Hemimegalencephaly: a fetal case with neuropathological confirmation and review of the literature.

Hemimegalencephaly (HME) is a developmental abnormality of the central nervous system, identified by an abnormal increase in the size of one cerebral hemisphere. HME may present as either a syndromic or isolated case. To date the literature on HME has focused primarily on non-fetal pediatric patients, largely related to surgical resection specimens of the HME hemisphere. We present the case of a male fetus at 22 weeks gestation with intracranial abnormalities identified on a follow-up ultrasound. Gross examination of the fetal brain confirmed the increased size of the right cerebral hemisphere. The ipsilateral brain stem and cerebellum were not involved. Light microscopy demonstrated the presence of accelerated cortical differentiation along with several migrational anomalies in the HME hemisphere. Based on the gross and microscopic findings, a diagnosis of fetal hemimegalencephaly was made. The periventricular proliferative zone of the abnormal hemisphere contained a normal population of neuroepithelial precursor cells. An exhaustive immunohistochemical study found immunoreactivity for calretinin and synaptophysin, while the Ki-67 proliferation labeling was not increased in the HME hemisphere. Our case is the first autopsied report on fetal hemimegalencephaly and confirms that the key pathogenic changes may present as early as 20-22 weeks gestation. The major pathological features of our case are in keeping with a disturbance in accelerated neuronal differentiation and migrational abnormalities.