Objective: To identify kinematic metrics that characterise and differentiate the motor manifestations of Cervical Dystonia (CD).

Background: Motor manifestations of CD include phasic torsional movements, painful tonic abnormal postures, and tremor, which combine to varying degrees within subjects and contribute to phenomenological heterogeneity. The current classification provides no formal definition of these terms and acknowledges that CD has not been consistently organised into distinct subtypes. Kinematic analysis provides an objective and quantitative approach to describing these motor features.

Method: CD subjects were recruited at least 3 months after their last botulinum toxin injection and compared with healthy controls. A wireless inertial measurement unit was used to record gyroscopic data pertaining to head movement. Voluntary movements were captured during head turns to the right and left side. Involuntary movements were captured holding a fixed neutral head position and in an unconstrained condition. Voluntary movement metrics included peak angular velocity, movement amplitude, movement smoothness, and asymmetry. Asymmetry represents within-subject direction differences for velocity (∆V), amplitude (∆A) and smoothness (∆S). Angular velocity of involuntary motion was analysed in time and frequency domains.

Results: 44 CD and 33 control subjects were enrolled. Voluntary movements showed reduced peak angular velocity (p<0.001), amplitude (p<0.001) and smoothness (p<0.001), with increased asymmetry (∆V: p=0.02; ∆A: p<0.001; ∆S: p=0.004). Cumulative involuntary movement was increased in all axes (pitch: 168.3 vs 43.8º; yaw: 256.9 vs 39.3º; roll: 151.5 vs 26.4º, all p<0.001). Power spectral density and position vector analysis revealed 3 patterns of involuntary movement: sinusoidal oscillations at 5.4±1.2 Hz affecting 73% of subjects, jerky oscillations at 2.9±0.7 Hz in 55%, and slow drift-saccade type movements at 0.6±0.4 Hz in 41%. 86.4% of subjects had least 1 involuntary movement pattern, with combinations frequently detectable within subjects.

Conclusion: Voluntary head movements in CD have impaired velocity, amplitude, smoothness, and symmetry. Several subtypes of involuntary movement are described with distinct kinematic patterns. This objective approach to describing motor phenomenology will aid classification, allow for quantification of treatment effects, and inform research into underlying disease mechanisms.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/kinematic-features-of-voluntary-and-involuntary-head-movements-in-cervical-dystonia/