A retrospective study of the clinical and electrophysiological characteristics of 32 patients with orthostatic myoclonus

Objectives: To review the electrophysiological and clinical characteristics of 32 patients with orthostatic myoclonus (OM), a relatively newly identified movement disorder, and compare these characteristics to those of primary orthostatic tremor (OT) patients and patients with similar gait and balance complaints without either hyperkinesia diagnosed during the same 30-month period.

Methods: The database of the Mayo Clinic Florida Movement Disorders Electrophysiology Laboratory (MDEL) was searched for all patients referred for possible OM or OT from 6/2010 to 12/2012. All available clinical records and archived surface electromyographical data for these patients were reviewed and analyzed.

Results: 32 patients with OM (mean age 74 years), 8 with primary OT (mean age 71), and 55 with neither orthostatic hyperkinesia (NOH) (mean age 68) were identified. All OT patients and 84% each of OM and NOH patients complained of involuntary leg movements while standing, e.g., "shaking," "trembling," or "jerking." All OM and OT patients experienced symptomatic and electrophysiological abatement or attenuation of their leg hyperkinesias by leaning forward onto an object while standing.

Conclusions: OM has some similarities to OT, including causing "shaky legs" subjectively in standing older patients. Novel data from this work include that, as in OT, OM essentially abates when patients remove their weight from their legs. This shared isometric phenomenon may reflect that OT and OM are on a pathophysiological continuum. Further, many patients who complain of their legs "shaking" while standing may have neither OT nor OM. Surface electromyography may be a useful adjunct in extrapolating patients complaining of "shaky legs."