Orthostatic tremor: an electrophysiological analysis.

Orthostatic tremor (OT) is a clinically defined syndrome of leg tremor while standing. Controversy surrounds whether OT is a distinct syndrome or is an essential tremor (ET) variant. We report two patients with OT. Electrophysiological testing included polymyography, accelerometry, nerve conduction, and evoked potential studies. The effects of various maneuvers and body positions on the tremor were assessed. The findings included rapid (15-17 Hz) lower-extremity tremor burst frequency evoked by standing but not by walking or swaying; rapid upper-extremity burst pattern synchronous with lower-extremity bursts; and failure of electrical stimulation or mental concentration to "reset" the tremor. Additionally, there was the novel finding of accelerometric recordings in the legs revealing the same rapid frequency (16-17 Hz) as the electromyographic tremor bursts. Some prior reports have suggested that OT is related to ET by emphasizing a considerable disparity and variability between the accelerometric tremor frequency and the electromyographic burst frequency. In our patients, however, the rapid (15-17 Hz) accelerometer-recorded tremor synchronous with the electromyographic bursts, and also the clinical improvement with clonazepam but not beta blockers or mysoline, and the lack of a family history of ET provide support that OT is distinct from ET.