What is HPE?
Holoprosencephaly (HPE) is a disorder that affects fetuses during the initial part of intrauterine life — typically within the second month of pregnancy. The defect is caused when the front part of the brain in an embryo, known as the prosencephalon, fails to properly divide into the double lobes of the cerebral hemispheres. The disorder results in a single-lobed brain structure and can cause skull and facial birth defects.
HPE affects about 1 in 5,000 to 10,000 live births. Since a very small number of HPE-diagnosed fetuses survive to term — less than five percent — the frequency of HPE may be as high as 1 in 200 or 250 pregnancies. Those that do reach birth, typically do not live for more than six months, although those with less severe abnormalities may go on to live a normal life span.
HPE children may be born with a number of symptoms of the defect including: a small head, excessive fluid on the brain, mental retardation, epilepsy or abnormalities of organ systems. In less severe cases, babies are born with normal or near-normal brains and facial deformities, which may affect the eyes, nose and upper lip. These facial abnormalities may include a flat nose that has only one nostril, eyes that are close together, cleft lip or palate or just one upper middle tooth, instead of two. The more severe deformities may include one eye that is centered on the face, a nose that is located on the forehead or a HPE child might be missing some facial altogether.
There are four classifications of HPE:
- Alobar is the most severe classification of HPE. It is characterized by a brain that has not divided. Typically, alobar comes with severe abnormalities in the skull and face formation.
- Semi-lobar is a more moderate classification of HPE. This form of the disorder exists when the brain partially divides into two hemispheres in the rear of the brain but not in the front. There are also moderate facial or skull abnormalities in this classification.
- Lobar is the mild classification, in which the brain divides and there are only mild facial and skull abnormalities.
- Middle Interhemispheric Variant (MIHV) occurs when the middle of the brain has not separated well.
The cause of HPE has yet to be determined, although risk factors include maternal diabetes, as well as infections or drug use during pregnancy. Miscarriages and bleeding in the first-trimester may also be indicators or symptoms of HPE. Abnormal chromosomes have been identified in some HPE children suggesting that HPE can be inherited.
Written by J.J. Babb
