Kinematic Features of Voluntary and Involuntary Head Movements in Cervical Dystonia

Background Cervical dystonia (CD) has a varied motor presentation, combining abnormal postures with complex involuntary head movements. Classification of these motor patterns remains imprecise, relying on descriptive terminology without robust definitions. Objectives To provide a kinematically‐grounded description of the motor phenomenology of CD. Methods Sixty‐two patients with CD and 32 age‐matched controls were recorded wearing inertial sensors. Voluntary movement was assessed during a head‐turning task, including velocity, range of motion, smoothness, and trajectory. Kinematic metrics were extracted from samples of involuntary movements and cluster analysis was applied to identify different patterns. These were then compared with clinical appraisal of movement phenotype. Exploratory factor analysis was conducted to investigate the relationship between motor characteristics. Results Voluntary deficits included impaired velocity, range, smoothness, and symmetry, with no evidence of sequence effect. Three distinct clusters of involuntary head movement were identified: low‐frequency oscillations with a sawtooth waveform consisting of alternating fast and slow phases (Cluster 1); erratic mid‐frequency movements with constant changes in speed, waveform profile, and axis (Cluster 2); and high‐frequency sinusoidal movements (Cluster 3). Cluster 2 was specifically associated with a ‘jerky’ clinical phenotype, while Cluster 3 was associated with a ‘tremulous’ phenotype. Factor analysis identified distinct latent components driving variation in voluntary deficits and involuntary movement. Conclusions CD is characterized by a heterogeneous combination of oscillatory movements and impairments in voluntary head control. Kinematic analysis has the potential to refine disease classification and support the development of objective instrumental outcome measures for clinical assessment and research.