Objective: To describe the axis characteristics for different patterns of oscillatory head movement in Cervical Dystonia (CD).

Background: CD is associated with a variety of involuntary movements, including slow jerky oscillations and faster sinusoidal movements. It is not known how these patterns differ in terms of their dominant axis, or how this axis varies over time.

Method: 61 CD subjects and 49 controls were asked to maintain a neutral head position while wearing motion sensors to record triaxial angular velocity pertaining to involuntary head movement. Sensor data was used to generate a time series of rotation matrices, Rn, from which axis characteristics were extracted. The axis of rotation was equal to the eigenvector of the matrix R at each time point corresponding to a real eigenvalue of 1. The median axis, axis variance, and axis stability index were calculated for samples in three frequency bands (low: 0-1.5 Hz, mid: 1.5-4.5 Hz, high: >4.5Hz) and compared with controls.

Results: Oscillatory head movements in controls were restricted to the low band with a median frequency of 0.31 Hz. These occurred almost exclusively around a transverse axis (i.e. nodding movements) with a variance of 3.16° and an axis stability of 82%. Movements in CD occurred across all three frequency bands. Low frequency movements (0.27 Hz) occurred around a wide variety of axes with a variance of 36.59° and median axis stability of 67%; mid frequency movements (2.61 Hz) occurred around a skewed vertical axis with a variance of 23.93° and a median axis stability of 55%; high frequency movements (5.07 Hz) were tightly clustered around a vertical axis with a variance of 15.67° and a median axis stability of 65%. Axis stability was significantly higher in controls than all CD frequency bands (p<0.001). Within CD subjects, mid frequency movements had significantly lower axis stability than both low (p<0.01) and high frequency movements (p<0.001).

Conclusion: Control subjects have kinematic evidence of very low frequency, low amplitude involuntary head movements with highly stable axis properties. By contrast, CD subjects display movements across multiple frequency bands with distinct axis characteristics. Our findings provide a richer account of involuntary motor patterns in CD. Such behavioural fingerprints have broad utility, for example their neural correlates may be explored experimentally and offer novel targets for therapeutic neuromodulatory interference.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/axis-characteristics-of-oscillatory-head-movements-in-cervical-dystonia/