Origin of somatosensory evoked responses recorded from the cervical skin surface

Abstract

Somatosensory evoked response from the cerevical skin surface over the spine (the cervical SER) was recorded in 7 patients undergoing spinal operations, and compared with the cord dorsum potential (CDP) simultaneously recorded from the posterior epidural space at the same segment. The cervical SER evoked by segmental nerve stimulation consisted of an initially positive spike (P1), the peak latency being the same as that of the P1 of the CDP, followed by a smaller negative wave with 2 peaks. The latency of the 2nd peak of the negative wave (N1) coincided with that of the N1 of the CDP. Subsequent to this negative wave, a slow positive wave (P2) with peak latency similar to that of the P2 of the CDP, could be noticed in some subjects. The cervical SER could not be evoked even by strong stimulation of the cauda equina. The cervical SER might reflect a segmental phenomenon rather than the conducted potential along the cord, and originate from the spinal root and cord in the same way as the segmentally evoked CDP. [Surface recordings may be used for the study of spinal cord diseases.].