Abstract
Myoclonus in infancy is often benign; however, persistent or progressive forms may indicate an underlying neurologic disease, including genetic movement disorders such as myoclonus-dystonia syndrome, a rare genetic condition most often associated with pathogenic variants in the Epsilon-Sarcoglycan (SGCE) gene. Early-onset cases are particularly challenging due to overlap with benign, epileptic, and metabolic conditions. We report a six-year-old girl with myoclonus beginning at 12 months of age, involving her head, neck, and arms, and a delayed onset of mild dystonia at five years. Neurological examination, neuroimaging, electroencephalography, and metabolic investigations were unremarkable. Genetic testing identified a heterozygous pathogenic SGCE variant, confirming the diagnosis. Trials with zonisamide, carbamazepine, sodium valproate, and clonazepam were ineffective; however, motor and cognitive development remained within the average range. Behavioral and attentional difficulties emerged later in childhood. This case highlights the diagnostic challenges of early-onset SGCE myoclonus-dystonia presenting as isolated myoclonus. Genetic testing was essential for diagnosis, pharmacologic response was limited, and functional outcome remained favorable. The later emergence of behavioral symptoms underscores the need for long-term multidisciplinary follow-up.