Orthostatic Myoclonus-Clinical and Electrophysiological Features in a Large Retrospective Cohort

Orthostatic myoclonus is characterized by irregular, lower limb myoclonic bursts during stance and is a major cause of postural instability and falls. However, studies are limited, and little is known about its pathophysiology.

We sought to define the clinical and electrophysiological features of orthostatic myoclonus in a large, single-center cohort.

We included 42 participants (24 males, 18 females) with a mean age of 74 years (range, 46-93) from Westmead Hospital presenting with orthostatic myoclonus from 2007 to 2023. Medical records were retrospectively reviewed for demographic details, symptoms, co-morbidities, and treatment. Lower limb surface electromyography (EMG) was analyzed using a custom-designed algorithm to automatically identify myoclonic bursts and measure their duration, synchronicity, and rhythmicity. Differences in burst parameters between muscles and associations between burst parameters and clinical characteristics were statistically evaluated.

Mean burst durations during standing were 77 to 90 ms across lower limb muscles. Maximum burst activity and bilateral synchronicity occurred in tibialis anterior. Only 12% of participants exhibited any rhythmicity. A total of 79% of participants had a coexistent neurological disorder including 26% with parkinsonism. There was no significant association between parkinsonism and burst parameters. However, there was a significant, inverse correlation between the presence of neuropathy or radiculopathy and synchronous activity (P = 0.02).

We provide a computationally robust clinical and electrophysiological analysis of orthostatic myoclonus in a large cohort. Our findings support the theory of a subcortical generator arising from protean secondary causes and subject to peripheral modulation. Further work is needed to clarify treatment outcomes.