Orthostatic myoclonus - A retrospective study of Asian patients.

Background/purpose: Orthostatic myoclonus (OM) is myoclonic jerks in both legs during standing. It may relate to gait unsteadiness and may be associated with various diagnoses, including neurodegenerative, systemic, and musculoskeletal diseases. Diagnosis is based on the surface electromyography (SEMG).

Methods: From January 2016 to June 2020, we retrospectively reviewed 35 patients diagnosed with OM based on the SEMG criteria and analyzed the electrophysiological data.

Results: The mean age was 75.3±8.9. Disease duration ranged from 2 days to several years. Genders were equally affected. The most common symptom was gait disturbance, and the next was leg tremulous sensation, followed by backward disequilibrium. 28.6% of our patients had falls. Electrophysiologically, bursting duration shorter than 100 ms supported the myoclonic origin from the cortex. The bursting activity occurred only on the upright and weight-bearing leg. The associated diagnoses included lumbosacral radiculopathy (28.6%), lumbosacral radiculopathy plus myasthenia gravis (2.9%), lumbosacral radiculopathy plus vascular parkinsonism (5.7%), diabetic polyneuropathy (5.7%), vascular parkinsonism (17.1%), Parkinson's disease (PD) (14.2%), normal pressure hydrocephalus (5.7%), medication-induced parkinsonism (2.9%), cervical myelopathy (2.9%), medication-related myoclonus (2.9%), and unknown (11.4%). In PD, OM was present before, along with, or after PD diagnosis. The myoclonic activity disrupted the parkinsonian tremor upon standing on SEMG.

Conclusion: OM is a syndrome instead of a diagnosis. It is more prevalent in the elderly with gait disturbance and patients will not necessarily fall. It is associated with central and peripheral nerve system disorders, systemic diseases, and medication use. We hypothesize that OM is a pathological disintegration of motor circuit.