Successful Monitoring of Transcranial Electrical Motor Evoked Potentials With Isoflurane and Nitrous Oxide in Scoliosis Surgeries

Study Design. This is a case series studying the efficacy of concomitant inhalational anesthesia and transcranial electrical motor-evoked potential (tceMEP) monitoring in spinal deformity surgery. Objective. To determine the affects of inhalational anesthesia on the efficacy of tceMEP monitoring. Summary of Background Data. Inhalational agents inhibit transmission of evoked potentials from the motor cortex. Consequently, many authors have recommended using total intravenous anesthesia during motor-evoked potential monitoring. Methods. A total of 247 consecutive patients, aged 1 to 83 years (156 patients <22 years), undergoing spinal fusion for scoliosis, excluding those with history of seizure or myelopathy, were monitored with tceMEP intraoperatively. Isoflurane with or without nitrous oxide (per anesthesiologist preference) was administered with vecuronium and i.v. agents including propofol and/or narcotic. Vecuronium was titrated for a goal of 2/4 twitches, and isoflurane was decreased (if necessary) to a maximum level at which tceMEP responses were monitorable (patient specific). Patients were grouped according to whether they received nitrous oxide and the anesthetic depth at which responses were monitored (<0.5, 0.5-0.9, 1-1.4, and >1.5 MAC). Results. A total of 232 (94%) patients received nitrous oxide. Of these patients, responses were obtained throughout the case in 20 (8.6%) at <0.5 MAC, 118 (50.9%) at 0.5 to 0.9 MAC, 85 (36.6%) at 1 to 1.4 MAC, and 9 (3.9%) at >1.5 MAC. Of the remaining 15 (6%) who received no nitrous oxide, responses were monitored in 3 (20%) at <0.5 MAC, 10 (66.7%) at 0.5 to 0.9 MAC, 2 (13.3%) at 1 to 1.5 MAC, and 0 at >1.5 MAC. No false-positive and 1 true-positive (transient) loss of responses occurred. No operations resulted in postoperative motor deficit. Conclusion. Although isoflurane and nitrous oxide diminish tceMEP responses, reliable monitoring can still be accomplished while using significant levels of inhalational anesthetic agents.