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聽ms; bursts per second 鈮?6).
Among 93 subjects studied, OM was the most frequent disorder (n聽=聽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聽=聽3) Parkinson's disease (n聽=聽2), Alzheimer's disease (n聽=聽2), mild cognitive impairment (n聽=聽2) and normal pressure hydrocephalus (n聽=聽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.