Methodology for intraoperatively eliciting motor evoked potentials in the vocal muscles by electrical stimulation of the corticobulbar tract

Objective: To establish a methodology for recording corticobulbar motor evoked potentials (CoMEPs) from vocal muscles after transcranial electrical stimulation (TES) and direct cortical stimulation (DCS). Methods: Twenty-four patients were included in this study (22 for TES, 2 for DCS. 3 for TES Plus DCS) that underwent different surgical procedures. We used two methods to elicit CoMEPs: (a) TES by stimulation over C3/Cz or C4/Cz and (b) DCS with a strip electrode placed over the primary motor area (M1) for laryngeal muscles. To record CoMEPs from vocal muscles we used two hook wire electrodes 76 pm of diameter passing through 27 gauge needle endotracheally placed in the vocal muscles after intubation. Results: Recording of CoMEPs in the vocal muscles after TES was successfully performed in 22 patients. TES over the right or left hemisphere elicit responses bilaterally. The onset latencies for the right vocal muscle was 12.4 +/- 3.1 ms (ipsilateral stimulation) and 12.7 +/- 2.2 ms (contralateral stimulation) while for the left vocal muscle, onset latency was 12.9 +/- 2.3 ms (ipsilateral stimulation) and 14.1 +/- 3.4 ms (contralateral stimulation). In five patients DCS elicited CoMEPs in right and left vocal muscle with latency of 16.6 +/- 4.7 and 15.6 +/- 3.7 ms, respectively. Conclusion: The method to elicit and record CoMEPs in vocal muscles shows reliable results and adds one more tool in the armamentarium of intraoperative neurophysiology. Significance: This method shows the ability to continuously monitor the functional integrity of corticobulbar pathways, vagal nucleus and laryngeal nerves. Published by Elsevier Ireland Ltd. on behalf of International Federation of Clinical Neurophysiology.