Transcranial motor evoked potentials (Tc-MEPs) are a valuable intraoperative modality in spinal and spinal cord surgery for predicting postoperative paralysis. Arm Tc-MEPs are generally easily induced, but leg Tc-MEPs have a lower induction rate, and patients in whom leg Tc-MEPs cannot be induced are often encountered in spinal and spinal cord surgery in particular. Different hospitals use different cranial stimulation sites for leg Tc-MEPs, and an investigation of the optimal stimulation site will be helpful for preventing postoperative paralysis. In this study, the optimal stimulation site in the temporal direction was investigated, with the objective of improving the leg Tc-MEP induction rate. The subjects were 200 patients undergoing spinal cord or spinal surgery. The stimulation sites were determined by measuring lateral distances of 3 cm, 5 cm, 7 cm, 9 cm, and 11 cm from the vertex (Cz) and conducting stimulation at the five points 2 cm anterior to each of these points. The results showed that the induction rate was highest and the stimulus threshold was lowest at the stimulation site 2 cm anterior to the point 7 cm lateral to the vertex. A similar level of induction was also possible at the stimulation site 2 cm anterior to the point 9 cm lateral to the vertex, but the induction rates were significantly lower, and the stimulus thresholds were higher at the stimulation sites 2 cm anterior to the points 3 cm, 5 cm, and 11 cm lateral to the vertex. When positioning electrodes in the parietal direction to induce leg Tc-MEPs, leaving a greater distance between the electrodes may allow the stimulus to be transmitted deeper within the brain, thus improving the leg Tc-MEP induction rate.
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