Orthostatic myoclonus: an underrecognized cause of unsteadiness?

Recently, orthostatic myoclonus (OM) has been suggested as a cause of gait impairment and unsteadiness in neurodegenerative diseases. The aim of this study was to investigate the frequency of orthostatic myoclonus, its clinical characteristics and the underlying associated neurological disorders.

A retrospective analysis of clinical data and electromyogram surface recordings from subjects with unexplained unsteadiness/gait impairment was performed. Diagnosis of OM was made when a pattern of non-rhythmic bursts was observed (duration range 20-100 ms; bursts per second ≤16).

Among 93 subjects studied, OM was the most frequent disorder (n = 16; 17.2%), followed by orthostatic tremor (13.9%) and low frequency tremors during orthostatism (12.9%). All patients with OM complained about unsteadiness during orthostatism and/or during gait. Leg jerking was only observed by visual inspection during orthostatism in four subjects and two also presented falls. Eleven out of 16 patients (68.7%) with OM had an associated neurodegenerative disease, such as multiple system atrophy (n = 3) Parkinson's disease (n = 2), Alzheimer's disease (n = 2), mild cognitive impairment (n = 2) and normal pressure hydrocephalus (n = 2). Although four subjects showed improvement of orthostatic myoclonus with antimyoclonic treatment, the follow-up was not systematic enough to evaluate their therapeutic effect on OM.

Orthostatic myoclonus is often underdiagnosed and can be considered a possible cause of unsteadiness in subjects with neurodegenerative diseases. Electromyography surface recording is thereby an aid for investigating unsteadiness of unknown origin.