Usefulness of Transcranial Motor Evoked Potential in Clipping Surgery for Cerebral Aneurysms-Introduction of a New Protocol for Stable Monitoring

The usefulness of transcranial motor evoked potentials (Tc-MEPs) in clipping surgery has been reported. However, numerous false positive and false negative cases were reported. We report the usefulness of a new protocol compared with direct cortical MEP (Dc-MEP). Materials were 351 patients who underwent aneurysmal clipping under simultaneous monitoring of Tc- and Dc-MEPs. A total of 337 patients without hemiparesis and 14 with hemiparesis were separately analyzed. Intraoperative changes of Tc-MEP thresholds were examined in the first 50 patients without hemiparesis. The stimulation strength of Tc-MEP was set at +20% of the stimulation threshold. As thresholds changed intraoperatively, thresholds were examined every 10 min and changed stimulation strength. Stimulation thresholds of Tc-MEP were significantly decreased after craniotomy and significantly increased after CSF aspiration. The recording ratios of Tc- and Dc-MEPs were 98.8% and 90.5%, respectively. Out of 304 patients without MEP change, 5 patients developed transient or mild hemiparesis with infarction of the territory of the perforating artery arising from the posterior communicating artery. Out of 31 patients whose MEP transiently disappeared, 3 patients developed transient or mild hemiparesis. The other two patients without MEP recovery manifested persistent hemiparesis. In 14 patients with preoperative hemiparesis, 3 patients whose healthy/affected ratio of Tc-MEP was large developed severe persistent hemiparesis. We clarified the intraoperative changes of Tc-MEP thresholds for the first time. A new protocol of Tc-MEP that followed thresholds and changed stimulation strength to +20% of thresholds is useful for stable monitoring. The usefulness of Tc-MEP is the same as that or better than that of Dc-MEP.