Focal dystonia: Current theories

Abstract

Dystonia is a syndrome characterised by abnormal involuntary sustained muscle contractions that often result in twisted and abnormal positions. Focal dystonia affects only a single body part with symptoms varying from permanent (e.g., torticollis) to task-specific (e.g., musician's cramp). The exact causes of focal dystonia have yet to be determined. Possible causative factors have been identified at all levels along the sensorimotor pathway, including anatomical constraints of the hand (musicians), abnormal co-contractions of the muscles due to reciprocal inhibition in the spinal cord, subcortical and cortical remapping, deficiencies in sensorimotor integration and perceptual deficits. A review of the current literature on these topics is provided with a special focus on musicians with focal dystonia. Also reviewed are current treatments of focal dystonia in musicians. On the basis of the currently available evidence, certain risk factors are identified for the development of task-specific focal dystonia, including number of practice hours, personality, genetic predisposition, performance factors and sensory effects. In addition, it is highlighted that dystonic movements occur predominantly in the context of perceptual-motor tasks involving emotions. When emotional and motor traces have become associated, they are difficult to change; it is suggested that this mechanism plays an important role in the preservation of dystonic symptoms.

Introduction

The term dystonia is used to describe a syndrome characterised by sustained muscle contractions, frequently causing twisting and repetitive movements, or abnormal postures (Fahn, 1998, Fahn et al., 1998, Fahn et al., 1987). In 1911, Oppenheim provided the first neurological description of hypotonic or spasmic muscle responses occurring at different times upon volitional movements. Focal dystonia can affect any part of the body including the arms and legs, trunk, neck, eyelids, face, or vocal cords (Berardelli et al., 1998, Deuschl and Hallett, 1998).

This syndrome may be classified according to three criteria: age of onset, etiology and distribution (Fahn et al., 1987, Fahn et al., 1998). According to Fahn (1998), classification by age of onset represents the best prognostic indicator as to whether there will be a spread of dystonic symptoms to other body parts. While it is acknowledged that an age-criterion is rather arbitrary, onset before 28 (median of 9 years) is classified as early-onset primary dystonia and thereafter as late-onset dystonia (median of 45 years).

Second, current classification for etiology divides dystonia into just two major categories, idiopathic or primary (including familial and sporadic forms), and symptomatic or secondary (Fahn, 1998). For an in-depth presentation of the subdivisions of dystonia by etiologic classification, see Fahn et al. (1998).

Third, dystonia may be classified according to its distribution: generalised dystonia involves at least one leg, the trunk and some other body part; hemidystonia affects one side; segmental dystonia affects adjacent body parts or a segment of the body; and focal dystonia affects a single body part (“Beschaftigungskrämpfe” as described by Oppenheim, cited in Deuschl & Hallett, 1998).

Many focal dystonias are named according to the affected region: spastic torticollis (cervical dystonia), blepharospasm (eye lids), oromandibular dystonia, lingual dystonia, spastic dysphonia, writer's or musician's cramp. For writer's or musician's cramp, the dystonic symptoms may affect a single finger up to an entire arm. It typically occurs without pain, although muscle aching can occur after prolonged spasms. Lack of pain distinguishes it from repetitive strain injury or occupational fatigue syndrome. Typically, the pain in the muscle associated with repetitive strain injury continues well after finishing the task.

Usually, these disorders do not progress beyond the focal, task-specific problem, although there are rare occasions where they do generalise to other tasks or other parts of the body. The precise symptoms and severity of writer's or musician's cramp vary from patient to patient. Dyscoordination and cramping are common complaints. Symptoms often start with holding the instrument (or pen), but become exaggerated when playing (or writing) begins (Chen & Hallett, 1998). Focal dystonia is sometimes further classified as simple or complex (Yazawa et al., 1999), depending on whether the symptoms only occur in one specific task (e.g., simple writer's cramp) or in multiple tasks (e.g., also when using utensils for eating, or when buttoning shirts).

According to Jankovic and Shale (1989), another characteristic of focal dystonia is tremor. Rosenbaum and Jankovic (1988) found task-specific tremors in 10 (36%) out of 28 patients, eight of whom had task-specific focal dystonia. Sheehy and Marsden (1982) found tremor in 14 (48%) out of 29 patients with various forms of (task-specific) focal dystonia.

The present paper reviews the current literature on the causes of dystonia with illustrative examples of musicians with focal dystonia. To date, explanations of focal dystonia have focussed on biomechanical limitations, peripheral neuropathy, spinal cord dysfunction, subcortical, diencephalic and cortical dysfunction, disturbed sensory feedback, sensory integration, remapping of the somatosensory cortex and decreased perceptual discrimination. Also reviewed are current treatments of focal dystonia in musicians. Finally, it will be argued that associated emotional and motor memories play an important role in the preservation of dystonic symptoms.