Biomechanical abnormalities in musicians with occupational cramp/focal dystonia.

Occupational factors and peripheral injuries are frequently implicated in the development of hand cramps and the syndrome of persistent manual incoordination among musicians and others most commonly given a diagnosis of focal limb dystonia. In an attempt to gain insight into the character and influence of risk factors in the evolution of this disorder, the authors conducted detailed evaluations of 33 individuals who responded to a questionnaire sent to university- and conservatory-level music schools in Germany in September 1989. Response was invited from any musician with complaints of impaired hand control. Of the 33 individuals accepted for evaluation, 18 were musicians who met clinical criteria for the diagnosis of occupational cramp/focal dystonia (OC/FD). Nineteen of the original 33 subjects underwent a quantitative biochemical assessment, comparing active and passive ranges of motion at all joints below the shoulder with those for cohorts of unimpaired musicians, matched for gender and musical instrument. Of the 19 tested biomechanically, 14 had OC/FD and the remaining five had either persistent pain or nonspecific movement idiosyncracies interfering with playing. Compared with the matched groups of normals, no consistent biomechanical abnormality was found in the non-OC/FD group; in the OC/FD group two thirds had marked limitation of passive and/or active abduction range between the central digits of both hands. Based on detailed training and performance histories in these subjects, the authors conclude that a specific biomechanical condition in the hand can interfere with certain high-speed digital movements required in musical instrument performance. Unintended muscle synergies, postures, and movement patterns can develop as attempts are made to increase the speed and fluency of such movements. As rehearsal is intensified, degraded movements are stabilized ("programmed"). In this situation, OC/FD appears to represent an aberrant outcome of normal motor learning whose physiologic correlates mimic neuropathologically based dystonia. The implications for prevention are discussed.