Bilateral high-frequency synchronous discharges: a new form of tremor in humans.

OBJECTIVES To report bilateral high-frequency synchronous discharges in a patient with a sporadic form of olivopontocerebellar atrophy; to investigate the electromyographic pattern, the coherence and cospectral density across limbs, and the resetting effects of electrical stimulation over the posterior fossa; and to highlight the clinical, electrophysiologic, and radiologic features of this new form of tremor associated with posterior fossa disorders. DESIGN Case study of a patient clinically exhibiting a sporadic form of olivopontocerebellar atrophy associated with cerebellar and brainstem atrophy. SETTING Research unit, university hospital. MAIN OUTCOME MEASURES Electromyographic studies, coherence and cospectral density analysis, and calculation of a resetting index based on the timing of measured bursts and predicted bursts for an electrical stimulus given over the posterior fossa at increasing delays. RESULTS Surface electromyographic recordings in forearm muscles revealed a low-frequency postural tremor in the upper limbs, with episodes of highly coherent tremor at a frequency of 14 Hz. Squared coherence and cospectral density was strong between agonist and antagonist muscles in the left and right upper limbs and across limbs for the high-frequency discharges. Electrical stimulation over the posterior fossa reset the explosive high-frequency bursts. The resetting index was 0.82 Hz. CONCLUSIONS Our results show that bilateral high-frequency synchronous discharges may be associated with the sporadic form of olivopontocerebellar atrophy. Bilateral coherent bursting and resetting of this explosive postural tremor following electrical stimulation over the posterior fossa strongly suggest that the brainstem plays a key role in the network involved in the genesis of rhythmic bursts. We suggest that the high-frequency discharges are due to repetitive discharges in the reverberating cerebello-precerebellar circuits.