Neurophysiological mechanism of possibly confounding peripheral activation of the facial nerve during corticobulbar tract monitoring

Objective: To improve the recognition and possibly prevent confounding peripheral activation of the facial nerve caused by leaking transcranial electrical stimulation (TES) current during corticobulbar tract monitoring. Methods: We applied a single stimulus and a short train of electrical stimuli directly to the extracranial portion of the facial nerve. We compared the peripherally elicited compound muscle action potential (CMAP) of the facial nerve with the responses elicited by TES during intraoperative monitoring of the corticobulbar tract. Results: A single stimulus applied directly to the facial nerve at subthreshold intensities did not evoke a CMAP, whereas short trains of subthreshold stimuli repeatedly evoked CMAPs. This is due to the phenomenon of sub-or near-threshold super excitability of the cranial nerve. Therefore, the facial responses evoked by short trains TES, when the leaked current reaches the facial nerve at sub-or near-threshold intensity, could lead to false interpretation. Conclusions: Our results revealed a potential pitfall in the current methodology for facial corticobulbar tract monitoring that is due to the activation of the facial nerve by subthreshold trains of stimuli. This study proposes a new criterion to exclude peripheral activation during corticobulbar tract monitoring. Significance: The failure to recognize and avoid facial nerve activation due to leaking current in the peripheral portion of the facial nerve during TES decreases the reliability of corticobulbar tract monitoring by increasing the possibility of false interpretation. (C) 2015 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.