Laryngeal myoclonus related to vagal nerve stimulation

Significance statement

Vagal nerve stimulators (VNS) have well known efficacy in the treatment of intractable epilepsy, and are relatively well tolerated. ^1,2 Most side effects of vagal nerve stimulation relate to the activation cycle, and include dysphonia. Dysphonia is usually manifested by left vocal fold paresis; ^1,3 however, the effects of VNS on laryngeal function can be diverse, and include laryngeal myoclonus. Early referral for otolaryngologic evaluation may be beneficial in any dysphonic patient after VNS implantation.

Clinic

A 44-year-old female with a history of partial complex seizure disorder and anxiety status post placement of a vagal nerve stimulator in 2012 presented to the otolaryngology clinic due to episodes of vocal tremor, dyspnea, throat tightness and anxiety intermittently over the past 2–3 years. She had visited the emergency department on at least one occasion due to her symptoms, and workup was unremarkable. She was not aware of any specific inciting events. She had experienced dysphonia immediately following VNS placement in 2012, though her symptoms improved with down-titration of signal amplitude. VNS interrogation prior to otolaryngologic evaluation revealed an output current of 1.25 mA with a 10 Hz signal frequency and a pulse width of 130 microseconds. Signal on period was 21 seconds and off period was 5 minutes.

Upon initial examination, the patient had an audible vocal tremor occurring intermittently and lasting an estimated 20 seconds. Her voice was normal in the intervals between episodes. There was no stridor or respiratory distress at baseline, but she did describe a strangling sensation during symptomatic intervals.

The patient underwent flexible laryngoscopy for further evaluation. At baseline, vocal fold lengths were symmetric. There was normal, symmetric vocal fold adduction with phonation and abduction with sniff. During the exam, the patient developed a gross tremor predominately affecting the left true vocal fold reminiscent of myoclonus. The tremor was worsened with adduction by phonation, but persisted at rest. There was complete abduction with sniffing. These events were found to correlate with activation of the VNS and were associated with the patient’s symptoms (see Supplementary Video 1).

Discussion

The patient’s symptoms and laryngoscopic exam are atypical for dysphonia from vagal nerve stimulation. Felisati. et al organizes the endoscopic effects of vagal nerve stimulation into three groups: (I) (a) Left vocal cord paresis at baseline, unchanged with stimulation; (I) (b) Left vocal cord paresis at baseline, adduction with stimulation; and (II) normal vocal cord mobility at baseline, adduction with stimulation. ³ Additional, less common, patterns have been documented, including near-complete glottic closure as a result of vagal nerve stimulation. ⁴ Our patient exhibits another unique pattern by demonstrating a rhythmic quality of left vocal fold movement, more reminiscent of myoclonus than paresis. The patient’s presentation is also unusual in the length of time after implantation that she began to experience distress, as patients are usually most symptomatic directly after implantation. ^{1 -3} While vagal nerve stimulators have been well-documented to cause dysphonia, our case demonstrates that presentation can be varied. Early referral for otolaryngologic evaluation and flexible laryngoscopy may be beneficial in any dysphonic patient after VNS implantation.

Supplemental Material