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.
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.
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.
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
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
Anesthesia and Other Factors
- Anesthesia Methods. EMG activity is not affected by gas anesthesia.
- Muscle Relaxants