Pathophysiological differences between musician's dystonia and writer's cramp

Focal hand dystonia (FHD) has been suggested to be a maladaptive response of the brain to repetitive performance of stereotyped and attentionally demanding hand movements. However, not all patients with FHD have a strict history of excessive hand use; for example, patients with musician's dystonia (MD) spend many hours per day with their attention focused on instrumental practice, whereas many patients with writer's cramp (WC) have a history of average hand use. The present experiments test whether seven MD and six WC patients have different pathophysiological deficits by examining the spatial pattern of sensorimotor organization in the motor cortex. Two control groups were used, eight healthy non-musicians and eight healthy musicians. The latter served to control for physiological adaptation of the brain to musical training. We used focal vibration of a single hand muscle to produce sensory input whilst the excitability of corticospinal outputs to the vibrated and other hand muscles was evaluated with transcranial magnetic stimulation. In healthy non-musicians, vibration increases the amplitude of motor-evoked potentials and decreases the short-latency intracortical inhibition (SICI) in the vibrated muscle, whilst having the opposite effect on the non-vibrated hand muscles. The pattern of sensorimotor interaction was abnormal in both patient groups. However, the nature of the deficit differed between them. While vibration had little effect on cortical excitability in WC, it strongly reduced SICI in all hand muscles irrespective of spatial organization in MD. In the healthy musicians we found an organization intermediate between that of healthy non-musicians and MD. The data are consistent with a model in which musical practice in healthy musicians leads to beneficial changes in organization of the motor cortex, but in MD these progress too far and begin to interfere with movement rather than assist it. The fact that sensory input had no effect on motor output in patients with WC suggests that sensory information from the hand may play a smaller role in provoking pathological changes in WC than in MD.