Holoprosencephaly is an abnormality of brain development in which the brain doesn't properly divide into the right and left hemispheres. The condition can also affect development of the head and face.

There are 4 types of holoprosencephaly, distinguished by severity. From most to least severe, the 4 types are alobar, semi-lobar, lobar, and middle interhemispheric variant (MIHV). In general, the severity of any facial defects corresponds to the severity of the brain defect. The most severely affected people have one central eye (cyclopia) and a tubular nasal structure (proboscis) located above the eye. In the less severe forms, the brain is only partially divided, and the eyes usually are set close together. Other signs and symptoms often include intellectual disability and pituitary gland problems.

Holoprosencephaly can be caused by mutations in any of at least 14 different genes; chromosome abnormalities; or agents that can cause birth defects (teratogens). It may also be a feature of several unique genetic syndromes. In many cases, the exact cause is unknown. Life expectancy for people with this condition varies, and treatment depends on the symptoms and severity in each person.

Holoprosencephaly is classified into into 3 main subtypes based upon the severity of the malformation: lobar, semilobar, and alobar, and a fourth subtype, known as the middle interhemispheric (MIH) variant:

• Alobar holoprosencephaly is when there is a complete failure of the brain to divide into right and left hemispheres which results in the loss of midline structures of the brain and face as well as fusion of the cavities of the brain, known as lateral ventricles and the third ventricle (which are normally separated). Facial findings may include a single eye (cyclopia) or very closely spaced eyes (ethmocephaly) or absent eyes (anophthalmia), or very small eye (microphthalmia) with a tubular-shaped nose (proboscis); or closely spaced eyes (hypotelorism) and a flattened nose or cleft lip that occurs in the middle of the lip (median cleft lip) or on both sides (bilateral cleft lip). In some cases the face make look almost normal (especially in persons with variants (mutations) in the ZIC2 gene)
• Semi-lobar holoprosencephaly occurs when the left side of the brain is fused to the right side in the areas of the brain known as the frontal (front) and parietal lobes (sides of the brain). Also, the dividing line between the right and left hemispheres of the brain (known as the interhemispheric fissure) is only present in the back of the brain. People with semi-lobar holoprosencephaly may have hypotelorism, microphthalmia or anophthalmia. Other features may include a flattened bridge and tip of the nose, one nostril, a median cleft lip or bilateral cleft lip, and a cleft palate.
• Lobar holoprosencephaly, is when there are two ventricles (right and left) but the cerebral hemispheres are fused in the frontal cortex. Features may include bilateral cleft lip , closely spaced eyes, depressed nose or an almost normal looking face.
• Middle interhemispheric variant results when the brain is fused in the middle. Signs may include closely spaced eyes, depressed and narrow nose or an almost normal looking face.

Other signs and symptoms may include seizures, hydrocephalus, neural tube defects, pituitary dysfunction, short stature, feeding difficulties, and instability of temperature, heart rate, and respiration. Most people with holoprosencephaly have developmental delay and intellectual disability, that varies in severity depending on severity of the brain malformation.

Holoprosencephaly may be part of several genetic syndromes which each having unique characteristics.

Holoprosencephaly (HPE) can be inherited, but it is not always inherited.

Inherited causes of holoprosencephaly may include:

• certain types of chromosome abnormalities
• single gene mutations that cause syndromic disorders
• mutation(s) in a gene associated with isolated (nonsyndromic) HPE

The risk for family members to have HPE depends on the specific cause of HPE in the family, if known. For example, nonsyndromic HPE is usually inherited in an autosomal dominant manner. This means that having a variation (mutation) in only one copy of the responsible gene in each cell is enough to cause the condition. When a person with a mutation that causes an autosomal dominant condition has children, each child has a 50% (1 in 2) chance to inherit that mutation. However, not all people with a gene mutation that causes HPE will have HPE. This is called reduced penetrance.

For people with HPE without a known cause, recurrence risk for family members is likely to be low, but may be as high as 50%.

People with personal questions about recurrence risks for themselves or family members are encouraged to speak with a genetic counselor or other genetics professional.