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. Normally, the forebrain is formed and the face begins to develop in the fifth and sixth weeks of pregnancy.

The exact cause of holoprosencephaly are yet to be determined, although the presence of toxins may be suspected. However, it often seems that there is no specific cause at all.

There are three classifications of holoprosencephaly:

• Alobar – in which the brain has not divided at all, is usually associated with severe facial deformities
• Semilobar – in which the brain’s hemispheres have somewhat divided, causes an intermediate form of the disorder
• Lobar – in which there is considerable evidence of separate brain hemispheres, is the least severe form. In some cases of lobar holoprosencephaly the baby’s brain may be nearly normal

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.

The least severe of the facial anomalies is the median cleft lip (premaxillary agenesis). The most severe is cyclopia, an abnormality characterized by a single eye located in the area normally occupied by the root of the nose, and a missing nose or a proboscis (a tubular-shaped nose) located above the eye. The least common facial anomaly is ethmocephaly, in which a proboscis separates closely-set eyes. Cebocephaly, another facial anomaly, is characterized by a small, flattened nose with a single nostril situated below incomplete or underdeveloped closely-set eyes.

Moderate to severe defects may cause:

• Mental retardation
• Spastic quadriparesis
• Athetoid movements
• Endocrine disorders
• Epilepsy
• Other serious conditions

Mild brain defects may only cause learning or behavior problems with few motor impairments.

Seizures may develop over time with the highest risk before 2 yrs of age and the onset of puberty. Most are managed with one medication or a combination of medications. Typically, seizures that are difficult to control appear soon after birth, requiring more aggressive medication combinations and doses.

Most children with holoprosencephaly are at risk of having elevated blood sodium levels during moderate-severe illnesses, that alter fluid intake/output, even if they have no previous diagnosis of diabetes insipidus or hypernatremia.

The prognosis for individuals with the disorder depends on the severity of the brain and facial deformities.

There is no standard course of treatment for holoprosencephaly. Treatment is symptomatic and supportive.