Smoothness (i.e. non-intermittency) of movement is a clinically important property of the voluntary movement with accuracy and proper speed. Resting head position and head voluntary movements are impaired in cervical dystonia. The current work aims to evaluate if the smoothness of voluntary head rotations is reduced in this disease. Twenty-six cervical dystonia patients and 26 controls completed rightward and leftward head rotations. Patients' movements were differentiated into "towards-dystonia" (rotation accentuated the torticollis) and "away-dystonia". Smoothness was quantified by the angular jerk and arc length of the spectrum of angular speed (i.e. SPARC, arbitrary units). Movement amplitude (mean, 95% CI) on the horizontal plane was larger in controls (63.8°, 58.3°-69.2°) than patients when moving towards-dystonia (52.8°, 46.3°-59.4°; P = 0.006). Controls' movements (49.4°/s, 41.9-56.9°/s) were faster than movements towards-dystonia (31.6°/s, 25.2-37.9°/s; P < 0.001) and away-dystonia (29.2°/s, 22.9-35.5°/s; P < 0.001). After taking into account the different amplitude and speed, SPARC-derived (but not jerk-derived) indices showed reduced smoothness in patients rotating away-dystonia (1.48, 1.35-1.61) compared to controls (1.88, 1.72-2.03; P < 0.001). Poor smoothness is a motor disturbance independent of movement amplitude and speed in cervical dystonia. Therefore, it should be assessed when evaluating this disease, its progression, and treatments.
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