In idiopathic cervical dystonia movement direction is inaccurate when reaching in unusual workspaces

When reaching movements are performed in an unusual area of work, normal subjects produce a rightward directional error. This has been considered to be caused by an impaired representation of limb configuration, which hampers the actual movement vector. Motor programming has been found to be impaired in dystonia. To understand how patients affected by idiopathic cervical dystonia (CD) perform reaching movements in an unusual area of work, we investigated 10 CD patients and 10 age-matched controls. Reaching movements on a digitized tablet were recorded both with the right arm aligned to the midline (central position) and shifted to the right (lateral position), but hidden from view. While differences in the main kinematic parameters were not affected by the position both in patients and controls, the directional error was significantly increased in dystonic patients for the lateral position. We hypothesise that an impaired integration of proprioceptive information with the motor output and egocentric spatial perception could be responsible for a greater error in spatial representation of hand location and consequently to an increased directional error in dystonic patients.