Vagal Nerve Stimulation: Clinical and Electrophysiological Effects on Vocal Fold Function

Abstract

More than 16,000 vagal nerve stimulators (VNSs) have been implanted for refractory epileptic seizures. The most commonly reported side effect is hoarseness. This study examines the effects of VNS placement on vocal fold function. Eleven patients who had undergone VNS placement at our institution were recruited. Subjective evaluation by a panel of speech and language pathologists of both connected speech and videolaryngoscopy recordings were used both at rest and during VNS activation. Additional subjective evaluation included use of the Voice Handicap Index for the study group. These results were compared to data from age- and sex-matched controls. Objective data included maximum phonation time in the study and control groups, as well as laryngeal electromyography performed on the VNS-implanted patients only. Motor unit potential morphology and recruitment, as well as spontaneous activity, were analyzed bilaterally for the cricothyroid and thyroarytenoid muscles. Significant differences were found between the study and control groups subjectively for vocal quality and videolaryngoscopy parameters. Vocal fold tension, supraglottic muscular hyperfunction, and reduced vocal fold mobility were the most common findings during VNS activation. Two of 10 patients had immobile left vocal folds in the absence of active stimulation. The maximum phonation time was generally reduced in the subject group, but this reduction did not reach statistical significance. Finally, 6 of 10 patients had abnormal electromyographic results, including large-amplitude polyphasic motor unit potentials and decreased recruitment. We conclude that implantation of a VNS can affect vocal fold function. The effects are magnified during periods of active stimulation. There is the potential for nerve degeneration after prolonged repetitive stimulation, and there may be a trend toward greater vocal fold dysfunction with higher stimulation parameters.