IONM in Vascular Surgery

Carotid endarterectomy

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.

Microvascular decompression

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.