HSC Section 6 Nov2016 Green Book

Annals of Otology, Rhinology & Laryngology 123(4)

maneuver (hard breath hold). 8 However, it has been shown that vocal fold closure is not consistently accomplished dur- ing Valsalva maneuvers up to 14% of the time, potentially leading to significant diagnostic error. 16 Other qualitative ratings such as decreased recruitment scales are not compa- rable across offices due to their highly subjective nature and lack of standardized between-office collection protocols. 17 Another commonly used clinical LEMG technique is comparison of recruitment against the contralateral muscle. Unfortunately, this technique does not take into account the notion that the contralateral TA muscle is dependent on the co-contraction of neighboring intrinsic muscles. Thus, TA contraction may be altered in the presence of a contralateral paresis or paralysis. In this scenario, compensatory muscle activation is a likely confounder. 18 In addition, a large-scale retrospective study reported unexpected contralateral neu- ropathy in 26% of patients with laryngeal movement disor- ders. 9 Electromyography studies of limb muscle also indicate significant contralateral differences in motor unit recruitment even during simultaneously controlled muscle contractions. 12 Because raw EMG signals are quasi-random in nature, they cannot be directly compared. Thus, a principle goal of this study was to use quantitative methodology with the addition of control parameters and measures, to character- ize the reliability of a primarily qualitative clinical evalua- tion. One such measure was quantification of the LEMG signal via calculation of the root mean square (RMS). Root mean square is considered to be the current “gold standard” for quantitative electromyographic analysis 11,12 and allows for rapid quantitative comparisons among groups of sig- nals. Root mean square was chosen as a measurement met- ric because it provides an indication of mean muscle activity and signal power and is the analog to voltage output. Because RMS is also considered a data smoothing tech- nique, it is not well suited for visualization of waveform transients and morphology characteristics such as polypha- sic or nascent potentials; however, it is useful to quantify and compare LEMG across samples in terms of signal volt- age and power. Because EMG is a time-varying signal con- taining positive and negative values, RMS is an ideal quantitative measure that can be easily calculated post hoc or in real time with many commercially available data acquisition software programs. Determining LEMG data reliability within the context of an in-office clinical environment is important to make care- ful and useful clinical interpretations and to potentially improve clinical protocols. To our knowledge, in-office clinical LEMG reliability has not been systematically inves- tigated in a cohort of vocally healthy adults. As such, the purpose of this study was to prospectively investigate LEMG signal reliability recorded from the thyroarytenoid muscle over multiple testing sessions using a common in- office clinical routine. We modeled our basic methodology

laryngoscopic evaluation of vocal fold mobility to assess a variety of laryngeal disorders such as dystonia and vocal fold paresis and paralysis, to differentiate among various neurological disorders, 5 and for guiding the placement of botulinum toxin for the treatment of spasmodic dyspho- nia. 1,8 Laryngeal electromyography may also be poten- tially useful as a tool for the prognosis of laryngeal nerve disorders. 9,10 In general, electromyography (EMG) recordings are affected by multiple confounding variables including elec- trode type and placement, level of muscle activation, left and right side dominance, artifact from electrode move- ment, and so on, which may all, or in part, compromise the accuracy of the data necessary for diagnostic evaluation. 11,12 Electromyographic investigations of between-session and intra-session reliability for some limb muscles have revealed high reliability for both between- and within-ses- sion measurements. 13,14 However, similar data for laryn- geal-based EMG are absent and cannot be directly interpolated from limb studies due to significant differences in anatomical structure and the ability to control for muscle length and loading. Unlike most limb muscles that have skeletal support and firm attachment points, the larynx is suspended in the neck, surrounded by soft muscle tissue, a series of membranes, and a somewhat yielding cartilaginous framework. Distinct muscle force and leverage points are difficult to determine in the laryngeal complex because of these flexible attach- ment points. Placing a consistent and measureable isotonic load on laryngeal muscles for accurate and reliable activa- tion is difficult, complicating replication of motor unit acti- vation in these muscles. Another factor to be considered regarding the reliability of clinical in-office LEMG is that phonation is an emergent behavior, arising through the complex interaction of respi- ratory, phonatory, and resonance subsystems of the vocal tract. These vocal subsystems function synergistically, inte- grating properties of tissue elasticity, muscle activation, and aerodynamics toward normal vocal function. A change in any subsystem’s dimension will potentially alter vocal out- put. These additional confounding variables have the poten- tial to further complicate in-office LEMG interpretation. 15 In general, reliable LEMG measurements are dependent on consistent muscle activation tasks. These tasks must be carefully controlled and performed for measurement reli- ability. For example, to describe relative recruitment of motor unit potentials for the thyroarytenoid muscle (TA), maximum voluntary contraction (MVC) strategies have been used for comparison. Typically, a maximal voluntary contraction is assigned a 100% possible recruitment value whereby subsequent muscle contractions during voicing tasks are given a percentage of decreased recruitment. Maximum voluntary contraction in the laryngeal system is typically accomplished through performance of a Valsalva

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