Dystonic contractions are often aggravated by purposeful actions and may be specific to a particular task. A patient may have dystonia when using the hand for writing but not for other tasks such as eating or typing. Occupational dystonias are those that occur in individuals with a particular occupation requiring repetitive and excessive fine motor activity. Most of these dystonias are task specific and fall under the rubric of primary focal dystonias. The occupations especially prone to have focal task-specific dystonia are listed inTable 9.1.
In this chapter we discuss in detail writer's cramp and musician's cramp, the two most common occupational dystonias, followed by a brief mention of other focal occupational dystonias.
Although the exact cause of these focal dystonias is not yet elucidated, it seems that an interaction of proprioceptive, behavioral, genetic, gestural, environmental, and psychological factors plays a role. Excessive activation of antagonists, overflow into synergists, and prolongation of muscle activation are thought to reflect deficiency of premotor cortical network inhibition (Hallett, 2000, 2006a, b). Decreased levels of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) are present in the contralateral sensorimotor cortex and lentiform nucleus in patients with writer's cramp (Levy & Hallett, 2002). Functional magnetic resonance imaging shows impaired activation of the primary sensorimotor and supplementary motor cortex during voluntary muscle relaxation and contraction (Oga et al., 2002). A genetic factor in the development of hand dystonia is possible, as up to 20% of patients with writer's cramp have family members with dystonia.