Monozygotic twins with severe myoclonic epilepsy in infancy discordant for clinical features.

Male monozygotic twins with genetically determined severe myoclonic epilepsy in infancy are described. Although seizure onset, clinical seizure symptomatology, and motor and mental development were almost identical until age 38 months, their clinical courses then became discordant. The emergence of myoclonus was delayed by 12 months in twin 1 compared with twin 2. Regression in language development, which is a common feature of severe myoclonic epilepsy in infancy, was obvious in twin 2 after the emergence of myoclonus, whereas twin 1 did not demonstrate any regression. The clinical-course discordance between twins was attributable to bacterial meningitis, which twin 1 developed at age 35 months. Bacterial meningitis may have affected the clinical course of severe myoclonic epilepsy in infancy in twin 1, resulting in delayed onset of myoclonus and more favorable language development in twin 1 than in twin 2, who did not experience bacterial meningitis.