Primary writing tremor.

We describe a patient who complained of jerking of the right forearm on writing. Active pronation of his arm produced several beats of pronation/supination tremor. A burst of tremor also could be elicited by tendon taps to the volar surface of the wrist, to the finger extensors, and to pectoralis major, and by forcible supination of the wrist delivered by a torque motor. The subject's writing difficulty and tremor were temporarily abolished by partial motor point anaesthesia of pronator teres. We conclude that the tremor was caused by an abnormal response to muscle spindle input from pronator teres.