Objective: To identify acoustic variables that characterize focal embouchure dystonia (FED) and use these measures to quantify FED severity.

Background: FED is a focal task-specific dystonia that impairs orofacial motor control in wind musicians, causing professional disability. It is common in brass musicians, with an estimated 1% prevalence in this population. Little is known about its specific features or mechanisms, in part because there are no quantitative measures or rating scales. As the deficit in FED is often “heard” better than it is “seen,” we used acoustic metrics to quantify specific features and global severity of FED.

Methods: Adult brass musicians with and without FED were studied while playing sustained tones and sequences. Sound recordings were analyzed in amplitude, frequency, and time domains using MATLAB. Pitch inaccuracy, stability (jitter and shimmer), sound breaks, and timing (inter-onset interval (IOI) standard deviation (SD) were quantified for each note. Subject FED acoustic severity (FAS) was calculated from mean normalized values of key variables. Clinical global impression (CGI) of severity was assessed independently by two expert movement disorders neurologists.

Results: We recruited 6 musicians with FED and 5 healthy musician controls (CTL). FED was characterized by greater pitch inaccuracy (pitch deviation: FED=88.7%, CTL=28.2%; P<.02), instability (Bin jitter: FED=1.8%, CTL=0.23%, P=0.004; Bin shimmer: FED=13.6%, CTL=6.2%; P=0.03); Cycle jitter: FED=1.1%, CTL=0.46%, P=0.25; Cycle shimmer: FED=3.3%, CTL=1.8%, P=0.17), and rhythmic variability (IOI SD: FED=0.15 s; Control=0.06 s; P<0.001). All FED but few CTL subjects had breaks (Breaks per note: FED=0.75, CTL=0.03, P=0.004). Subjects with FED differed in clinical phenotype as well as patterns of impairment across variables. FAS correlated highly with CGI (R²=0.83; DF=9, P=0.0001).

Conclusions: Quantified acoustic variables distinguish musicians with FED from those without and can be used to measure severity. The relative contributions of these variables to overall severity differed between subjects, but a global severity score incorporating all key variables correlated highly with CGI. These quantitative measures can be used to assess response to treatment and as a measure of severity in other studies. In addition, our results provide the foundation for developing a clinical rating scale for FED.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/quantitative-measurements-of-deficit-type-and-severity-in-focal-embouchure-dystonia/