A coloboma (from the Greek , 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. Ocular coloboma is relatively uncommon, affecting less than one in every 10,000 births. The classical description in medical literature is of a keyhole-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 vision 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). Signs and symptoms Visual effects may be mild to more severe depending on the size and location of the coloboma. If, for example, only a small part of the iris is missing, the vision may be normal; when a large part of the retina or (especially) optic nerve is missing, the vision may be poor. Commonly posterior colobomata affect the inferior retina, with resultant deficit in the superior visual field. Other conditions can be associated with a coloboma. Sometimes, the eye may be reduced in size, a condition called microphthalmia. Glaucoma, nystagmus, scotoma, or strabismus may also occur. Related conditions Other ocular malformations that include coloboma or are related to it: CHARGE syndrome, a term that came into use as an acronym for the set of unusual congenital features seen in a number of newborn children. The letters stand for: oloboma of the eye, eart defects, tresia of the nasal choanae, etardation of growth and/or development, enital and/or urinary abnormalities, and ar abnormalities and deafness. Although these features are no longer used in making a diagnosis, the name has remained. 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. The term "cat eye" was coined because of the particular appearance of the vertical colobomas in the eyes of some patients. Patau syndrome (trisomy 13), a chromosomal abnormality that can cause a number