An Open Label Evaluation of MIDI to Quantify Performance Change in Subjects With Musician's Dystonia After Treatment With Botulinum Toxin Type B (Myobloc ®).

Dystonia represents a group of clinical disorders characterized by various combinations of sustained involuntary muscle contractions, abnormal postures and movements, tremors and pain. Dystonia can occur at rest but is more likely to appear during voluntary activity.

Focal dystonia affects one body area and includes blepharospasm, oromandibular dystonia, spasmodic dysphonia, torticollis, and limb dystonia. Focal dystonia typically presents as task-specific muscle spasms or "occupational cramps" in which learned or repetitive motor tasks (such as writing or playing a musical instrument) trigger muscle spasms and interfere with performance while other actions remain normal. Writer's cramp is the most common form of idiopathic limb dystonia [1-3] where involuntary muscle activity and abnormal postures affect the arms and hands, but virtually any part of the body may be affected, even the lips when playing a woodwind or brass instrument [4]. Patients may develop two focal dystonias but rarely does focal dystonia progress to more generalized forms.

As originally defined by Oppenheim [5], dystonia refers to the slow, sustained, writhing, contorting movements of dystonia musculorum deformans. Dystonic movements, however, are often rapid [6] and this can be a cause for misdiagnosis. Electromyography (EMG) may be helpful in corroborating dystonia, but is not essential for diagnostic purposes. Nerve conduction studies, short and long loop reflexes and analysis of motor units are normal [7, 8]. Ballistic movements, which are normally tri-phasic in pattern with alternating agonist-antagonist bursts, may show disrupted patterns with co-contraction of agonist and antagonist muscles and excessively long EMG bursts in dystonia [3].

Dystonic spasms are intriguing in that they may be suppressed (or triggered) by sensory input such as postural change, tactile stimuli, alternative movements or even thought processes [9]. Studies are revealing that the involuntary muscle spasms may be due, at least in part, to abnormal sensory processing of spindle afferent information [10-12]. This may help explain the nature of these sensory "tricks" as well as why the effect of treatment using botulinum toxin usually outlasts the weakness it creates.

Though the pathophysiology of musicians' dystonia has yet to be determined fully, the motor learning associated with playing a musical instrument probably results in both functional and structural changes in the brain [13]. This plastic reorganization, including the rapid unmasking of existing neural circuitry and the establishment of new connections, is probably fundamental to the accomplishment of skillful playing, but also may result in focal, task-specific dystonia. When musicians get dystonia, their playing abilities can become severely compromised, to the point where they may not be able to perform professionally, and possibly not even teach. While botulinum toxin injections can be highly successful in allowing musicians to perform again, there are no objective methods to evaluate improvement.