ORTHOSTATIC TREMOR - AN ELECTROPHYSIOLOGICAL ANALYSIS

Orthostatic tremor (OT) is a clinically defined syn drome of leg tremo r while standing. Controversy surrounds whether OT is a distinct syndr ome or is an essential tremor (ET) variant. We report two patients wit h OT. Electrophysiological testing included polymyography, acceleromet ry, nerve conduction, and evoked potential studies. The effects of var ious maneuvers and body positions on the tremor were assessed. The fin dings included rapid (15-17 Hz) lower-extremity tremor burst frequency evoked by standing but not by walking or swaying; rapid upper-extremi ty burst pattern synchronous with lower-extremity bursts; and failure of electrical stimulation or mental concentration to ''reset'' the tre mor. Additionally, there was the novel finding of accelerometric recor dings in the legs revealing the same rapid frequency (16-17 Hz) as the electromyographic tremor bursts. Some prior reports have suggested th at OT is related to ET by emphasizing a considerable disparity and var iability between the accelerometric tremor frequency and the electromy ographic burst frequency. In our patients, however, the rapid (15-17 H z) accelerometer-recorded tremor synchronous with the electromyographi c bursts, and also the clinical improvement with clonazepam but not be ta blockers or mysoline, and the lack of a family history of ET provid e support that OT is distinct from ET.