An 11-year-old girl with nonketotic hyperglycinemia who typically presented with a picture of early myoclonic encephalopathy in the neonatal period is presented in this article. Treated early with sodium benzoate and dextromethorphan, she became seizure-free, while myoclonus persisted. During examination, multifocal rhythmic myoclonic jerks in gamma frequency enhanced by motor activity were noted. Coherence analysis of the electroencephalography−electromyography relationship indicated a cortical origin of the myoclonic jerks. Observation of this case suggests that rhythmic cortical myoclonus may represent a late evolution of this rare disorder.
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