Threshold-level repetitive transcranial electrical stimulation for intraoperative monitoring of central motor conduction

Object. The authors conducted a study to evaluate repetitive transcranial stimulation (TES) to assess spinal cord motor tract function in individuals undergoing spine surgery, with emphasis on safety and efficacy. Methods. Somatosensory evoked potentials (SSEps) were elicited using standard technique. Muscle electromyographic values were measured in response to a three- or four-pulse train of stimulation delivered to the motor cortex via subdermal electrodes. They also evaluated whether changes in the minimum stimulus intensity (that is, threshold level) needed to elicit a response from a given muscle predict motor status immediately postoperatively. Anesthesia was routinely induced with intravenous propofol and remifentanil, supplemented with inhaled nitrous oxide. Use of neuromuscular block was avoided after intubation. Satisfactory monitoring of muscle response to threshold-level repetitive TES was achieved in all but nine of the 194 patients studied. In contrast, cortical SSEP responses could not be elicited in 42 of 194 individuals. In cases in which response were present, TES-based evoked responses proved to be extremely accurate for predicting postoperative motor status. Somatosensory evoked potential monitoring was nearly as accurate for predicting postoperative sensory status. There were frequent instances of postoperative motor or sensory deficit that were not predicted by SSEP- and TES-based monitoring, respectively. There were no adverse events attributable to TES-based monitoring, although since the study ended we have had a single adverse event attributable to threshold-level repetitive TES. Conclusions. Intraoperative threshold-level repetitive TES-based monitoring of central motor conduction has proven to be a simple, safe, and highly accurate technique for the prevention of minimization of inadvertent motor deficit during surgery involving the spine or spinal cord.