From null to midline: changes in head posture do not predictably change head tremor in cervical dystonia

Abstract

Introduction: A common view is that head tremor (HT) in cervical dystonia (CD) decreases when the head assumes an unopposed dystonic posture and increases when the head is held at midline. However, this has not been examined with objective measures in a large, multicenter cohort.

Methods: For 80 participants with CD and HT, we analyzed videos from examination segments in which participants were instructed to 1) let their head drift to its most comfortable position (null point) and then 2) hold their head straight at midline. We used our previously developed Computational Motor Objective Rater (CMOR) to quantify changes in severity, amplitude, and frequency between the two postures.

Results: Although up to 9% of participants had exacerbated HT in midline, across the whole cohort, paired t-tests reveal no significant changes in overall severity (t = -0.23, p = 0.81), amplitude (t = -0.80, p = 0.43), and frequency (t = 1.48, p = 0.14) between the two postures.

Conclusions: When instructed to first let their head drift to its null point and then to hold their head straight at midline, most patient's changes in HT were below the thresholds one would expect from the sensitivity of clinical rating scales. Counter to common clinical impression, CMOR objectively showed that HT does not consistently increase at midline posture in comparison to the null posture.

Keywords: Head tremor; TWSTRS; computer vision; task dependency; video.

Figure 1.

Changes in HT as patients go from the null posture to holding their head straight at midline (N = 80). Rows correspond to: (upper, A-C) parallel plots showing magnitudes in null and midline postures for every participant, and (lower, D-F) distributions of changes in magnitude (vertical lines represent no change (solid), and thresholds for one-point change in HTS (dashed) and TWSTRS-2 HT (dotted)). Columns correspond to CMOR measures of: (A, D) overall HT severity, (B, E) log peak power (“amplitude”) and (C, F) frequency. F) One participant had identical peak frequency between the null and midline postures and was excluded from the plot.

Figure 2.

Log peak power and frequency changes from Drift to Straight (N = 80). Horizontal and vertical dashed lines represent changes in CMOR metric that corresponds to a one-point change in HTS severity rating. Horizontal and vertical solid lines represent no change in log peak power and frequency respectively.