Postnatal evolution of cortical malformation in the "non-affected" hemisphere of hemimegalencephaly.

A boy with epilepsy of neonatal onset was diagnosed with hemimegalencephaly (HME) based on the finding of an enlarged left cerebral hemisphere with dysplastic cortex over the frontal, parietal, and temporal lobes. Magnetic resonance imaging (MRI) showed an essentially unremarkable gyration pattern in the contralateral hemisphere. Ictal electroencephalography and magnetoencephalography revealed epileptic foci in the left hemisphere. Hyperperfusion and hypermetabolism were noted in comparison to the contralateral hemisphere, and these findings were consistent with the diagnosis of HME. Soon after hemispherotomy was performed at age 3 months, an epileptic focus appeared in the right hemisphere. The epilepsy remained intractable. Unexpectedly, dysplastic changes emerged over the right frontoparietal cortex at age 7 months as shown by MRI, and the cortical volume in this area was observed to be markedly increased at age 1 year and 8 months. This increase was associated with a distorted gray-white matter boundary. These findings contribute to our understanding of the pathogenesis of HME and may be helpful in determining the management approach for epilepsy in this entity.