Fig. 1.2. Normal somatosensory evoked potentials detected after stimulation of the right median nerve. The central conduction time is calculated as the difference between the N13 and N20 peaks.

20% developed significant post-operative deficit. This number may have been larger had not a number of the patients with SSEP changes undergone shunting as a result of those changes

[6]. Severe, irreversible SSEP changes appear to be a rare but ominous sign. This occurred in less than 1% of cases in a series of 994 CEAs; however, all awoke with neurological sequelae

[7]. In contrast to EEG, no study has been performed with the intent to delineate the false-positive rate of SSEP monitoring - as it relates to stroke - if a shunt is not placed.

When cerebral ischemia occurs with the application of a clamp upon the ICA, characteristic changes occur in the N20, P25 and N30 components of the SSEP. A defined sequence of alterations, or stages, that occur with progressive ischemia has been described. Amplitude reduction combined with latency progression of N30 represents mild, or stage 1, ischemic change. Stage 2, or moderate, changes include the disappearance of N30 as well as amplitude reductions of N20 and P25 up to 50%. Severe, or stage 3, changes are defined by the loss of P25 with the concomitant progression of increasing latency and amplitude reduction of N20. Guerit describes a similar system, which recommends shunt placement whenever moderate-to-severe SSEP alterations occur within 7 minutes after cross-clamping, and he suggests that some cases of mild-to-moderate SSEP change may be due to drops in blood pressure rather than to the ischemic effects of cross-clamping [8].

Experience of others has supported the sensitivity of N20 and P25 to ischemic insults, and amplitude reductions have proven to be more predictive than latency increases. Most surgeons who rely on SSEPs place a shunt when the N20-P25 complex decreases by 50% or rapidly disappears with clamping of the ICA. These changes typically recover when flow is re-established through the shunt. Overall, the correlation of SSEP changes to clinical outcome is quite good. Neurological dysfunction is remarkably rare in the setting of SSEP with little or no change. In the series by Haupt and Horsch, only one of the 994 patients suffered a stroke in the face of normal SSEPs. As with EEG, clamp-induced changes in SSEPs occur in about 20-30% of monitored cases. It is important to note that temporary ischemia, whether due to intended or accidental vessel occlusion, does not imme

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