Restoring balance in focal limb dystonia with botulinum toxin.

Focal task-specific dystonia of the hand is rare in the general population, where it usually manifests as writer's cramp, but seems relatively common among musicians. The disability may be so severe as to prevent writing altogether or to end a professional musician's career. The cause is usually unknown but it is thought to be primarily a basal ganglia disorder with dysfunction of cortical-striatothalamic-cortical circuits. Abnormalities have been found in cortical movement preparation, intracortical inhibition, sensory and motor maps, and patterns of cortical activation during movement. Much evidence supports disordered processing of sensory information with disturbed sensorimotor integration. Underlying this may be maladaptive neural plasticity mechanisms. Treatment is difficult. Oral medications are generally ineffective and have troublesome side-effects. Intensive rehabilitation techniques based on neural plasticity theory show promise but are rarely available and are time-intensive. Botulinum toxin injections appear to be effective in writer's cramp and musician's dystonia, at least initially; long-term benefit is less common. Despite definite improvement, some patients abandon treatment because the gain is insufficient for meaningful function: this is particularly so for musicians. Much of the benefit from botulinum toxin injection comes from simply reducing muscle overactivity through muscle paralysis, restoring balance to motor control. However, some evidence suggests that botulinum toxin injections can produce transient improvement in some of the various cortical abnormalities described, probably through alteration of sensory input from the periphery, by direct and indirect means. These changes in cortical function might be usefully combined with those brought about by sensorimotor retraining programs, but such studies are awaited.