Spinal cord monitoring during intraspinal extramedullary tumor operations (peroneal nerve evoked responses).

Longterm scalp recording of early SEP components triggered by peroneal or tibial nerve stimulation detects functional disturbances of spinal cord transmission due to mechanical trauma. We confirm previous observations that preoperative SEP patterns reflect neurological deficits and clearly show functional disturbances even on the side where they are not manifest. Peroneal nerve SEP have a well-known P40-peak corresponding to activities of neurons at the postcentral cortical layers. The P40-peak was identified in only 55% of our recordings. We therefore, tried to use the P50-peak that could be identified in 100% of the recordings under the difficult recording circumstances in the operating room. Other authors demonstrate that the P40 component varies spontaneously about 7 ms during operations; we found variation of P50-latencies of 9 ms. We found the P50 component to be a valid and reproducable SEP event for intraoperative monitoring. Decline of SEP amplitudes during the operation occurred especially during dissection of the tumor. We once found intermittent loss of the P50 component with complete recovery after several minutes. This study shows that peroneal nerve SEP monitoring during intraspinal tumor operations is a feasible and valid measure. It provides the surgeon with a new alarm parameter for spinal cord lesions.