Definition

Holoprosencephaly is a birth defect caused by failure of the forebrain (prosencephalon) to grow as two separate hemispheres in the first few weeks of fetal life. The more complete the failure to divide, the worse the resulting abnormalities of brain, skull, and face. In its most severe form, holoprosencephaly entails the development of a tiny, undivided forebrain and is fatal before birth. Equivalent terms are arhinencephaly, holotelencephaly, and telencephalosynapsis. The prefix holo means undivided.

Description

There are three degrees of severity of holoprosencephaly: (1) alobar holoprosencephaly, in which a tiny, single-lobed, nonfunctional forebrain brain develops, along with other severe cerebral abnormalities and severe facial deformities including cyclopism, or formation of a single, nonfunctional eye where the bridge of the nose should be; (2) semilobar holoprosencephaly,...