|Classification and external resources|
|ICD-10||Q10.3, Q12.2, Q13.0, Q14.2, Q14.8|
|ICD-9||377.23, 743.4, 743.46, 743.52, 743.57|
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 between two structures in the eye, 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 about 1 in every 10,000 births.
The effects a coloboma has on the vision can be mild or more severe depending on the size and location of the gap. If, for example, only a small part of the iris is missing, vision may be normal, whereas if a large part of the retina or optic nerve is missing, vision may be poor and a large part of the visual field may be missing. This is more likely to cause problems with mobility if the lower visual field is absent. Other conditions can be associated with a coloboma. Sometimes the eye may be reduced in size, a condition called microphthalmia, or there may be glaucoma, nystagmus, scotoma or strabismus.
Related conditions 
Other ocular malformations that include coloboma, or are related to it, are:
- 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: Coloboma of the eye, Heart defects, Atresia of the nasal choanae, Retardation of growth and/or development, Genital and/or urinary abnormalities, and Ear abnormalities and deafness. Although these features are no longer used in making a diagnosis, the name has remained.
- Cat Eye Syndrome (CES), 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, a chromosomal abnormality which can cause a number of deformities, some of which include structural eye defects, including microphthalmia, Peters anomaly, cataract, iris and/or fundus coloboma, retinal dysplasia or retinal detachment, sensory nystagmus, cortical visual loss, and optic nerve hypoplasia.
Perhaps the most famous people with coloboma are John Ritter, Henry Cavill, Danielle Minton, New York Times columnist Andrew Ross Sorkin, tennis player Arnaud Clément, alternative rock singer songwriter Lachi and Madeleine McCann. McCann, a British girl, disappeared from the vacation resort rented by her parents in May 2007 just before her fourth birthday. Posters and online campaigns promoting the search for Madeleine use the word "Look" with the first "O" in the word being drawn in the shape of a coloboma radius extending from the pupil at the 7 o'clock position.
There is no treatment for the visual impairment caused by coloboma at present. Specialized contact lenses can be used later in life for colobomas of the iris, and glasses can be used to help with vision problems. 
- coloboma, Mosby's Medical, Nursing and Allied Health Dictionary, Fourth Edition, Mosby Year-Book, 1994, p. 361
- Pagon RA, Graham JM, Zonana J, Yong SL (1981). "Coloboma, congenital heart disease, and choanal atresia with multiple anomalies: CHARGE association". J. Pediatr. 99 (2): 223–7. doi:10.1016/S0022-3476(81)80454-4. PMID 6166737.
- Cunliffe HE, McNoe LA, Ward TA, Devriendt K, Brunner HG, Eccles MR (October 1998). "The prevalence of PAX2 mutations in patients with isolated colobomas or colobomas associated with urogenital anomalies". J. Med. Genet. 35 (10): 806–12. doi:10.1136/jmg.35.10.806. PMC 1051454. PMID 9783702.
- Hornby, SJ et al. "Visual acuity in children with coloboma". Ophthalmology 107(3):511-20
- RNIB database - Supporting blind and partially sighted people - Coloboma; http://www.rnib.org.uk/xpedio/groups/public/documents/PublicWebsite/public_rnib003643.hcsp#P92_9303