subcortical mapping of the corticospinal tracts - the Penfield and Taniguchi techniques. Sufficient stimulation of the motor cortex along the homunculus elicits compound muscle action potentials in lower motor neurons that innervate contralateral muscle tissue. Electromyography is used to record these electrical responses. The Penfield technique uses low frequency stimulation. The current is delivered between the two points of the probe with trains of monophasic cathodal pulses. In contrast, the Taniguchi technique uses high frequency stimulation. The current is delivered between the probe and a reference electrode placed on the skull close to the motor strip with trains of monophasic anodal pulses. In either case, it is essential to determine the motor response threshold - the lowest intensity required to elicit a motor response in muscle tissue. Determination of the motor response threshold provides information on (1) the level of excitation of the motor cortex and (2) the distance between the stimulation site and the motor cortex.

The following are stimulation parameters and information for the Penfield and Taniguchi techniques:

Penfield technique
(1) Stimulator type: Bipolar
(2) Pulse type: Bi or Monophasic cathodal
(3) Frequency: 50 Hz
(4) Pulse width: 300-1000 microsec
(5) Pulse number: 5
(6) Intensity: 2-20 mA
(7) Duration: 2-5 sec with a 10-20 sec interval

The Penfield technique is particularly effective at eliciting motor responses in tumors that do not infiltrate the motor cortex extensively and have sharp margins (reference).

The Penfield technique may induce intraoperative seizures or result in false negative mapping (reference).

Taniguchi technique
(1) Stimulator type: Monopolar
(2) Pulse type: Monophasic anodal
(3) Frequency: 250-500 Hz
(4) Pulse width: 500 microsec
(5) Pulse number 5
(6) Intensity: 2-20 mA
(7) Duration: 20 microsec

Some difficult cases may require adjustment of these parameters, such as increasing the number of pulses to 7-9 or increasing the pulse width up to 800 microsec).

Alternatively, some have suggested using a bipolar probe with the Taniguchi technique to increase the spatial resolution of stimulation. This method may be useful for cases involving the primary motor cortex or supplementary motor cortex (Rossi et al., 2021).

==Subcortical mapping==
Subcortical mapping is used for tumors that are located within or near the descending subcortical motor pathways. A monopolar stimulating electrode is used to determine the motor threshold, indicating the relative distance from the electrode tip to the motor pathways. Every 1 mA change is approximately 1 mm distance change from the motor pathways. It is recommended that the surgeon not proceed further if a motor threshold of 7 mA is reached. Resection past this point may result in postoperative motor deficits.

The following are stimulation parameters and information for subcortical mapping:
Intensity: 2-20 mA, Stimulation type: monopolar cathodal stimulation, pulse duration: 0.5-1.0 ms, Frequency: 250-500 Hz, Pulse number: 4-5, Interstimulus interval: 3-4 ms.

Monopolar cathodal stimulation has been shown to be more effective than bipolar stimulation for eliciting subcortical MEPs (Szelenyi et al., 2011).

==Skull base or CP angle tumors==

==Deep brain stimulation==

==Intracranial vascular procedures==

==Neurovascular information==
The brain is very sensitive to changes in blood flow. Cerebral blood flow is maintained and regulated by a homeostatic process called autoregulation. Cerebral blood flow is maintained at approximately 40-70 ml per minute for every 100 g of brain tissue, which occurs over a wide range of arterial blood pressures (~60 - 160 mm Hg) in a healthy brain. To maintain constant cerebral blood flow, several homeostatic mechanisms converge to maintain a balance between vasoconstriction and vasodilation, which includes myogenic, neurogenic, metabolic, and endothelial components (Armstead, Anesthesiol Clin. 2016; 34(3): 465–477).

==Cranial nerve monitoring==
The cranial nerves are monitored with for a variety of surgical procedures. The modalities that are monitored typically include spontaneous and triggered EMG recordings and motor evoked potentials.

1. The recurrent laryngeal nerve, a branch of the vagus nerve (CN X), is monitored for thyroidectomies. The recurrent laryngeal nerve, one of approximately 11 branches of CN X, innervates the intrinsic muscles of the larynx. This nerve is monitored using an endotracheal tube that is lined with recording electrodes.

2. The facial nerve (CN VII) is monitored for parotidectomies, tympanoplasties, mastoidectomies, microvascular decompressions, etc. Major branches of CN VII include the temporal, zygomatic, buccal, marginal mandibular, and cervical. The temporal branch innervates the frontalis, orbicularis oculi and corrugator supercilii muscles. The zygomatic branch innervates the orbicularis oculi muscle. The buccal branch innervates the orbicularis oris, buccinator and zygomaticus muscles. The marginal mandibular branch innervates the mentalis muscle. The cervical branch innervates the platysma, a sheet of muscle fibers extending from the collarbone to the jaw. Pairs of recording electrodes are typically placed at the muscles of each branch.

3. The glossopharyngeal (CN IX) can be monitored for microvascular decompressions and acoustic neuroma resections. For acoustic neuroma resections, multiple cranial nerves could be at risk of injury depending on the size of the tumor, including the vestibulocochlear (CN VIII), facial (CN VII) nerve, vagus nerve (CN X), hypoglossal nerve (CN XII), and accessory nerve (CN XI).

