Noninvasive Motor Evoked Potential Monitoring During Neurosurgical Operations on the Spinal Cord

We present the results of monitoring descending pathways with motor evoked potentials (MEP) in 50 patients during neurosurgical operations on the spinal cord. The electromyographic responses of the anterior tibial muscles were recorded. In addition, in 24 patients responses of the thenar muscles after transcranial electrical stimulation of the motor cortex were recorded. Usually, the averages of 5 to 15 signals were evaluated. Although potentials were obtained preoperatively in all 50 patients, during neuroleptanesthesia intraoperative recording from the anterior tibial muscles was possible in 43 patients (86%) and from the thenar muscles in 21 patients (87.5%). Amplitudes were superior to latencies as evaluation criteria for intraoperative changes in potentials. On the basis of acceptable changes in amplitudes of up to 50% at the end of the operation, 16 recordings from the thenar muscles (76.2%) and 35 from the anterior tibial muscles (81.4%) correlated correctly with the postoperative neurological status; there were false positive results in 5 (23.8%) and 8 (18.6%) patients, respectively. We did not observe false negative findings. Postoperative neurological complications coincided in every case with permanent reduction in amplitudes of more than 50% of the base lines or with intraoperative loss of potentials as observed in 3 and 1 patient, respectively. MEP monitoring during neurosurgical operations on the spinal cord is a sensitive method for early detection of impending neurological complications. Major problems are the influence of anesthesia and the definition of acceptable limits for changes in amplitudes.