Is writer's cramp caused by a deficit of sensorimotor integration?

Writer's cramp is a highly specific movement disorder in which handwriting is impaired while most other manual skills are often unaffected. On the basis of abnormal findings in experiments measuring the control of grip forces, it has been suggested that writer's cramp is caused by a deficit of sensorimotor integration. The aim of our study was to determine whether there is a functional link between sensory deficits, abnormalities in the control of grip force, and handwriting disorders. We compared the grip force and handwriting performance of writer's cramp patients with that of control subjects and with that of a stroke patient suffering a purely somatosensory deficit of his dominant hand (patient S1). We found that: (1) writer's cramp patients and patient S1 had elevated grip-force levels; (2) training reduced the grip force to near-normal levels in all writer's cramp patients but not in S1; (3) effortful writing performance also induced increased grip-force levels in healthy subjects; and (4) patient S1 had normal handwriting movements. These findings suggest that the elevated pretraining gripforce levels of writer's cramp patients might be a consequence of their effortful writing style and do not reflect a deficit of sensorimotor integration. Moreover, the good handwriting performance of patient S1 shows that a severe somatosensory deficit is not a sufficient condition for a handwriting disorder. These findings disagree with the sensorimotor explanation of writer's cramp.