The Cutoff Amplitude of Transcranial Motor-Evoked Potentials for Predicting Postoperative Motor Deficits in Thoracic Spine Surgery

Study Design. Prospective clinical study of intraoperative transcranial motor-evoked potentials (TcMEP) amplitudes and postoperative motor deficits. Objective. To determine the cutoff amplitude during intraoperative TcMEP monitoring for predicting postoperative motor deficits after thoracic spine surgery. Summary of Background Data. Several alarm points when monitoring with TcMEP have been advocated, but there have been no reports on an actual cutoff amplitude of TcMEP for predicting the occurrence of postoperative motor deficits. Methods. Among 80 consecutive surgical cases, 28 had a deterioration in TcMEP amplitude in at least 1 monitored muscle during surgery. We examined intraoperative electrophysiological changes and postoperative motor deficits in 270 monitorable muscles in those 28 patients. Through receiver operating characteristic curve analysis, we identified the cutoff amplitudes at the intraoperative point of deterioration and at the end of surgery for predicting postoperative motor deficits in both relative and absolute values. Results. The relative and the absolute cutoff amplitudes of TcMEP at the intraoperative point of deterioration and at the end of thoracic spine surgery were 12% of control amplitude and 1.9 mu V and 25% of control amplitude and 3.6 mu V, respectively. Sensitivity/specifi city for those cutoff points are 88%/64%, 69%/83%, 90%/64%, and 70%/82%, respectively. Conclusion. We determined the cutoff amplitude for predicting postoperative motor deficits in thoracic spine surgery. The results may help establish the alarm criteria for thoracic spine surgery.