Vagus nerve stimulation for epilepsy activates the vocal folds maximally at therapeutic levels.
OBJECTIVE
Vagus nerve stimulation (VNS) for medically refractory epilepsy can give hoarseness due to stimulation of the recurrent laryngeal nerve. For a group of VNS-therapy users this side-effect interferes severely with their daily activities. Our goal was to investigate the severity of intra-operative VNS-related vocal fold contraction at different pulse widths and current output parameters. We investigated electromyographic and morphometric alterations on the vocal folds during VNS.


METHODS
Vocal fold EMG experiments were conducted intra-operatively during the implantation of a VNS system. During surgery the VNS pulse generator was programmed to stimulate at different pulse durations. At each pulse width the EMG-threshold current was determined by electrical stimulation of the vagus nerve with increasing stimulation currents. Laryngostroboscopic examination was performed after surgery to analyze the effects of spontaneous stimulation on the larynx.


RESULTS
The vocal fold EMG and morphodynamic changes in the larynx have been analyzed in eight patients. In all patients left vocal fold EMG-threshold was between 0.25 and 0.50 mA. Pulse duration had little influence on the EMG-threshold level. Vocal fold EMG saturation levels were reached between 0.75 and 1.00 mA. Video stroboscopic monitoring showed that stimulation induced an adductory spasm of either the ipsilateral vocal fold or the vestibular fold, and was present remarkably irrespective of the presence of hoarseness.


CONCLUSIONS
VNS causes pronounced effects on the vocal folds even at low stimulation amplitudes. At therapeutic levels even at the lowest stimulation pulse durations, the vocal fold contract, however, this does not necessarily give hoarseness.
Ardesch
J J
JJ
Department of Neurology, Medisch Spectrum Twente, Haaksbergerstraat 55, 7513 ER Enschede, The Netherlands. jjardesch@hotmail.com
Sikken
J R
JR
Veltink
P H
PH
van der Aa
H E
HE
Hageman
G
G
Buschman
H P J
HP
D000293:Adolescent; D000328:Adult; D004576:Electromyography; D004827:Epilepsy / Q000628:therapy*; D005260:Female; D006685:Hoarseness / Q000209:etiology / Q000503:physiopathology*; D006801:Humans; D007821:Laryngeal Muscles / Q000294:innervation / Q000503:physiopathology*; D007828:Laryngoscopy; D008297:Male; D008875:Middle Aged; D016896:Treatment Outcome; D014630:Vagus Nerve / Q000503:physiopathology*; D055536:Vagus Nerve Stimulation / Q000009:adverse effects* / Q000379:methods; D014827:Vocal Cords / Q000294:innervation* / Q000503:physiopathology