Somatosensory evoked potential monitoring in carotid surgery. I. Relationships between qualitative SEP alterations and intraoperative events

This paper presents the results of intraoperative median nerve SEP monitoring in 205 successive patients undergoing isolated carotid endarterectomy (CE) (N=172) or CE followed by coronary bypass (CBP) and/or vascular replacement (VR) (N=33). The left and right median nerves were alternately stimulated and recordings performed on 4 channels: cervical, ipsi- and contralateral parietal, and frontal. SEPs were qualitatively rated in terms of mild, moderate, or severe ipsilateral, contralateral, or bilateral abnormalities. The SEP abnormalities were subdivided into 5 categories as a function of their relationships with intraoperative events: no alterations (67.3%), early or late SEP alterations after carotid cross-clamping (15.6%), SEP alterations after a drop in blood pressure (occurring outside of or within the cross-clamping period) (15.1%), SEP alterations of a most likely embolic origin (2.4%), SEP changes after head positioning (1%), and SEP changes after a modification of the anesthetic regimen (1.5%). Only moderate to severe SEP alterations occurring soon after carotid cross-clamping justified shunt installation in 16% of the cases. SEP alterations after a drop in blood pressure were reversed merely by restoring blood pressure. The neurological outcome was uneventful in 94.2% of cases. Of the 12 patients who developed neurological sequellae, only one case presented transient sequellae after isolated CE without SEP changes while most cases either had undergone combined CE and CBP and/or VR (6 cases) or had presented SEP alterations of embolic origin (3 cases). We conclude that our system of qualitative rating of SEPs proved very sensitive to intraoperative hemodynamic disturbances or macroembolisms.