Subcortical strokes from intracranial aneurysm surgery: implications for intraoperative neuromonitoring

Forty-five patients underwent surgery for anterior circulation aneurysms using intraoperative neurophysiologic monitoring at the Johns Hopkins Hospital during 1996. There were seven intraoperative strokes. Two were cortical strokes associated with irreversible somatosensory evoked potential (SEP) changes during temporary arterial occlusion. The remaining five were subcortical strokes, one of which was associated with transient SEP changes during temporary arterial occlusion, but the other four occurred despite normal SEPs throughout surgery. Somatosensory evoked potential monitoring is not sensitive for the detection of subcortical ischemia and infarction in the distribution of the deep perforating arterial branches during intracranial aneurysm surgery. Although attenuation of loss of cortical SEP responses may indicate cerebral ischemia from inadequate collateral circulation during temporary arterial occlusion, normal SEPs can not exclude subcortical ischemia sufficient to cause significant postoperative deficits, and may therefore provide a false sense of security during these surgeries.