VOLUME 29, ISSUE 4 • DECEMBER 2025.
Myoclonus is a sudden, brief, shock-like, involuntary movement caused by abnormal neuromuscular contraction or inhibition. Physiologic myoclonus such as startle response, hiccups, and sleep myoclonus, typically occurs in healthy people without disability or progression. A new under-recognized manifestation of myoclonus, which might be defined “benign idiopathic myoclonus” (BIM), was identified.
Adolescent patients with a clinical diagnosis of isolated distal myoclonic jerks and a subjective complaint of mild/moderate jerky movements of the hands, in absence of clinical progression, underwent clinical and neurophysiological evaluations. The exclusion criteria were presence of other neurological signs and the use of drugs known to induce myoclonus i.e., serotonin reuptake inhibitors, antiepileptic drugs that enhance GABAergic transmission, tricyclic antidepressants. Fifteen patients (4 men [26.7%]; age at onset, 18.1±3.6 years; disease-duration, 5.3±3.7 years) were assessed.
All patients did not show any positive family history. Laboratory tests investigating treatable causes of myoclonus (e.g., glucose, renal, hepatic, and thyroid function tests) and additional brain MRI were performed: All tests came back negative. Patients underwent genetic testing, mostly consisting of genetic panels based on whole-exome sequencing (WES) with subsequent analysis of the relevant genes, with negative results. Neurophysiological examinations, including EMG, somatosensory evoked potentials (SEPs), EEG-EMG witch back-averaging (BA), and cortico-muscular coherence (CMC) confirmed the clinical diagnosis of myoclonus in all patients. All patients had an EMG burst duration of <100 ms with a mean burst duration of 63 ms (±13.7, range 39-95 ms), and a cortical correlate was demonstrated by BA or CMC or by the presence of giant SEPs in six patients.
These findings suggest a cortical myoclonus in some but not all patients, indicating a possibly heterogeneous origin of BIM. Follow-up at 0.5-8 years depicted clinically stable conditions or complete remission in all patients, except one that reported slight progression.
The results suggest the existence of a new nosologically myoclonus entity that might be called “benign idiopathic myoclonus.” These jerky movements manifest predominantly in women during the second decade, with a prevalent distal upper limb distribution and benign course and prognosis in nearly all cases without requiring pharmacological treatment. The possible etiological causes were ruled out and no triggers were identified.
If a movement disorder specialist recognizes clinically mild or moderate distal myoclonus, in absence of other neurological signs, especially during the second decade of life, BIM should be considered. Routine laboratory tests should be done to rule out treatable causes and a clinical-follow up is recommend. As the disease course seems favorable, with no need for medication at follow-up in nearly all cases, a therapeutic management is recommended only on patients’ complaints about the interference of jerky movements on daily activities. Moreover, the possibility of spontaneous improvement or complete remission of complaints should be shared with patients. This phenotype might represent a particular subgroup of physiologic myoclonus, to be substantiated in multicenter cohorts. This new observation may shed light on a newly defined subtype of myoclonus requiring specific and limited diagnostic and therapeutical management.
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