Holoprosencephaly

Holoprosencephaly is a type of cephalic disorder. This is a disorder characterized by the failure of the prosencephalon (the forebrain of the embryo) to develop. During normal development the forebrain is formed and the face begins to develop in the fifth and sixth weeks of human pregnancy. (The condition also occurs in other species, as with Cy, the Cyclops kitten.) Holoprosencephaly is caused by a failure of the embryo's forebrain to divide to form bilateral cerebral hemispheres (the left and right halves of the brain), causing defects in the development of the face and in brain structure and function.

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Symptoms

Symptoms of holoprosencephaly range from mild (anosmia, or a single central incisor) to moderate (cleft lip or palate) to severe (synophthalmia proboscis or cyclopia).

There are three classifications of 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.

Presentation

Holoprosencephaly, once called arhinencephaly, consists of a spectrum of defects or malformations of the brain and face. At the most severe end of this spectrum are cases involving serious malformations of the brain, malformations so severe that they often cause miscarriage or stillbirth. At the other end of the spectrum are individuals with facial defects - which may affect the eyes, nose, and upper lip - and normal or near-normal brain development. Seizures and mental retardation may occur.

The most severe of the facial defects (or anomalies) is cyclopia, an abnormality characterized by the development of a single eye, located in the area normally occupied by the root of the nose, and a missing nose or a nose in the form of a proboscis (a tubular appendage) located above the eye. The condition is also referred to as cyclocephaly or synophthalmia and is very rare.

Causes

Armand Marie Leroi ^[1] describes the cause of cyclopia as a genetic malfunctioning during the process by which the embryonic brain is divided into two. Only later does the visual cortex take recognizable form, and at this point an individual with a single forebrain region will be likely to have a single, possibly rather large, eye (at such a time, individuals with separate cerebral hemispheres would form two eyes).

Increases in expression of such genes as Pax-2, as well as inhibition of Pax-6, from the notochord have been implicated in normal differentiation of cephalic midline structures. Inappropriate expression of any of these genes may result in mild to severe forms of holoprosencephaly.

Incidence/prevalence

Leroi goes on to state that:

[Holoprosencephaly] is, in all its manifestations, the most common brain deformity in humans, afflicting 1 in 16,000 live-born children and 1 in 200 miscarried fetuses.

Prognosis

The more severe forms of holoprosencephaly are usually fatal. People born with the mildest form (only one incisor) may have mild to moderate developmental delay and pituitary dysfunction.^[2]

References

1. ^ Armand Marie Leroi, Mutants: On the Form, Varieties and Errors of the Human Body, 2003, Harper Perennial, London. ISBN 0-00-653164-4
2. ^ Jeanty, Philippe; Sandra R. Silva, Juliana Leite (2006-01-18). Holoprosencephaly, overview. TheFetus.net. MediaMacros. Retrieved on 2007-02-03.

See also

• cephalic disorder