Objective: To investigate postural control and head–trunk dynamics in isolated cervical dystonia (CD).

Background: Postural control requires integration of visual, proprioceptive and vestibular inputs and coordinated motor output across body segments. Head–trunk coordination shows frequency-dependent dynamics, with inverted-pendulum behaviour at low frequencies (<0.7 Hz), resonance at 2–3 Hz, and active vestibular-mediated stabilization at higher frequencies. It is unclear if these control strategies are affected in CD, or if they represent a primary dysfunction contributing to dystonic movement.

Method: 62 CD and 32 age-matched controls were assessed during quiet stance (eyes open, eyes closed) and one-leg stance. Inertial measurement units recorded acceleration and angular velocity from the head and sternum. Postural sway was quantified using adjusted mean acceleration (m/s²) and sway ellipse area (m²/s⁴). Head-on-trunk dynamics in pitch and roll were analysed using coherence, transfer function (TF) and power spectral density ratio (PSDR). Regression slopes for head-trunk angles were calculated after filtering (<0.7 Hz, 0.7–3 Hz, >3Hz). Data are reported as median [interquartile range].

Results: Postural sway was increased in CD across all tasks. During quiet stance (eyes open), adjusted mean acceleration (0.25 [0.16–0.42] vs 0.12 [0.08–0.16]), and sway ellipse area (1.25 [0.58–3.20] vs 0.32 [0.22–0.46]) were greater compared to controls (both p<0.001). Similar differences were seen with eyes closed and one-leg stance (all p<0.001). Sway metrics correlated with involuntary head movement and remained elevated in a subgroup (n=13) without detectable head movement. During standing, PSDR was increased at all frequencies. Coherence was reduced <0.7 Hz and >4 Hz, while TF was increased between 0.7–3 Hz. The normal transition to negative regression slopes at high frequencies was lost in CD. During one-leg stance, coherence was reduced across all frequencies, but TF, PSDR and regression slopes were similar to controls.

Conclusion: Postural instability and head movements in CD have a complex relationship. Abnormalities in both low-frequency and high-frequency modes suggest disrupted sensory processing and integration involving proprioceptive and vestibular pathways. Excess head motion at 2–3 Hz likely reflects parameters measures, suggesting balance-demanding tasks modulate dystonic head movement and could inform rehabilitation strategies.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/frequency-dependent-disruption-of-head-trunk-control-in-cervical-dystonia/