diately give rise to alterations in the relevant SSEP [7].

SSEP monitoring has also been shown to be useful during intracranial procedures that directly or indirectly contribute to ischemia, including aneurysm clipping or manipulation and retraction of various brain structures. Isolated vascular territories can be assessed through judicious selection of the stimulus location to be used for SSEP. Regions of cortex subserved by the ICA and MCA can be monitored by stimulation through the median nerve; in addition, the median nerve can be used to assess flow to the thalamic segment of the somatosensory pathway, an area provided for by the PCA. Posterior tibial nerve SSEP has been used for monitoring the territory of the ACA, although concurrent monitoring of the median nerve may be necessary for adequate detection of ischemia involving the dependent regions of the recurrent artery of Huebner. In assessing the posterior circulation, isolated monitoring of either SSEP or BAEP during vertebrobasilar aneurysm clipping may be of little use, as ischemia due to basilar perforator occlusion may not affect the auditory or somatosensory pathways traversing the brainstem; however, if used in combination, SSEP and BAEP monitoring may enhance the ability to detect brainstem ischemia.

Currently, prospective data comparing EEG and SSEP monitoring for reversible ischemia and patient outcomes do not exist. On a theoretical level, EEG monitors a larger area of the cerebral cortex and does not require time averaging of signals. However, Fava et al. have suggested that SSEP monitoring, in addition to EEG, enhances the overall predictive value of monitoring during CEA. Patients (n = 151) with EEG changes indicating significant ischemia were shunted only if severe SSEP changes occurred within the first few minutes after vessel occlusion. Fewer shunts were placed using this protocol than if EEG were used independently. No patient with significant EEG changes in conjunction with insignificant SSEP changes had a post-operative deficit. Patients who were shunted did well, with the exception of subjects whose ischemia was felt to be embolic and who awoke with new deficit [9]. Guerit suggests that SSEP may be superior to EEG in the determination of ECS in cases using deep hypothermic circulatory arrest, as the SSEP is much less sensitive to environmental electrical noise and is therefore better suited to identifying true ECS [8].

SSEP has also been used for functional localization in the cerebral cortex, most particularly in defining the central sulcus, via the use of phase reversal. SSEPs recorded simultaneously from the precentral and postcentral gyri exhibit typical responses of reversed polarity (Fig. 1.3). The evoked potential from the precentral gyrus is a biphasic positive-negative waveform, compared with the mirror image of the postcentral gyrus, which is negative-positive. The typically recorded response in the postcentral gyrus following median nerve stimulation is a negative deflection with a latency period of 20 ms (N20) followed by a positive deflection at 30 ms (P30). Precentral recordings reveal somewhat lower amplitude deflections that mirror the sensory strip recordings (characteristically, a P22 component followed by an N33 deflection). The precise etiology of these potentials and phase reversal is not fully understood. Brodman's area 3b, located on the primary sensory cortex along

Fig. 1.3. Phase reversal across the central sulcus in response to contralateral median nerve stimulation. The reversal in polarity is evident when comparing leads 2 and 3, positions that bridge the central sulcus (darkened for emphasis).

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