Holoprosencephaly (HPE) may be rare in absolute terms, but in terms of brain abnormalities, it is one of the most common defects, at about 1 in 16,000 live births. Persons with the condition often have closely spaced eyes, a small head size, clefts of the lip and roof of the mouth, together with other birth defects.
The condition is caused in utero when the prosencephalon – the embryonic forebrain – fails to develop normally. The forebrain usually divides and forms the double lobes of a regular human brain, but in fetuses with holoprosencephaly, the forebrain does not separate completely, causing an abnormal continuity between both lobes. As a result, affected persons have a brain structure that is single-lobed – that is, not split into the usual left and right hemispheres of a human brain, as well as skull and facial defects that vary in severity from person to person. Generally, the more severe one’s holoprosencephaly is, the more severe their facial and skull defects are.
The word holoprosencephaly originates from a combination of the words holo (whole or entire), prosō (forward) and enkephalos (of the brain).
Holoprosencephaly occurs soon after conception, usually during the first few weeks of pregnancy. Although the exact cause of holoprosencephaly is not confirmed for many individuals, the following has been understood about the condition:
- It can be caused by mutations in any one of at least 14 different genes, which results in the gene and its proteins functioning abnormally, thus affecting the development of the brain.
- About one third of those born with holoprosencephaly have an abnormality of the chromosomes, which points to a genetic cause. The most common chromosomal defect associated with holoprosencephaly is trisomy 13, meaning that there are three copies of chromosome 13 (instead of the usual two).
- Holoprosencephaly can also occur in certain other genetic conditions as one of the medical issues affecting organs in addition to the brain and face. One example of such a condition is Smith-Lemli-Opitz syndrome, a developmental disorder that affects many parts of the body.
- It may be genetic, i.e. those who are relatives of someone with holoprosencephaly may be at higher risk of having a child with holoprosencephaly themselves.
- Holoprosencephaly may also potentially occur with exposure to teratogens, agents that can cause birth defects.
- It is also believed that diabetes in the mother during pregnancy can increase the risk of holoprosencephaly in the fetus.
The understanding of the causes of holoprosencephaly is not definite. It is possible that there are other genetic causes of holoprosencephaly in addition to the ones listed above.
The most noticeable signs of holoprosencephaly are its facial defects, including a small head, abnormalities of the facial features, tooth abnormalities in the form of a single central incisor, a cleft lip and/or palate, and defects in the formation of the nose. Individuals with severe cases may have cyclopia, a very rare single central eye.
Other symptoms of holoprosencephaly include intellectual disability, seizures and/or epilepsy, endocrine abnormalities and excessive fluid in the brain (hydrocephalus). Additionally, other systems in the body may also be affected, such as the pituitary gland, which can result in unusually low levels of sugar in the blood (hypoglycemia), low levels of sodium in the blood or genital abnormalities. There may also be neural tube defects, short stature, feeding difficulties and instability of temperature, heart rate and respiration. Development may be delayed in some affected individuals.
Holoprosencephaly is usually diagnosed through MRI or CT scans of the brain. In some cases, it can be detected prenatally during an ultrasound or MRI scan, although this is not always true for milder forms of holoprosencephaly.
There are four main types of holoprosencephaly, in order from most to least severe: alobar, semi-lobar, lobar and middle interhemispheric variant (MIHV).
- In alobar holoprosencephaly, the most severe form of the condition, there is no separation between the left and right hemispheres at all. As a result, the parts of the brain that are normally separated (lateral ventricles) are fused together. A person with alobar holoprosencephaly may have a single eye (cyclopia), very closely spaced eyes, very small eyes or no eyes. However, there are also people with alobar holoprosencephaly whose faces look almost normal.
- In the semi-lobar form, there is some separation between the two halves. The left side of the brain is fused to the right side in the front and sides of the brain. Additionally, the divide between the two halves of the brain is only present at the back of the brain. Individuals with semi-lobar holoprosencephaly may have ocular features that are similar to alobar holoprosencephaly, such as closely spaced eyes, very small eyes or no eyes.
- The lobar form is less severe, with most of the brain having separated into the two hemispheres, although the division is still incomplete. The frontal cortex of the brain is still fused together. Individuals with lobar holoprosencephaly may have more normal-looking features, although they may still have bilateral cleft lip, closely spaced eyes or a flattened nose.
- Middle interhemispheric variant occurs when the brain is fused only in the middle. Individuals with middle interhemispheric variant may have closely spaced eyes, a flattened and narrow nose or otherwise normal-looking features.
The condition itself cannot be corrected, but supportive treatment and care for the related issues associated with holoprosencephaly can be provided according to each individual’s needs.
- An endocrinology evaluation is normally performed to assess if there are any pituitary abnormalities.
- A neurologist helps in treating seizures if there are any.
- A developmental pediatrician helps in developmental therapies.
- If necessary, plastic reconstructive surgery may be conducted to correct cleft lip, palate or other facial abnormalities.
Apart from the above, other treatments may be administered where appropriate. A variety of specialists are usually involved in monitoring the progress of a child with holoprosencephaly, including pediatricians, neurologists, dentists, surgeons, therapists, psychologists and special education teachers.
The prognosis usually depends on the type of holoprosencephaly. In some cases of holoprosencephaly in which the malformations are very serious (usually in alobar types), the affected babies may either be stillborn or die during the first six months of life.
It was previously thought that individuals with holoprosencephaly had a poor prognosis, but in recent times, studies have shown that there is still hope. A significant number of affected children are able to survive past 12 months, especially those with milder forms of holoprosencephaly. For less severe cases, the babies survive with normal or near-normal brain development, and may have facial deformities affecting the eyes, head size, nose and upper lip.