The Effect of an Anesthetic Induction Dose of Midazolam on Motor Potentials Evoked by Transcranial Magnetic Stimulation in the Monkey

Abstract

The effect of a hypnotic dose (0.5 mg/kg) of midazolam (MDZ) on motor evoked potentials (MEPs) was examined in 12 monkeys. MEPs were elicited by transcranial magnetic stimulation (TMS) and the resultant potentials recorded from abductor pollicis brevis (APB) and anterior tibialis (AT) muscles contralateral to the stimulation site. After administration of MDZ, sequential MEP recordings were obtained at postinduction, hypnosis, awakening, emergence, and recovery periods. The results were compared with control values using one-way analysis of variance and Tukey's post-hoc test. Under hypnosis, MEP reproducibility was problematic as the potentials were occasionally ill identified and questionable. MDZ resulted in marked MEP scalp field reduction, coil demography alteration, stimulation threshold elevation, and amplitude suppression (p <0.01). Latency response was unaltered. During hypnosis, awakening, and recovery periods, the mean APB and AT thresholds were elevated by 39, 23, and 0% and by 60, 34, and 4% respectively; while APB and AT amplitudes were depressed by 95, 86, and 53% and by 99, 91, and 60%, respectively. We conclude that an induction dose of MDZ can produce profound and prolonged attenuation of TMS MEPs. The drug inhibitory effect on MEPs may persist after recovery. Anesthetic doses of MDZ should cautiously be used in the settings of MEP monitoring.