Quantitative, clinically relevant acoustic measurements of focal embouchure dystonia

Abstract

Background: Focal embouchure dystonia impairs orofacial motor control in wind musicians and causes professional disability. A paucity of quantitative measures or rating scales impedes the objective assessment of treatment efficacy.

Objectives: We quantified specific features of focal embouchure dystonia using acoustic measures and developed a metric to assess severity across multiple domains of symptomatic impairment.

Methods: We recruited 9 brass musicians with and 6 without embouchure dystonia. The following 4 domains of symptomatic dysfunction in focal embouchure dystonia were identified: pitch inaccuracy, sound instability and tremor, sound breaks, and timing variability. Musicians performed sustained tones and sequences, and then acoustic variables within each domain were quantified. A composite brass acoustic severity score composed of these variables was validated against clinical global impressions of severity.

Results: Musicians with dystonia performed worse in acoustic domains of pitch inaccuracy (median: dystonia = 100%, control = 62%), instability (median shimmer: dystonia = 3%, control = 2%), and breaks (median: dystonia = 0.34%, control = 0.05%). Tremor in embouchure dystonia was 5 to 8 Hz, intermittent, and variable in amplitude. Rhythmic variability did not differ between groups. Participants with embouchure dystonia had different patterns of impairment across variables. Composite severity scores strongly predicted clinical global impression of severity (R² = 0.95).

Conclusions: Acoustic variables distinguish musicians with embouchure dystonia from controls and reflect different types of symptomatic impairments. Our composite acoustic severity score predicts severity of clinical global impression for musicians with different patterns of symptomatic impairment and may provide a foundation for developing a clinical rating scale. © 2018 International Parkinson and Movement Disorder Society.

Keywords: clinical assessment; musician dystonia; task-specific dystonia.

Figure 1

Study design & key acoustic variables. Top: Sustained tones were selected from the lowest pitch register of the instrument (very low pitch), highest pitch register of the instrument (very high pitch) and in between and each of the 5 pitches was played as long as possible at soft, moderate, and loud volumes. Each note was analyzed for measures of instability of the fundamental frequency, inaccuracy, and sound breaks. A: Diagram of the fundamental frequency period (T0) and amplitude. B: Inaccuracy is the F0 deviation from the initial 100ms and central 50% of each note. C: A sound break (gray) characterized by a loss of periodicity and amplitude during a sustained tone. Bottom: Evenly-spaced note sequences consisting of a one-octave ascending, articulated musical triad in each instrument’s low, middle, and high pitch registers were performed at soft and loud volumes. Inter-onset-intervals (IOIs) were measured between notes of each sequence. Subject mean IOI and IOI CoV were calculated from IOI values across all 6 sequence conditions.

Figure 2

Summary of group-level analyses for key acoustic variables. Musicians with FED have increased attack inaccuracy (A), sound instability (B–C), break severity (D), and low-frequency instability of A0 (F) compared with MC musicians. IOI CoV (E) did not differ between groups, however two outliers in the FED group reflect individuals with difficulty initiating notes. All musicians with FED had breaks (D) though they occurred rarely in MC subjects, such that the best musician with FED had greater break severity than the most impaired MC. The shaded region from y = 0 to y = 0.3 is expanded below the plot to show the distribution of FED and MC subjects in this severity range. Error bars represent the 95% confidence interval. In all graphs, each data point represents a subject. *p<0.05, **p<0.01, ***p<0.001.

Figure 3

Tremor analyses in a MC & 3 FED musicians. Normalized, concatenated note A0 time series were analyzed for tremor in the range of 3–20 Hz. A: FFT of A0 reveals a dominant tremor frequency in the range of 5–8 Hz with increased low-frequency FFT noise in some musicians with FED. B: STFT analysis of the A0 time series (above each spectrogram) depicts patterns of tremor. The representative control (top) has no dominant tremor frequency and minimal low-frequency noise. Each FED participant had a different pattern: regular, nearly continuous tremor (FED8), intermittent tremor with substantial low-frequency noise (FED9), or predominantly noise with a visible tremor band in one note only (FED2, inset). White spaces indicate concatenated note transitions.

Figure 4

Summary of composite score analyses. A: Correlation matrix of values for key acoustic variables (breaks, inaccuracy, jitter, and shimmer) in musicians with FED. Spearman ρ values are shown for each pair and significant results (p<0.05, corrected) are denoted in bold. Jitter and shimmer were highly correlated in musicians with FED. B: COmposite BRass Acoustic Severity (COBRAS_ALL) scores of global FED severity were calculated for FED and MC musicians. COBRAS_ALL is significantly higher in the FED group. Only one FED subject falls within the 95% confidence interval of controls. C: Linear regression of COBRAS_FED and average CGI severity. COBRAS_FED is predictive of average CGI severity in musicians with FED. The shaded region represents the 95% confidence interval. **p<0.01