Why are voluntary head movements in cervical dystonia slow?

Introduction: Rapid head movements associated with a change in fixation (head saccades) have been reported to be slow in cervical dystonia (CD). Such slowing is typically measured as an increase in time to complete a movement. The mechanisms responsible for this slowing are poorly understood. Methods: We measured head saccades in 11 CD patients and 11 healthy subjects using a magnetic search coil technique. Results: Head saccades in CD took longer to reach a desired target location. This longer duration was due to multiple pauses in the trajectory of the head movement. The head velocity of each segment of the (interrupted) head movement was appropriate for the desired total movement amplitude. The head velocity was, however, higher for the amplitude of the individual interrupted movements. These results suggest that brain programs the proper head movement amplitude, but the movement is interrupted by pathological pauses. Conclusion: Voluntary head saccades have a longer duration in CD due to frequent pauses. The frequent pauses reflect pathological interruptions of normally programmed intended head movement.