Hemimegalencephaly syndrome

Abstract

Hemimegalencephaly (HME) or unilateral megalencephaly is an uncommon but major congenital malformation of the brain, characterized by hamartomatous overgrowth limited to one cerebral hemisphere. HME differs from all other cerebral dysgeneses because of its extreme asymmetry not corresponding to any normal stage of human brain development. HME is a syndrome because it presents a recognizable constellation of multiple symptoms and signs, although the etiology is unknown. Immunocytochemical studies have shown that HME is an early disorder of cellular lineage, beginning at the time of the differentiation of neuroepithelium, but long before the initiation of neuroblast migration. HME can occur as an isolated disorder, but is more often associated with neurocutaneous syndromes. The three most commonly involved syndromes are epidermal nevus syndrome (ENS), Proteus syndrome, and Klippel–Trenaunay syndrome (KTS). A common pathogenesis for neurocutaneous syndromes can be related to abnormalities in the migration and differentiation of the neural crest cells. This new concept may explain many features of neurocutaneous syndromes and also the problem of overlapping manifestations.