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Holoprosencephaly (HPE) is a birth defect that occurs during the first few weeks of intrauterine life. HPE is a disorder in which the fetal brain does not grow forward and divide as it is supposed to during early pregnancy (incomplete cleavage of the embryonic forebrain/failure of the prosencephalon to cleave into the cerebral and lateral hemispheres).
This brain malformation can range from mild to severe and is classified into three types:
- Lobar (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).
- Semi-Lobar (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).
- Alobar (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 thalmi and absent third ventricle, olfactory bulbs and tracts and optic tracts).
Children diagnosed with HPE may have a small head (microcephaly), excessive fluid in the brain (hydrocephaly), variable degrees of intellectual disabilities, epilepsy, endocrine abnormalities or abnormalities of other organ systems such as cardiac, skeletal, genitourinary and gastrointestinal. Mildly affected children may exhibit few symptoms and may live a normal life.
Facial deformities are often present in many children diagnosed with HPE. Mild forms of facial abnormalities may include a flat single-nostril nose (cebocephaly), close set eyes (hypotelorism), cleft lip and/or palate, or just one upper middle tooth (single maxillary central incisor). More severe facial deformities may include a single central eye (cyclopia), a nose located on the forehead (proboscis) or missing facial features.