Somatosensory Evoked Potential and Brainstem Auditory Evoked Potential Monitoring in Cerebral Aneurysm Surgery under Hypotension

Summary

We report our experiences with somatosensory evoked potential (SSEP) monitoring during cerebral aneurysm operations under hypotension. In the first part of this study the surgical technique was intentionally not changed as a result of SSEP monitoring; in this way, we established what changes in intraoperative SSEP were consistent with preservation of function during surgical manipulations. We recorded both short-latency SSEP to computer central conduction time (CCT) and long-latency cortical SSEP, with a timebase of 200 ms, on admission of the patients to the hospital and intraoperatively. We found that 54% (7 of 13) patients with preoperative signs of arterial spasm, i.e., increased rate of blood flow in the middle cerebral artery, had ipsilateral prolongation of CCT and depression of the cortical late waves. During the operation a CCT exceeding 9 ms for at least 10 minutes and an irreversible loss of the cortical late waves were the only parameters predictive of postoperative deterioration in function. In addition, the patients whose neurological status deteriorated postoperatively had preoperative CCT equal to or longer than 7.5 ms and depression of the cortical late waves over the affected hemisphere. These patients are not good candidates for operations under hypotension. On the basis of these results, patients in the second part of this study who were not candidates for operation under hypotension were operated upon with the technique of temporary clipping under normotension, with evoked potential monitoring. No patients operated upon according to this protocol showed postoperative deterioration in function. Brainstem auditory evoked potentials (BAEP) were recorded, in addition to SSEP, when aneurysms of the vertebral artery were operated upon. Prolongation of the wave I-V interval was caused by brainstem retraction, but even more severe BAEP changes developed when hypotension was induced. These changes soon reversed when blood pressure was raised after the aneurysm had been clipped.

In conclusion, we found that for monitoring during cerebral aneurysm operations the late waves of cortical SSEP provide useful information to supplement that provided by CCT measurements. Moreover, SSEP were valuable in helping to identify those patients who were not suitable for surgery under hypotension because of the risk of brain ischemia, and in planning a safer technique for surgical management of the lesions in these patients.