Eye-hand coordination in essential tremor

Patients with essential tremor (ET) or with cerebellar lesions have in common oculomotor abnormalities, with the exception of saccadic eye movements, which do not seem to be involved in ET. Since grasping is prolonged in ET and might be related to saccadic dysmetria, we tested whether simultaneous hand pointing could unmask it. Twelve ET patients and 14 controls performed saccades with and without simultaneous pointing movements to the same targets, and with and without a gap between the disappearance of the fixation point and the appearance of the target. Eye movements were recorded with the magnetic search-coil method, hand movements with an ultrasound-emitting probe. ET patients did not have saccadic dysmetria, and contrary to normal subjects their saccadic latency did not decrease during combined eye-hand movements compared with saccades performed in isolation. Hand movements had a longer duration in ET patients, with decreased peak acceleration, an increased latency of the peak velocity, and peak deceleration. In conclusion, this first study on eye-hand coordination in ET revealed abnormal kinematic changes in the early phase of pointing movements. These changes might be related to cerebellar disease but they are independent of the intention tremor component and saccade performance.