BACKGROUND: Myoclonus is a brief shock-like, involuntary movement, which can be distinguished in physiologic, essential, epileptic, and symptomatic, according to its etiology. Physiologic myoclonus typically occurs in healthy people without disability or progression. OBJECTIVES: We suggest a new nosological entity in the physiologic group: "benign idiopathic myoclonus." METHODS: We present a cohort of patients with isolated adolescent-onset, distal limb myoclonus at rest and during action, in absence of a known cause and disabling progression, who underwent both clinical and neurophysiological examination in our tertiary Movement Disorders Expertise Center Groningen. RESULTS: Fifteen patients (4 men [26.7%]
age at onset, 18.1 ± 3.6 years
disease-duration, 5.3 ± 3.7 years) were assessed. Neurophysiological examinations, including electromyography (EMG) (n = 14), somatosensory evoked potentials (SEPs, n = 4)
electroencephalography (EEG)-EMG with back-averaging (BA, n = 11) and cortico-muscular coherence (CMC, n = 10), confirmed the clinical diagnosis of myoclonus in all patients. Mean EMG burst duration was 62.6 ± 13.7 ms and a cortical origin of myoclonus was demonstrated in six cases (40%). No genetic causes were found. Follow-up at 0.5 to 8 years depicted clinically stable conditions in eight patients (61.5%), complete remission in four (30.8%), whereas one patient (7.7%) reported slight progression. CONCLUSIONS: We suggest a new phenotype of physiologic myoclonus, which might be called "benign idiopathic myoclonus." It is characterized by distal myoclonus with onset during adolescence and benign course, without requiring treatment. Clinically and neurophysiologically these jerky movements were compatible with cortical myoclonus in some patients. We were unable to establish any genetic causes in explored cases. This phenotype might represent a particular subgroup of physiologic myoclonus, to be substantiated in multicenter cohorts.