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Excerpted from the GeneReview: Holoprosencephaly Overview
Holoprosencephaly (HPE) is a structural anomaly of the brain in which there is failed or incomplete separation of the forebrain early in gestation. Classic HPE encompasses a continuum of brain malformations including (in order of decreasing severity): alobar, semilobar, lobar, and middle interhemispheric variant (MIHV) type HPE; a septopreoptic type has also been described. Other CNS abnormalities not specific to HPE may also occur. HPE is accompanied by a spectrum of characteristic craniofacial anomalies in approximately 80% of individuals with HPE. Developmental delay is present in virtually all individuals with the HPE spectrum of CNS anomalies. Seizures and pituitary dysfunction are common. Most affected fetuses do not survive; severely affected children typically do not survive beyond early infancy, while a significant proportion of more mildly affected children survive past 12 months. Mildly manifesting individuals without appreciable brain anomalies on conventional neuroimaging may be described as having "microform" HPE.

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