Primary writing tremor: a selective action tremor.

Progressive difficulty in handwriting due to jerking movements precipitated by the act of writing beginning between the ages of 8 to 54 is reported in six patients. There was no rest tremor, but three had mild postural tremor. Specific muscle activity (especially pronation of the wrist or abduction of the fingers) elicited the tremors that persisted as long as the evocative posture or muscle activity was maintained. None had a family history of tremors, but two had a history suggestive of hypoxia at birth. Unlike benign essential tremor, the movements did not respond to propranolol HCl, but most patients were benefited both acutely and chronically by centrally active anticholinergic agents.