Holoprosencephaly is a disorder caused by the failure of the prosencephalon (the embryonic forebrain) to sufficiently divide into the double lobes of the cerebral hemispheres. The result is a single-lobed brain structure and severe skull and facial defects. In most cases of holoprosencephaly, the malformations are so severe that babies die before birth. In less severe cases, babies are born with normal or near-normal brain development and facial deformities that may affect the eyes, nose, and upper lip.
Symptoms of holoprosencephaly range from mild (no facial/organ defects, anosmia, or only a single central incisor) to moderate (cleft lip or cleft palate) to severe (synophthalmia proboscis or cyclopia). There are four classifications of holoprosencephaly. Gross pathology specimen from a case of alobar holoprosencephaly. * Alobar holoprosencephaly, the most serious form in which the brain fails to separate, is usually associated with severe facial anomalies. * Semilobar holoprosencephaly, in which the brain's hemispheres have a slight tendency to separate, is an intermediate form of the disease. * Lobar holoprosencephaly, in which there is considerable evidence of separate brain hemispheres, is the least severe form. In some cases of lobar holoprosencephaly, the patient's brain may be nearly normal. * Middle Interhemispheric Variant of Holoprosencephaly (MIHV) -- where the middle of the brain (posterior frontal and parietal lobes) are not well separated.
The cause of HPE is currently unknown. Often, no specific cause can be identified. Suggested risk factors include maternal diabetes, infections during pregnancy (syphilis, toxoplasmosis, rubella, herpes, cytomegalovirus), and various drugs taken during pregnancy (alcohol, aspirin, lithium, thorazine, anticonvulsants, hormones, retinoic acid). Women with previous pregnancy loss and first trimester bleeding are also more likely to have a child diagnosed with HPE. Although many children with HPE have normal chromosomes, specific chromosomal abnormalities have been identified in some patients. There is evidence that in some families, HPE is inherited (autosomal dominant as well as autosomal or X-linked recessive inheritance). Several genes have been identified that play a role in holoprosencephaly.
The prognosis for individuals with the disorder depends on the severity of the brain and facial deformities.