IONM in Vascular Surgery
Introduction and Anatomy
Blood supply to the spinal cord. The spinal cord is supplied by blood flow from two major sources: vertical arteries and segmental spinal arteries.
1. The vertical blood supply arises from the vertebral arteries. The subclavian artery gives rise to the vertebral arteries, which run bilaterally through the transverse foraminal processes of C1-C6 towards the brain. The right and left vertebral artery then join together (anastomose) to form the basilar artery, which continues into the skull via the magnus foramen (see below). The vertebral artery branches into two posterior spinal arteries and one anterior spinal artery, and these arteries run the length of the spinal cord.
2. The segmental spinal arteries supply each segment of the spinal cord and arise from different vessels along the spine. Along the cervical spine, the vertebral artery and the deep cervical artery give off segmental arteries. Along the thoracic spine, the posterior intercostal arteries give off segmental arteries. Along the abdomen, the lumbar and sacral arteries give off the segmental arteries. Each segmental artery also gives rise to other smaller arteries of the spinal cord, including the anterior and posterior radicular arteries, which supply the nerve roots. At some spinal levels, there are additional arteries that branch off the segmental spinal artery called the segmental medullary artery. The segmental medullary arteries feed into the anterior and posterior spinal arteries. There about 6-10 segmental medullary arteries along the spine in an adult, the largest of which is the artery of Adamkiewicz, typically found between T9-T12.
A carotid endarterectomy is a procedure to remove plaque buildup from the carotid artery, which supplies the brain with blood and oxygen. The carotid arteries lie on both sides of the neck and each branches into the internal and external carotid arteries. The region where the internal and external carotid arteries branch (or bifurcate) is susceptible to plaque buildup. The procedure involves making a small incision in the carotid artery and removing the plaque buildup, which is categorized by the degree of stenosis (mild, moderate, severe, total occlusion) and diameter of arterial reduction (<50%, 50-69%, 70-99%, 100%). The procedure requires a bypass shunt or artery clamp in order to work on the region containing the plaque buildup. A bypass shunt is commonly used to maintain cerebral blood flow but increases the risk of embolic stroke if plaque, air or debris is released in the vessels during shunt placement. The risk of an embolic stroke is low, approximately 4% (Woodworth et al. Neurosurgery, 2007;61:1170-6).
EEG and SSEPs are the most common modalities used to monitor ischemic events in the brain during a carotid endarterectomy. EEG measures the spontaneous electrical activity of the brain. Changes in the alpha and beta band frequencies of the EEG are an indicator of changes in cerebral blood flow and ischemia (Visser et al. J Clin Neurophysiol, 2001;18:169-77).
The N20 generator for the median nerve SSEP lies within the cortical region perfused by the middle cerebral artery. Therefore, monitoring the median nerve SSEP provides an indication of ischemia within the region perfused by the middle cerebral artery. By contrast, the P37 generator for the posterior tibial nerve SSEP lies within the cortical region perfused by the anterior cerebral artery, making the tibial nerve SSEP a good indicator of ischemia within the region perfused by the anterior cerebral artery.
Disorders, such as trigeminal or glossopharyngeal neuralgia, can cause blood vessels in the brain to compress the cranial nerves, resulting in chronic pain and muscle spasms. A microvascular decompression is a procedure to relieve vascular pressure on these nerves. The vascular compression often occurs at the level of the nerve roots. The majority of trigeminal neuralgia cases arise from compression of the trigeminal nerve root, and Multiple Sclerosis is associated with a higher incidence of trigeminal neuralgia (Putzki et al. Eur J Neurol, 2009;16:262–7). Hemifacial spasms are caused by vascular compression of the facial nerve roots in the brainstem.
Neuromonitoring of microvascular decompression involves the use of brainstem auditory evoked responses (BAERs), free-running EMG, and facial motor-evoked potentials. BAERs are used to monitor the auditory nerve (CN VIII), which is at risk for injury due to the surgical manipulations.