==References==

IONM in Cranial Surgery

2021-09-06T03:33:50Z

Wdoyon: /* Subcortical mapping */

==Introduction==

==Cortical mapping==
The resection of tumors located within or near the eloquent cortex can result in postoperative sensory, motor and language deficits. Cortical brain mapping techniques have been used for many years to help the neurosurgeon identify and avoid these important brain structures. For tumors that impact the primary motor cortex, the main concern includes loss of motor functions in the contralateral face and upper and lower extremities. Two techniques have been developed for intraoperative cortical and subcortical mapping of the corticospinal tracts - the Penfield and Taniguchi techniques. Sufficient stimulation of the motor cortex along the homunculus elicits compound muscle action potentials in lower motor neurons that innervate contralateral muscle tissue. Electromyography is used to record these electrical responses. The Penfield technique uses low frequency stimulation. The current is delivered between the two points of the probe with trains of monophasic cathodal pulses. In contrast, the Taniguchi technique uses high frequency stimulation. The current is delivered between the probe and a reference electrode placed on the skull close to the motor strip with trains of monophasic anodal pulses. In either case, it is essential to determine the motor response threshold - the lowest intensity required to elicit a motor response in muscle tissue. Determination of the motor response threshold provides information on (1) the level of excitation of the motor cortex and (2) the distance between the stimulation site and the motor cortex.

The following are stimulation parameters and information for the Penfield and Taniguchi techniques:

Penfield technique
(1) Stimulator type: Bipolar
(2) Pulse type: Bi or Monophasic cathodal
(3) Frequency: 50 Hz
(4) Pulse width: 300-1000 microsec
(5) Pulse number: 5
(6) Intensity: 2-20 mA
(7) Duration: 2-5 sec with a 10-20 sec interval

The Penfield technique is particularly effective at eliciting motor responses in tumors that do not infiltrate the motor cortex extensively and have sharp margins (reference).

The Penfield technique may induce intraoperative seizures or result in false negative mapping (reference).

Taniguchi technique
(1) Stimulator type: Monopolar
(2) Pulse type: Monophasic anodal
(3) Frequency: 250-500 Hz
(4) Pulse width: 500 microsec
(5) Pulse number 5
(6) Intensity: 2-20 mA
(7) Duration: 20 microsec

Some difficult cases may require adjustment of these parameters, such as increasing the number of pulses to 7-9 or increasing the pulse width up to 800 microsec).

Alternatively, some have suggested using a bipolar probe with the Taniguchi technique to increase the spatial resolution of stimulation. This method may be useful for cases involving the primary motor cortex or supplementary motor cortex (Rossi et al., 2021).

==Subcortical mapping==
Subcortical mapping is used for tumors that are located within or near the descending subcortical motor pathways. A monopolar stimulating electrode is used to determine the motor threshold, indicating the relative distance from the electrode tip to the motor pathways. Every 1 mA change is approximately 1 mm distance change from the motor pathways. It is recommended that the surgeon not proceed further if a motor threshold of 7 mA is reached. Resection past this point may result in postoperative motor deficits.

Stimulation parameters:
Monopolar cathodal stimulation is more effective than

==Skull base or CP angle tumors==

==Deep brain stimulation==

==Intracranial vascular procedures==

==Neurovascular information==
The brain is very sensitive to changes in blood flow. Cerebral blood flow is maintained and regulated by a homeostatic process called autoregulation. Cerebral blood flow is maintained at approximately 40-70 ml per minute for every 100 g of brain tissue, which occurs over a wide range of arterial blood pressures (~60 - 160 mm Hg) in a healthy brain. To maintain constant cerebral blood flow, several homeostatic mechanisms converge to maintain a balance between vasoconstriction and vasodilation, which includes myogenic, neurogenic, metabolic, and endothelial components (Armstead, Anesthesiol Clin. 2016; 34(3): 465–477).

==Cranial nerve monitoring==
The cranial nerves are monitored with for a variety of surgical procedures. The modalities that are monitored typically include spontaneous and triggered EMG recordings and motor evoked potentials.

1. The recurrent laryngeal nerve, a branch of the vagus nerve (CN X), is monitored for thyroidectomies. The recurrent laryngeal nerve, one of approximately 11 branches of CN X, innervates the intrinsic muscles of the larynx. This nerve is monitored using an endotracheal tube that is lined with recording electrodes.

2. The facial nerve (CN VII) is monitored for parotidectomies, tympanoplasties, mastoidectomies, microvascular decompressions, etc. Major branches of CN VII include the temporal, zygomatic, buccal, marginal mandibular, and cervical. The temporal branch innervates the frontalis, orbicularis oculi and corrugator supercilii muscles. The zygomatic branch innervates the orbicularis oculi muscle. The buccal branch innervates the orbicularis oris, buccinator and zygomaticus muscles. The marginal mandibular branch innervates the mentalis muscle. The cervical branch innervates the platysma, a sheet of muscle fibers extending from the collarbone to the jaw. Pairs of recording electrodes are typically placed at the muscles of each branch.

3. The glossopharyngeal (CN IX) can be monitored for microvascular decompressions and acoustic neuroma resections. For acoustic neuroma resections, multiple cranial nerves could be at risk of injury depending on the size of the tumor, including the vestibulocochlear (CN VIII), facial (CN VII) nerve, vagus nerve (CN X), hypoglossal nerve (CN XII), and accessory nerve (CN XI).

==References==