MODULATION OF PRIMARY ORTHOSTATIC TREMOR BY MAGNETIC STIMULATION OVERTHE MOTOR CORTEX

Objectives-To study the role of corticomotor neuronal pathways in prim ary orthostatic tremor. Methods-Transcranial magnetic stimuli at an in tensity 10% above the resting motor threshold were delivered over the leg motor cortex in two patients with primary orthostatic tremor while standing still. Electromyographic responses in both tibialis anterior muscles were recorded after 20 stimuli given randomly at intervals of 120 to 180 seconds. Differences between predicted and actual times of occurrence of tremor bursts after the stimuli were used to calculate a resetting index, with a value of 0 representing no resetting and a v alue of 1 representing complete resetting. Results-Transcranial magnet ic stimulation evoked EMG responses in both tibialis anterior muscles, followed by transient suppression of tremor before reappearance of rh ythmic EMG activity. Analysis of the timing of tremor bursts from EMG recordings before and after the magnetic stimuli disclosed that the ph ase of orthostatic tremor could be reset by brain stimulation (mean re setting indices 0.93 and 0.82). Conclusion-The results suggest that a central oscillator, involving the motor cortex, has a crucial role in either the generation or modulation of orthostatic tremor.