Hemimegalencephaly--morphological and immunocytochemical study.

Hemimegalencephaly (HME), a rare congenital abnormality characterized by unilateral enlargement of the cerebral hemisphere, is one of the less common causes of intractable seizures. We report a 6-month-old infant with uncontrolled seizures who was diagnosed to have a large mass lesion based on a CT scan. Postmortem examination revealed left-sided HME with pachygyria, widened cortex, indistinct grey-white junction, and distorted deep nuclear masses. Histological features included loss of cortical lamination, large atypical neurons with argyrophilic accumulations, ballooned cells, neuronal heterotopia, and astrocytosis with dystrophic calcification. The heterotopic neurons in the white matter were present in a radial pattern suggestive of aberrant neuronal migration. Several large neurons were dystrophic with cytoskeletal abnormalities like phosphorylated high molecular weight neurofilament and ubiquitin in the cytoplasm. However, typical neurofibrillary tangles with Congo red and tau positivity were not observed. Synaptophysin labelling was found to be decreased in the cortex, but some of the abnormal neurons had dense perisomatic label. The majority of the balloon cells were astrocytic in origin, being positive for glial fibrillary acidic protein and negative for the neuronal markers. Although the etiology of HME is not known, it provides an opportunity to study anomalous development of the brain and neuronal developmental abnormalities.