Congenital brainmalformation occurring during embryonic development in which the two halves of the prosencephalon or forebrain fail todivide properly into left and right hemispheres, the outcome being a single-lobed brain . Four types include alobar (no hemispheric division usually associated with severe facial defects, e.g., cyclopia, a single eye, and stillbirth), semi-lobar (some hemispheric division and intermediate in its effects), and lobar (clear separation of hemispheres and least severe in terms of effects, e.g., cleft palate)), as well as a variety of middle interhemispheric variants. The causes of the malformation are unknown, but there is recent evidence to suggest that it could be associated with mutations in the sonic hedgehog gene The incidence is between 1 in 16,000 and 1 in55,000.
See Congenital abnormalities, Neurulation, Prosencephalon