Cervical Dystonia: Effect of botulinum toxin on trajectory formation

Objective: To ascertain whether the abnormal processing of information from muscle spindles present in asymptomatic muscles of patients with focal dystonia impair trajectory formation and whether botulinum toxin has restorative effects. Method: Ten patients with cervical dystonia (CD) (age range: 35−65 years) were tested with a motor task before (baseline) and 3 weeks after (Test) botulinum toxin injections in the overactive neck muscles. Ten aged-matched normal controls were tested twice three weeks apart. During the motor task, subjects performed out-and-back reaching movements on a digitizing tablet from a central starting point to one of eight targets displayed on a computer screen. Screen cursor was blanked and vision of the limb was blocked to prevent corrections during movement. Results: At baseline, hand-paths of controls were straight, with sharp reversals and overlapping out-and-back strokes. In patients with CD hand paths were more curved (p < 0.001) with increased area (p < 0.001) and longer reversal lags (time between end of the out motion and the beginning of the return, p < 0.0001) and increased asymmetry in the temporal velocity profile (p < 0.001). Botulinum treatment improved head posture in all patients. At test, hand-paths curvature, normalized areas, reversal lags and symmetry indexes decreased in CD although without reaching the controls’ range. Conclusion: Patients with CD, without clinically evident involvement of the upper limbs, display defects in trajectory formation that partially recover after treatment. Altogether, these findings suggest the presence of abormal interjoint coordination. They support the hypothesis that CD impairs the processing of proprioceptive input from upper limb. Treatment with botulinum toxin is helpful in restoring sensory processes.