Transcranial stimulation parameters to elicit motor evoked potentials

Purpose: We studied effectiveness of single, paired, and repetitive transcranial electrical stimulation to activate motor cortex in patients during spine surgery. Methods: In ten consented patients, the latency and amplitude of the muscle action potential from abductor pollicis brevis muscle after cortical stimulation was measured. Results: The number of responses evoked by double (pair) pulse stimulation was significantly higher than that evoked by single pulse (p = 0.0191). A similar finding was obtained between the repetitive and single pulse stimulation (p = 0.0001). The repetitive stimulation was more effective than the double pulse stimulation (p = 0.0253). An increase of interstimulus interval (ISI), between pulses, from 1-3 msec did not significantly alter latency (p > 0.05) or amplitude (p > 0.05) with the double pulse or repetitive stimulations. The latency or amplitude did not significantly change with varying number of electrical pulses from 3-9. Conclusions: This study shows that the intraoperative repetitive transcranial stimulation is the most effective method to activate descending motor tracts under anesthesia.