Value of Stable and Changing Somatosensory Evoked Potentials (SSEP) During Aneurysm Surgery
Since it was established that monitoring somatosensory evoked potentials (SSEP) is a good method for evaluating the intactness of sensory afferent conduction, this method has been more and more frequently used during aneurysm surgery. This chapter describes the technical aspects of this method, including technical and anatomical limitations to its use, and the clinical value of this technique, with special consideration of those cases in which the preservation of SSEP or the disappearance of SSEP provided information considered helpful by the surgeon. The data presented were obtained during monitoring of 157 operations to manage 177 aneurysms out of a total of 190 cerebral vascular operations.
One of the remarkable findings in this study was that SSEP may be lost not only following impaired blood flow due to vascular occlusion, but also following retraction of the brain. The incidence of loss of potentials with occlusion of blood vessels is variable: in this study, 58 patients had vessel occlusions and in 19 of these evoked potentials changed. Other events associated with an evoked potential change were retraction of the cerebellum and splitting of the Sylvian fissure.
In an interesting subgroup of 17 cases, in which the vessel occlusion occurred accidentally, the loss of SSEP was the only sign of compromised blood flow in 2 cases. In the majority of these cases the surgeon detected the accidental vessel occlusion, either quickly or after some further dissection without the help of SSEP monitoring.
The lack of SSEP changes in cases where vessel occlusion was necessary was found to provide helpful information to the surgeon. Aneurysm dissection and clipping proceeded smoothly in several cases because the lack of SSEP change even when a major cerebral vessel was clipped signaled good collateral blood flow.
If the surgeon is sure that the supply area of the vessel is being monitored by the chosen SSEP monitoring modality, an absence of SSEP abnormalities during clipping of supratentorial aneurysms may be considered a sign that the blood supply to the brain regions being monitored is satisfactory. Monitoring SSEP during aneurysm surgery provides the surgeon with valuable information, especially during difficult procedures such as surgery on giant aneurysms; management of large, multilobed middle cerebral artery aneurysms, which require temporary clipping; and permanent vessel occlusion to treat unclippable aneurysms.
KeywordsIschemia Fentanyl Midazolam Dura Barbiturate
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