Holoprosencephaly (HPE) is a relatively common birth defect of the brain, which often can also affect facial features, including closely spaced eyes, small head size, and sometimes clefts of the lip and roof of the mouth, as well as other birth defects. Holoprosencephaly is a disorder caused by the failure of the prosencephalon (the embryonic forebrain) to sufficiently divide into the double lobes of the cerebral hemispheres. The result is a single-lobed brain structure and severe skull and facial defects. In most cases of holoprosencephaly, the malformations are so severe that babies die before birth. In less severe cases, babies are born with normal or near-normal brain development and facial deformities that may affect the eyes, nose and upper lip.
This birth defect occurs soon after conception. It has a prevelance of 1 in 250 during early embryo development, and 1 in 10,000 to 1 in 20,000 at term.
There are three classifications of holoprosencephaly:
Alobar, in which the brain has not divided at all, is usually associated with severe facial features.
Semilobar, in which the brain's hemispheres have somewhat divided, causes an intermediate form of the disorder.
Lobar, in which there is considerable evidence of separate brain hemispheres, is the least severe form. In some cases of lobar holoprosencephaly the baby's brain may be nearly normal.