Coloboma
A coloboma (also part of the rare Cat Eye syndrome) is a hole in one of the structures of the eye, such as the lens, eyelid, iris, retina, choroid or optic disc. The hole is present from birth and can be caused when a gap between two structures in the eye, which is present early in development in the uterus, fails to close up completely before a child is born. A coloboma can occur in one or both eyes.
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 or strabismus (squint).
Some children with coloboma of the eye also have malformations in other parts of the body. There is a rare condition called CHARGE syndrome, in which coloboma is associated with cleft lip and/or palate, ear abnormalities and hearing impairment, choanal atresia, delays in growth and development, central nervous system anomalies and congenital heart defects.
The incidence of coloboma is estimated at around 0.5 to 0.7 per 10,000 births, making it a relatively rare condition. 

# Differentiating coloboma from Other Diseases
Coloboma must be differentiated from retinoblastoma, Norrie disease, and Retinopathy of prematurity (ROP).
For more information on differential diagnosis of coloboma please click here

# Notes
-  Hornby SJ, Adolph S, Gilbert CE, Dandona L, Foster A (March 2000). "Visual acuity in children with coloboma: clinical features and a new phenotypic classification system". Ophthalmology. 107 (3): 51120. PMID 10711890..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"\"""\"""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{display:none;font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}
-  Butros LJ, Abramson DH, Dunkel IJ (March 2002). "Delayed diagnosis of retinoblastoma: analysis of degree, cause, and potential consequences". Pediatrics. 109 (3): E45. PMID 11875173.
-  Sachdeva R, Schoenfield L, Marcotty A, Singh AD (June 2011). "Retinoblastoma with autoinfarction presenting as orbital cellulitis". J AAPOS. 15 (3): 3024. doi:10.1016/j.jaapos.2011.02.013. PMID 21680213.
-  Singh, Arun (2015). Clinical ophthalmic oncology : retinoblastoma. Heidelberg: Springer. ISBN 978-3-662-43451-2.
-  Howard GM, Ellsworth RM (October 1965). "Differential diagnosis of retinoblastoma. A statistical survey of 500 children. I. Relative frequency of the lesions which simulate retinoblastoma". Am. J. Ophthalmol. 60 (4): 6108. PMID 5897773.