Effects of Voice Therapy on Laryngeal Motor Units During Phonation in Chronic Superior Laryngeal Nerve Paresis Dysphonia
Introduction
Injury to the superior laryngeal nerve (SLN) can result in dysphonia, and in particular, reduced fundamental frequency range and a reduction in the highest obtainable fundamental frequency.1, 2
The SLN innervates the cricothyroid (CT) muscle, which mainly controls the longitudinal tension of the vocal folds and voice pitch.3 Superior laryngeal nerve paresis (SLNP) is difficult to diagnose because of a lack of consistent laryngeal findings, and its effects on voice likely extend beyond simple pitch elevation control.4 The prevalence of SLN injury and dysfunction is unknown as many cases go undiagnosed, and little has been written about SLNP in regard to its specific effects on voice.5 This is particularly important as alterations in singing voice by SLNP can be a serious detriment to professional singers.2
SLNP can be a difficult problem to address with either voice therapy (VT) or surgical intervention. A wide variety of treatment options have been previously reported, ranging from steroids for acute potential viral-related injury, VT, and a variety of surgical procedures.4 Although not commonly performed, several small case series by Shaw et al6 and Nasseri and Maragos2 have described surgical treatment for SLN injury using a “muscle-nerve-muscle” pedicle with a nerve cable graft.7 VT is the prescribed treatment for long-standing isolated SLN paralysis, although the actual improvements in voice appear to be quite variable. This type of therapy has been reported to target the building of CT muscle strength through activities such as glissando maneuvers.8 Alternatively, adjunctive neuromuscular electrical stimulation (NMES) is a well-known modality in the field of orthopedics and physical medicine,9 and some clinicians and scientists have suggested that combining vocal exercises with NMES may enhance the positive effects of VT for SLNP.1 The combined approach of physical therapy and NMES has been reported to enhance treatment outcomes in sports medicine10, 11 and in stroke rehabilitation.12 However, the effects of VT without NMES are unknown.
The purpose of this retrospective study was to demonstrate the clinical effectiveness of VT for rehabilitating chronic SLNP dysphonia in two subjects, by assessing the changes in motor unit firing rate on laryngeal electromyography (LEMG).
Section snippets
LEMG
We used a Neuropack MEB 2200 with 30 mm × 0.45 mm NM-121T needle electrodes (all from Nihon Kohden Corporation, Tokyo, Japan). Both CT and thyroarytenoid (TA) muscles were tested. Needle placement and position were confirmed with standard percutaneous LEMG techniques.13, 14, 15 Placement into the TA muscles was confirmed by the presence of recruitment during phonation and silence during passive and active inspiration. TA muscle activity was evaluated both during speech and during sustained
Case 1
A 66-year-old female non-singer homemaker presented with an 18-month history of dysphonia. She had no history of vocal abuse, and no memory of having a cold around the time she first noticed her dysphonia. Her main complaint was occasional voice change during speech with lowered pitch. Vocal assessments and stroboscopic examination demonstrated a contraction of pitch range and vocal fold vibratory phase asymmetry (Table 1). Her total score on the Voice Handicap Index (VHI)-10 was 15 out of 40.
Vocal assessments
Results from the aerodynamic vocal assessments pre- and post VT are summarized in Table 1. Case 1 showed an improvement in her pitch range, intensity, jitter, shimmer, and VHI-10. Case 2 showed an improvement in maximum phonation time (MPT), pitch range, intensity, jitter, shimmer, and VHI-10. Stroboscopic examinations showed more periodic and greater vocal fold vibration compared with initial assessment in both cases.
Case 1
Before VT, the patient was assessed as having 70% recruitment of the left CT;
Discussion
Results from the current case studies identified several changes in voice production, voice quality parameters, LEMG, and mean ISIs following VT for SLNP dysphonia. Although several reports have described the effects of VT without NMES for functional voice disorders, it was unknown if VT without NMES is effective for patients with dysphonia with chronic SLNP.
Electrical stimulation therapy (ES) is a well-known modality in the field of orthopedics and physical medicine.9 The combined approach of
Conclusions
The current study demonstrated the objective effectiveness of VT for rehabilitating chronic SLNP dysphonia in two subjects, using ISI variability of laryngeal motor units by LEMG. VFE-based stimulation training may result in improved paralyzed laryngeal musculature in chronic SLNP dysphonia.
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This article won the David W. Brewer Award for Best Poster at the Voice Foundation's 45th Annual Symposium, 2016.