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to 0.2% frequency of such occurrences. Further analysis of these cases showed the cortical cerebral ischemia to consistently lie in the border area of regions supplied by the middle and posterior or anterior cerebral arteries. These areas are not assessed by the median nerve SSEPs.
In a comparative study of TCD ultrasonographic and SSEP assessment during carotid surgery, SSEPs were clearly superior. Two examples to demonstrate this are shown in Figures 11 and 12. The first case demonstrates intraoperative uneventful SSEPs with abnormal TCD ultrasonography (absent flow over the middle cerebral artery) during clamping. The second example demonstrates the superiority of SSEPs in recording intraoperative complications such as an embolus. Despite shunt insertion, there was no improvement in the SSEP amplitudes.
Current discussions among specialists emaphasize the fact that carotid surgery has to have low morbidity and mortality to beneficially alter the natural history. Perioperative monitoring has an important role in decreasing surgical risk. Monitoring assists in the assessment of collateral blood flow, the requirement for a shunt, evaluation of the effectiveness of the shunt, and ensurance of operative quality. The development of intra- or postoperative cerebral ischemia is due mainly to embolic events. Embolic events or inadequate cerebral perfusion during the clamping period can be detected with high sensitivity and specificity by the SSEP. In some rare situations (in 0.1%-0.2% of cases in our experience), new neurologic deficits may be observed, despite unaltered SSEP amplitudes during carotid reconstruction.
In agreement with most specialists, we see the necessity for an intraluminal shunt in the following situations: (1) loss of SSEP amplitude (N20/P25), (2) a more than 50% reduction in SSEP amplitude, and (3) late alterations of SSFP to a significant degree.
EMG
Intraoperative stimulation of the facial nerve and direct observation of facial musculature movement have been routine during surgery at the CPA, particularly of acoustic tumors. The same goal can be obtained, however, through more advanced and refined methods, by recording the compound action potential from several groups of facial muscles. This recording can be made either by placing disc electrodes over appropriate muscles or by inserting a routine EMG needle directly into facial muscles. Recordings from these electrodes can be played on an ordinary EMG machine that includes an audio channel. Bipolar stimuli help locate the facial nerve. Auditory feedback to the surgeon provides assurance that the nerve is intact or has been located . Techniques are available to remove auditory signals resulting solely from stimulus artifacts, helping the surgeon definitely identify the facial nerve. This technique helps identify the nerve and may alert the surgeon to nerve manipulation or heating, which may result in audible injury potentials, and can be heard in the absence of the stimulation. Nerve function can also be (Continued on page 113)
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