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 (except for one case, where it developed within the first few months of the child's life) 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. The level of visible impairment of those with a coloboma can range from having no vision problems to being able to see only light or dark, depending on the position and extent of the coloboma (or colobomata if more than one is present). Ocular coloboma is relatively uncommon, affecting less than one in every 10,000 births.