What is Holoprosencephaly?

Holoprosencephaly (HPE) is a congenital anomaly in which there is incomplete development of the brain. In utero, the developing forebrain (prosencephalon) fails to divide into two separate hemispheres and ventricles. Holoprosencephaly is classified into 4 subtypes: Alobar, Semilobar, Lobar and MIHV.

HPE can range from mild to severe and is classified into four types:

  • Alobar Holoprosencephaly (severe)--where the brain is not divided and there are severe abnormalities (there is an absence of the interhemispheric fissure, a single primitive ventricle, fused thalami, and absent third ventricle, olfactory bulbs and tracts and optic tracts).
  • Semi-Lobar Holoprosencephaly (moderate)--where the brain is partially divided and there are some moderate abnormalities; where there are two hemispheres in the rear but not the front of the brain (there are partially separated cerebral hemispheres and a single ventricular cavity).
  • Lobar Holoprosencephaly (mild)--where the brain is divided and there are some mild abnormalities (there is a well developed interhemispheric fissure however there is some fusion of structures).
  • Middle Interhemispheric Variant of Holoprosencephaly (MIHV) -- where the middle of the brain (posterior frontal and parietal lobes) are not well separated.

Commonly, parents of children with HPE are given a very dire prognosis and are told that their child has a very small chance of survival. However, the reality is that many children do survive and lead meaningful lives, some of those well into adulthood.