Magnetic stimulation for monitoring of motor pathways in spinal procedures

Transcranial magnetic stimulation was used for intraoperative motor evoked potential monitoring during surgery of intramedullar, extramedullar, and extradural spinal tumors in 13 patients. Anesthesia was based on etomidate. Magnetic stimulation for motor evoked potential monitoring was successful in 10 of 13 patients, 12 of whom were neurologically impaired. Motor evoked potentials were recorded from limb muscles or from the fibers of the cauda equina. Amplitudes of baseline recordings (the initial recording obtained after induction of anesthesia) were decreased by 64 +/- 34% (mean +/- SD) and baseline latencies were increased by 7 +/- 8% compared with the preoperative recordings. Subsequent recordings were analyzed for amplitude and latency changes in comparison to baseline. Amplitude changes exceeding 50% and latency changes higher than 3 ms compared with the baseline correctly indicated an impending lesion of motor pathways with increased paresis postoperatively. In cases where motor evoked potential monitoring was successful prediction of short-term postoperative motor outcome was always correct. There were no "false-negatives" or "false-positives."