Holoprosencephaly

What is Holoprosencephaly?

This rare disease is a brain abnormality in which the brain fails to divide into the right and left hemispheres, as it should. There are four types of the syndrome, identified by the severity of the abnormality. The more severe the brain abnormality, generally the more severe the unique facial features and symptoms will be.

What gene changes cause Holoprosencephaly?

Some research suggests that up to 14 genes may be responsible for the syndrome. Another possible cause is attributed to what are called teratogens, or chromosomal abnormalities responsible for birth defects. Another possibility that it is the result of a combination of several unique genetic syndromes.

The exact mode of inheritance was unknown at the time this entry was recorded.

What are the main symptoms of Holoprosencephaly?

The symptoms are defined by the type of Holoprosencephaly identified.

Alobar Holoprosencephaly: when both sides of the brain fail to divide at all. This is the most severe form of the syndrome. Individuals may develop just a single eye (cyclopia), absent eyes, eyes that are very close together or very small eyes. A tubular-shaped nose is also common with this form of the syndrome. A cleft lip that occurs in the middle of the face, or on both sides of the lip is another feature. Life expectancy with this type of the syndrome may vary, but generally patients with this form of the syndrome are still born, die at birth, or rarely make it past six months.

Semi-lobar Holoprosencephaly: when the left side of the brain fuses to the right side at the front and sides of the brain. Widely-spaced eyes, a flattened bridge and tip of the nose, one nostril, cleft palate and a cleft lip in the middle or both sides of the lip are unique facial features of the syndrome.
Lobar Holoprosencephaly: with this type the brain divides into the right and left ventricles but cerebral hemispheres are fused at the front. A cleft lip on both sides, closely spaced eyes and a depressed nose. In some cases there may be no unique facial features present or obvious.

Middle interhemispheric variant: when the brain fuses in the middle. Closely spaced eyes, a depressed and narrow nose may be present.

How does someone get tested for Holoprosencephaly?

The initial testing for Holoprosencephaly can begin with facial analysis screening, through the FDNA Telehealth telegenetics platform, which can identify the key markers of the syndrome and outline the need for further testing. A consultation with a genetic counselor and then a geneticist will follow.

Based on this clinical consultation with a geneticist, the different options for genetic testing will be shared and consent will be sought for further testing.

Medical information on Holoprosencephaly

In the report by Begleiter and Harris (1980) two brothers were described with Holoprosencephaly (both sibs had a premaxillary agenesis), a micropenis, and normal chromosomes. Both had head circumferences above the 50th centile. At post-mortem in one there was a large, posteriorly-situated cyst in the ventricular system, an agenesis of the corpus callosum, and an absence of pituitary tissue. The adrenal glands were hypoplastic. The other sib had marked hypoglycemia and a low level of serum insulin. As stated by the authors, it is difficult to know what the frequency of endocrine dysgenesis is in Holoprosencephaly and whether this makes autosomal recessive inheritance more likely.
The patients reported by Chan et al., (2005) and Kourti et al., (2008)had in addition diabetes insipidus. Note too, the patients (see other refs in Savasta et al., 2008), with neurogenic hypernatremia, without diabetes insipidus.

* This information is courtesy of the L M D.
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