Essential tremor and cerebellar dysfunction: abnormal ballistic movements.

BACKGROUND Clinical characteristics reminiscent of cerebellar tremor occur in patients with advanced essential tremor. Ballistic movements are known to be abnormal in cerebellar disease. The hypothesis was proposed that ballistic movements are abnormal in essential tremor, reflecting cerebellar dysfunction. OBJECTIVE To elucidate the role of the cerebellum in the pathophysiology of essential tremor. METHODS Kinematic parameters and the triphasic electromyographic (EMG) components of ballistic flexion elbow movements were analysed in patients assigned to the following groups: healthy controls (n = 14), pure essential postural tremor (ET(PT); n = 17), and essential tremor with an additional intention tremor component (ET(IT); n = 15). RESULTS The main findings were a delayed second agonist burst (AG(2)) and a relatively shortened deceleration phase compared with acceleration in both the essential tremor groups. These abnormalities were most pronounced in the ET(IT) group, which had additional prolongation of the first agonist burst (AG(1)) and a delayed antagonist burst (ANT). CONCLUSIONS Abnormalities of the triphasic pattern and kinematic parameters are consistent with a disturbed cerebellar timing function in essential tremor. These abnormalities were most pronounced in the ET(IT) group. The cerebellar dysfunction in essential tremor could indicate a basic pathophysiological mechanism underlying this disorder. ET(PT) and ET(IT) may represent two expressions within a continuous spectrum of cerebellar dysfunction in relation to the timing of muscle activation during voluntary movements.