An Examination of Surface EMG for the Assessment of Muscle Tension Dysphonia
Introduction
Muscle Tension Dysphonia (MTD) is one of the most prevalent diagnoses in patients frequenting the ear, nose, and throat (ENT) department for voice disorders.1, 2, 3 MTD is the pathological condition in which an excessive tension of the (para)laryngeal musculature, caused by a diverse number of etiological factors, leads to a disturbed voice.4, 5, 6 Several causes such as (1) technical misuses of the vocal mechanism in the context of extraordinary voice demands, (2) learned adaptations after upper respiratory tract infections, (3) laryngopharyngeal reflux, and (4) certain psychological and/or personality traits, have been cited as contributing factors to the imbalanced muscle activity.7 However, the underlying etiology of the increased muscular tension of the extralaryngeal muscles in patients with MTD is still poorly understood. It has been hypothesized that the altered tension of the extrinsic musculature in patients with MTD results in a changed position of the larynx in the neck. Because of this altered position, the inclination of the cartilaginous structures of the larynx (hyoid, thyroid, cricoid, and arytenoid) is disturbed, which affects the intrinsic musculature.8 Tension of the vocal folds is altered and the voice becomes disturbed.
MTD can be divided into two categories. Primary MTD involves a dysphonia in the absence of concurrent organic vocal fold pathology and is associated with excessive, atypical, or abnormal laryngeal movements during phonation, without obvious psychogenic or neurologic etiology.9 Primary MTD occurs primarily in women and accounts for 10–40% of the clinical caseloads at a voice center.7, 10, 11 Secondary MTD indicates a dysphonia in the presence of an underlying organic condition.9 This study focuses on primary MTD.
A few studies have shown that there is a measurable difference in the tension (eg, an increased tension) of the (para)laryngeal musculature between patients with MTD and healthy subjects.12, 13 These studies used surface electromyography (sEMG), to measure the tension of the supra- and infrahyoidal muscles. Electromyographic investigation enables recording of the collective behavior of motor units lying under the surface electrode. Hočevar-Boltežar et al12 evaluated 11 patients with MTD and five normal speakers. Their results showed a 6- to 8-fold increase of EMG activity and an alternation of the EMG activity in the perioral and supralaryngeal muscles before and during phonation in most patients with MTD in comparison with normal speakers. Redenbaugh and Reich13 examined seven normal and seven vocally hyperfunctional speakers with surface EMG and concluded that the hyperfunctional speakers showed significantly higher EMG values than the normal speakers during rest, vowel phonation, and reading.
The use of sEMG in a population of patients with vocal hyperfunction (e.g. MTD and vocal nodules) has also been investigated by Stepp and colleagues.14, 15, 16 Stepp et al14 investigated the possible role of vocal hyperfunction in 10 singers with vocal fold nodules, 8 nonsingers with nodules and 10 normal speakers using anterior neck sEMG. Surface EMG during vocal tasks did not differentiate singers or nonsingers with vocal fold nodules from healthy controls. Stepp and colleagues15 also investigated if sEMG of the anterior neck is sensitive to changes in vocal hyperfunction associated with injection laryngoplasty. Thirteen patients were examined with sEMG before and after injection laryngoplasty for glottal phonatory insufficiency. Anterior neck sEMG was not significantly reduced despite significantly reduced perceptual ratings of strain and false vocal fold compression, reflecting a decrease of vocal hyperfunction. Furthermore, Stepp et al16 examined 16 participants with vocal hyperfunction (MTD and vocal nodules) with surface EMG and current neck tension rating systems before and after a single session of voice therapy. They concluded that correlations between palpation ratings and anterior neck sEMG were generally low.
This study was undertaken because currently there is no objective tool to evaluate patients with MTD. Surface EMG has not found its way into daily practice despite the positive results of previous research, therefore, this technique was evaluated, to establish if sEMG could function as an objective diagnostic tool in the diagnosis of MTD. To date, videostroboscopy is the main technique of investigation, but this does not always allow one to differentiate between patients with and without MTD because the laryngoscopic features of MTD (such as a incomplete posterior glottic closure) may also be present in normal speakers.11 Furthermore, interrater reliability from videostroboscopy is known to be average to poor.17 The use of sEMG to monitor changes in neck tension and/or laryngeal position in patients with MTD could lead to more standardized diagnosis and improved information about patient progress. The purpose of this study was to compare the tension of the (para)laryngeal musculature between MTD patients and normal speakers using surface EMG to evaluate the use of sEMG as a diagnostic tool in distinguishing patients with MTD from normal subjects. However, one has to keep in mind that surface EMG only reflects the tensions of the extrinsic laryngeal muscles, and no conclusions can be made regarding the effect on the intrinsic laryngeal muscles.
Section snippets
Participants
The patients with MTD were recruited at the ENT Department of the University Hospital in Ghent, Belgium. Eighteen patients with MTD (six men and 12 women, mean age 38.2 years, age range 22–57 years) participated in the study. All patients included in this study were newly diagnosed patients with primary MTD. None of the patients were involved in any (medical or behavioral) treatment aimed at treating MTD.
Diagnosis of MTD was based on following key features: (1) a history of vocal misuse/abuse
Characteristics of the MTD patients and normal speakers
The results reported are based on 18 patients with MTD and 44 normal speakers. The control population consisted of 13 men and 31 women; they ranged in age from 20 to 63 years (mean = 35.7, SD = 13.8). The MTD patients consisted of 12 women and six men; they ranged in age from 22 to 57 years (mean = 38.2, SD = 9.1). There were no significant differences regarding age (P = 0.209) or BMI (P = 0.185) between the patients with MTD and the control group. Women and men are equally distributed in both groups.
Discussion
This study compared relative sEMG levels of three sites of the (para)laryngeal musculature in 18 patients with MTD with a control group of 44 normal speakers. Each subject’s rest, vowel, and speech sEMG levels were normalized by using the absolute levels to derive proportions of his/her maximal EMG. Aerodynamic and acoustic measurements, DSI, VHI, and videostroboscopy were obtained, to distinguish between patients with MTD and normal speakers. An increased muscle tension of the laryngeal
Conclusion
The present study evaluated sEMG characteristics of patients with MTD in comparison with normal speakers, at three different sites of the anterior neck during silence, phonation, and reading tasks. This study did not show any differences in muscle tension, using sEMG between patients with MTD and the control group. Although sEMG is noninvasive and inexpensive, the results of this study do not support its use as a diagnostic tool for distinguishing patients with and without MTD. MTD remains
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