Cyclopia

Cyclopia (also cyclocephaly or synophthalmia) is a rare form of holoprosencephaly and is a congenital disorder (birth defect) characterized by the failure of the embryonic prosencephalon to properly divide the orbits of the eye into two cavities. Its incidence is 1 in 16,000 in born animals, and 1 in 250 in embryos.

Presentation
Typically, the face is either missing or replaced with a non-functioning nose in the form of a proboscis. Such a proboscis generally appears above the central eye, or on the back, and is characteristic of a form of cyclopia called rhinencephaly or rhinocephaly.

Although cyclopia is very rare, several cyclopic human babies are preserved in medical museums (e.g. The Vrolik Museum, Amsterdam). There are also two known cases of children with Down Syndrome being born with one eye.

Some extreme cases of cyclopia have been documented in inbred farm animals (horses, sheep, pigs, and sometimes chickens). In such cases, the nose and mouth fail to form, or the nose grows from the roof of the mouth obstructing airflow, resulting in suffocation shortly after birth.

Causes
Genetic problems or toxins can cause problems in the embryonic forebrain-dividing process. One highly teratogenic alkaloid toxin that can cause cyclopia is cyclopamine or 2-deoxyjervine, found in the plant Veratrum californicum (also known as corn lily or false hellebore). The mistake of ingesting Veratrum californicum while pregnant is often because hellebore, a plant with which it is easily confused, is recommended as a natural treatment for vomiting, cramps, and poor circulation, three conditions that are quite common in pregnant women. Cyclopia can occur in the womb when certain proteins are inappropriately expressed and causes the brain to stay a whole instead of having two distinct hemispheres, which also means one optic lobe and one olfactory lobe resulting in one eye.

Shh (Sonic Hedgehog Gene Regulator, named after the effects a mutation in the gene had on the forming embryo of fruit flies studied by scientists; a spiky appearance under a microscope, similar to that of its video game character namesake) is involved in the separation of the single eye field into two bilateral fields. Although not proven, it is thought that shh emitted from the prechordal plate suppresses Pax6 which causes the eye field to divide into two. If the shh gene is mutated, the result is cyclopia, a single eye in the center of the face (Gilbert, 2000).

Notable cases
A British description from 1665 of a colt apparently suffering from cyclopia reads:

First, That it had no sign of any Nose in the usual place, nor had it any, in any other place of the Head, unless the double Bagg CC that grew out of the midst of the forehead, were some rudiment of it. Next, That the two Eyes were united into one Double Eye which was placed just in the middle of the Brow.

On December 28, 2005, a kitten with cyclopia, "Cy", was born in the Redmond, Oregon, United States and died about one day after birth.