Intraoperative transcranial electrical motor evoked potential monitoring during spinal surgery under intravenous ketamine or etomidate anaesthesia

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Abstract
Motor evoked potentials (MEPs), monitoring the motor function directly, are superior to somatosensory evoked potentials (SSEPs) in monitoring the motor system during spinal surgery. Reliable MEPs are difficult to elicit under normal anaesthesia. Using intravenous anaesthesia with either ketamine or etomidate infusion, we performed intraoperative MEP monitoring in 12 spinal operations for 11 cases from February 1992 to May 1992. For anaesthesia, ketamine was used in 5, etomidate in 7, fentanyl was supplemented in all, muscle relaxation at 30% to 50% of pre-anaesthetic muscle power was maintained with atracurium or vencuronium infusion. Transcranial bipolar electrical stimulation was used to induce MEPs. Concomitant SSEP monitoring was performed in 3. No significant anaesthesia related side effects were noted except one episode of unpleasant dream occurred in the ketamine anaesthesia group. Successful monitoring was achieved in 10 sessions. In 5 of which warning to the surgeons was made due to sudden MEP deterioration, which recovered followed by definite management in four and persisted in one. In the other 5 sessions, no warning was made due to stationary or gradual change in MEPs. Bilateral two-channel recordings were used in 3 sessions. In 2 of which unilateral transient change was noted. Loss of SSEPs was noted in one despite unchanged MEPs, in whom only new sensory deficits occurred postoperatively. Compared to the baseline MEPs in terms of latency and amplitude, the final MEPs improved in 5 sessions, did not change significantly in 4 sessions, deteriorated in one session, and were correlated well with the immediate postoperative motor status. In our small series, the intraoperative MEP monitoring showed neither false negative nor false positive result. It is concluded that the intraoperative MEP monitoring is feasible under intravenous ketamine or etomidate anaesthesia and valuable in spinal surgery.