A Carotid Endarterectomy (CEA) is a surgical procedure used to remove the abnormal buildup of material inside the Common or Internal Carotid artery. This material, called plaque, causes a narrowing inside the artery, which can increase the risk of stroke. Removal of this plaque allows bloodflow to the brain to return to its normal levels. Intraoperative monitoring (IOM) is used during these procedures to reduce the risk of perioperative stroke. By closely watching the brain’s activity in order to quickly detect any decrease in cerebral blood flow, known as ischemia, long-term impairment can be avoided.
During a CEA, blood flow through the affected vessel is temporarily stopped. During this time other vessels can continue supplying the brain with blood, but this may not be sufficient. If blood flow drops too far there is risk for ischemic stroke and the irreversible damage related to it. The IOM modalities used are sensitive enough to detect changes early on, which allows the surgeon to take action to restore cerebral blood flow to safe levels
The gold standard for neurophysiologic monitoring during CEA is EEG, but a multimodality approach affords an even better view of a patient’s neurologic function. SSEPs in particular are a valuable complement to EEG, as they are less prone to suppression from anesthetic agents. They only cover a relatively limited region of the brain compared to EEG, but it is the region directly influenced by flow from the carotid artery. Occasionally TcMEPs are monitored in addition to SSEPs and EEG, depending on the patient’s status and surgeon preference.
Should ischemia occur, there will be corresponding changes in the patient’s EEG patterns and an observable decrease in evoked potential (SSEP and/or TcMEP) amplitudes. These changes often occur within 20 seconds of the vessel being closed, with 80% of changes observed within 2 minutes. These changes are immediately relayed to the surgeon, who will then choose how to proceed. The most common response to a change in IOM responses is to insert a shunt that will divert blood flow around the portion of the vessel being operated on so that it may complete its path to the brain. After the shunt is placed properly, IOM responses should return to baseline within a few minutes and the surgeon can proceed with confidence.
Selective shunting in this way is favorable to routine shunting without the use of IOM for several reasons. Shunt placement itself does come with risks, including thrombosis, embolism, and unintended occlusion due to improper placement or kinking of the shunt material. Any chance to avoid unnecessary shunting avoids these risks altogether. Additionally, constant monitoring throughout the entire procedure can catch changes in cerebral blood flow that may occur later in the case whether a shunt was placed or not. Catching these unexpected changes gives the anesthesia team a chance to employ necessary neuroprotective agents via their stroke protocol, thus avoiding post-operative issues.
Overall, the use of IOM during Carotid Endarterectomy is an important tool a surgeon can use to protect their patients and improve the outcome of their procedures.