Modulation of primary orthostatic tremor by magnetic stimulationover the motor cortex

OBJECTIVES—To study the role of corticomotorneuronal pathways in primary orthostatic tremor.
METHODS—Transcranial magnetic stimuli at anintensity 10% above the resting motor threshold were delivered overthe leg motor cortex in two patients with primary orthostatic tremorwhile standing still. Electromyographic responses in both tibialisanterior muscles were recorded after 20 stimuli given randomly atintervals of 120 to 180 seconds. Differences between predictedand actual times of occurrence of tremor bursts after the stimuli wereused to calculate a resetting index, with a value of 0 representing no resetting and a value of 1 representing complete resetting.
RESULTS—Transcranial magnetic stimulation evokedEMG responses in both tibialis anterior muscles, followed by transientsuppression of tremor before reappearance of rhythmic EMG activity.Analysis of the timing of tremor bursts from EMG recordings before and after the magnetic stimuli disclosed that the phase of orthostatic tremor could be reset by brain stimulation (mean resetting indices 0.93 and 0.82).
CONCLUSION—The results suggest that a centraloscillator, involving the motor cortex, has a crucial role in eitherthe generation or modulation of orthostatic tremor.