Somatosensory evoked potentials in cerebral aneurysm surgery.

Monitoring of median nerve somatosensory evoked potentials (SSEP) during surgery for a basilar artery aneurysm under moderate hypothermia revealed an unexpected loss of the first cortical peak. This was due to compression of the middle cerebral artery under the retractor during the surgical approach to the aneurysm and would have continued unnoticed for some time in the absence of monitoring, possibly resulting in infarction in the territory of the middle cerebral artery. When the surgeon was alerted the artery was released and the evoked potential returned within about 24 min. There was no new focal neurological deficit postoperatively.