Irregular jerky tremor, myoclonus, and thalamus: a study using low-frequency stimulation.

High-frequency thalamic stimulation alleviates tremor in Parkinson's disease (PD) and essential tremor (ET). The origin of thalamic myoclonus is unexplained and the effects of low-frequency thalamic stimulation on movement control are still unknown. We studied the effects of stimulation at a low frequency of 15 Hz in five drug-free patients (3 PD, 2 ET) 6 months after thalamic implantation of quadripolar electrodes (unilateral in four patients, bilateral in one patient). Clinical, electrophysiological, and videotaped assessment, using a monopolar 15 Hz frequency (3 V, 90 micros) stimulation current applied simultaneously through two adjacent contacts of the electrode, was performed. We observed myoclonus and irregular jerky tremor in the upper limb contralateral to the site of stimulation. The jerks lasted less than 200 ms, were irregular and not synchronous with stimulation, were superimposed on rest or postural tremor, and increased in response to tactile, proprioceptive, or vibratory stimuli. The fact that this complex movement disorder can be induced by low-frequency stimulation in the ventral intermediate nucleus (Vim) of the thalamus suggests that it results, at least partly, from dysfunction of the Vim and possibly adjacent nuclei of the thalamus.