Somatosensory evoked potential(SEP) have characteristic which are of value in intraoperative monitoring in situations where the brain is at risk from ischemia. The median nerve SEP provides the information form median nerve innervated dermatome to the part of the sensory cerebral cortex representing the distribution of the median nerve (the hand) which is supplied by the middle cerebral artery.

Intraoperative SEP monitoring of median nerve was carried out in two aneurysm cases of middle cerebral. artery where temporary occlusion of the internal carotid artery was employed selectively. Base line SEPs wee obtained shortly after the induction of general anesthesia and were recoreded continuously throughout operation. Their absolute latencies and amplitudes were subsequently determined. Delay or disappearance of the N20 potential following occlusion is regarded as a 'danger' signal. We consider that these phenomena possibly result in infarction in the territory of the clipped cerebral artery. The following two cases were undertaken to elucidate more carefully the utility of SEP monitoring in order to reduce such morbidity. In the first case, the surgeon was alerted by decreased amplitude of the evoked potentials follwed by the artery was released and the evoked potential returned. There was no new focal neuroligical deficit postoperatively. While in the second case, transient new deficits were seen due to prolonged clipping

Delay or disappearance of the SEP should therefore be capable of giving an early warning of regional ischemia and the there should be a "window of therapeutic opportunity" in which effective remedial measured can be taken in order to avoid permanent neurological damage.