A coloboma (from the Greek koloboma, meaning defect,) is a hole in one of the structures of the eye, such as the iris, retina, choroid, or optic disc. The hole is present from birth and can be caused when a gap called the choroid fissure, which is present during early stages of prenatal development, fails to close up completely before a child is born. The classical description in medical literature is of a key-hole shaped defect. A coloboma can occur in one eye (unilateral) or both eyes (bilateral). Most cases of coloboma affect only the iris. People with coloboma may have no vision problems or may be blind, depending on severity. It affects less than one in every 10,000 births.One manifestation of colobomas is as part of the Cat eye syndrome, "caused by the short arm (p) and a small section of the long arm (q) of human chromosome 22 being present three (trisomic) or four times (tetrasomic) instead of the usual two times."
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