ABSTRACT
Per-operative use of S.E.P.s for vascular disease is based on: 1. The relationship between electrical cortical responses and cerebral blood flow. 2. The existence of a reversible threshold of cerebral ischemia. Intra-operative monitoring S.E.P.s were used during 30 procedures for aneurysms of the middle cerebral artery (M.C.A.). In 18 cases, a temporary occlusion of M.C.A. was necessary. Occlusion times ranged from 1 to 30 minutes. The central conduction time delay (C.C.T.), i.e. the delay N14-N20 and the cortical peak (N20) amplitude, elicited by median nerve stimulation was bilaterally monitored. A prolongation of the C.C.T. by 1 millisecond (compared with the baseline induction value) and a progressive decrease or a disappearance of the N20 peak were considered as "significant" changes. In response to these changes, immediate corrective actions (interruption of temporary M.C.A. occlusion (T.O.), repositioning of brain retractors, reapplication of aneurysm clips ...) were implemented. The reversibility of the S.E.P.s alterations during surgery was correlated with the post-operative outcome. Significant changes were found in 20 cases (including 13 T.O.). They were totally reversible in 11 cases: 5 of them developed a new but transient immediate post-operative deficit, none had a definitive deficit, and 6 patients had no new deficit. In 9 cases, the per-operative S.E.P.s alterations were not reversible: 3 cases (including 1 T.O.) had a transient deficit, 4 a permanent deficit, and 1 died (aneurysm rupture during craniotomy). An irreversible N20 peak disappearance predicted a permanent post-operative deficit in 4 of 4 patients (100%), whereas an isolated irreversible C.C.T. increase was only followed by a transitory deficit. Only 1 of 9 patients with no change in S.E.P.s (during a 15 min. T.O.), had a transient hemiparesis: this "false-negative" case will be discussed. This study confirms that S.E.P.s monitoring provides useful warning during aneurysm surgery. Median nerve S.E.P.s reflect the functional integrity of cortical M.C.A. territory; it is the pathway a risk during M.C.A. aneurysm surgery. S.E.P.s changes are not real-time information (an average of 500 responses need about 2 min), but these delays allow the surgeon to reverse the situation by immediate intra-operative adjustment, especially during temporary M.C.A. occlusion.
