Holoprosencephaly

Holoprosencephaly (HPE) is a structural brain malformation resulting from incomplete or absent division (cleavage) of the embryonic forebrain (prosencephalon) into two distinct cerebral hemispheres. It is the most common congenital forebrain anomaly in humans. HPE is classified into several subtypes of decreasing severity: alobar (the most severe, with no separation of the hemispheres), semilobar (partial separation), lobar (nearly complete separation), and the middle interhemispheric variant (syntelencephaly). The condition affects the central nervous system primarily but is frequently associated with characteristic craniofacial anomalies, as the brain influences facial development — often summarized by the phrase 'the face predicts the brain.' Facial features can range from cyclopia (a single central eye) and proboscis in the most severe forms to milder findings such as hypotelorism (closely spaced eyes), a single central incisor, cleft lip and palate, or a flat nasal bridge. Key symptoms depend on the severity of the brain malformation and may include intellectual disability of varying degrees, seizures, motor impairments, feeding difficulties, endocrine dysfunction (particularly diabetes insipidus and growth hormone deficiency due to hypothalamic-pituitary axis involvement), hydrocephalus, and temperature instability. Many infants with severe alobar HPE do not survive beyond the neonatal period, while individuals with milder forms may survive into childhood or adulthood with variable degrees of neurological impairment. Holoprosencephaly can be caused by mutations in several genes, including SHH, ZIC2, SIX3, TGIF1, PTCH1, GLI2, and FGF8, among others. It can also occur in the context of chromosomal abnormalities, most commonly trisomy 13. Environmental risk factors include maternal diabetes, alcohol exposure, and certain teratogenic medications. Inheritance is most commonly autosomal dominant with highly variable expressivity and incomplete penetrance, though autosomal recessive and X-linked forms have been described. There is no cure for HPE; treatment is supportive and symptomatic, including management of seizures with antiepileptic medications, hormone replacement therapy for endocrine deficiencies, surgical intervention for hydrocephalus (such as ventriculoperitoneal shunting), and multidisciplinary rehabilitation including physical, occupational, and speech therapy.

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