Vibro-tactile stimulation as a non-invasive neuromodulation therapy for cervical dystonia: a case study

Abstract

Background: Cervical dystonia (CD) is a type of focal dystonia characterized by involuntary neck postures. The underlying neurophysiology mechanism of CD is unknown, but it has been long hypothesized to be associated with somatosensory and proprioceptive deficits. As a form of somatosensory stimulation approach, vibro-tactile stimulation (VTS) is known to alter afferent signals from the vibrated mechanoreceptors in muscles and tactile receptors in the skin. Previous studies have shown that VTS can be an effective neuromodulation therapy for treating laryngeal dystonia. Objectives: This proof-of-concept study examined the effect of VTS on alleviating the involuntary cervical muscle contractions in two adolescents with different presentations of CD – a male with consistent retrocollis and a female intermittent torticollis. Method: VTS was applied sequentially on four neck positions: bilateral trapezius (TRP) and bilateral sternocleidomastoid (SCM). Each VTS site was vibrated continuously for six minutes. The kinematics and underlying neck muscle activities during dystonic neck movements were examined with acceleration and surface electromyography (sEMG). Two acceleration features and two sEMG features were derived: (1) number of peaks per minute; (2) peak amplitude of acceleration (PAA); (3) change in power of sEMG after VTS; (4) cumulative density function of sEMG between 3-10 Hz (CDF10). Results: First, the application of neck muscle VTS did not induce meaningful symptom relief for the participant with constant retrocollis. Second, the frequency of dystonic neck movements decreased by 60% after VTS in the participant who presented with intermittent torticollis. In addition, PAA during dystonic episodes was significantly lower in post VTS when compared to baseline. Third, the effectiveness of VTS in alleviating dystonic muscle spasms depended on the site of vibration. For the patient with right torticollis, the left trapezius muscle was the optimal vibration site reducing sEMG signal power by 15% across all recorded muscles. During VTS on the optimal vibration site, sEMG power of left trapezius dropped rapidly within the first minute and then continued to decline over time. As an additional result, the mean CDF10 of left trapezius in post VTS condition was found significantly lower than baseline. Discussion: This case study offered preliminary insight into the assumed effectiveness of neck muscle VTS as a treatment for CD. One participant responded positively to VTS. The frequency and extent of the dystonic postures were markedly reduced during and immediately after VTS application. A systematic study with larger sample size is required in the future to validate the effectiveness of VTS for treating symptoms in CD.