Objective: Our aim was to objectively quantify how head tremor (HT) changes when cervical dystonia (CD) patients are instructed to hold their head at midline in contrast to the null point.

Background: A common view is that HT amplitude decreases when the head assumes an unopposed dystonic posture and increases when holding the head at midline. However, this has not been examined with objective measures in a large, multicenter cohort.

Method: Movement disorders neurologists assigned ordinal scores of 0-4 for the TWSTRS-2 HT item and 0-10 for a HT severity (HTS) score. 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; Vu et al. 2022 J Neurol Sci) to quantify HT for both video segments and we evaluated the changes in overall HT severity, log peak power, and peak frequency. To put those changes in the context of sensitivity of clinical severity assessments, we compared each participant’s CMOR measures of change to the level of change that would correspond to a one-point change (OPC) in both HTS and TWSTRS-2 HT. We also estimated the minimal detectable change in HT between the two postures.

Results: Paired t-tests reveal no significant changes in HT severity (t = -0.23, p = 0.81), log peak power (t = -0.80, p = 0.43), and peak frequency (t = 1.48, p = 0.14) between the two positions. Of 80 participants, the number exhibiting absolute changes greater than the OPC in HTS was 22 for overall HT severity, 20 for log peak power, and 3 for frequency. The number exhibiting absolute changes greater than the OPC in TWSTRS-2 was 1 for overall HT severity, 1 for log peak power, and 0 for frequency. The minimal detectable change between positions was estimated to be 1.12 for HTS-based overall HT severity, 0.45 for TWSTRS-based overall HT severity, 0.72 for log peak power, and 1.71 Hz for frequency.

Conclusion: When instructed to first let their head drift to its null point and then to hold their head straight at midline, for most patients, changes in HT were below the thresholds one would expect from the sensitivity of clinical rating scales. Our results indicate that the widely held view that HT is exacerbated when holding the head in midline may be limited to a small portion of patients.

References: Vu JP, Cisneros E, Lee HY, Le L, Chen Q, Guo XA, et al. Head tremor in cervical dystonia: Quantifying severity with computer vision. J Neurol Sci. 2022 Mar 15;434:120154.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/how-does-head-tremor-in-cervical-dystonia-change-with-head-position/