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Library of Medicine®| On this page: Reviewed November 2011 What
is coloboma? Coloboma is an eye abnormality that occurs before
birth. Colobomas are missing pieces of tissue in structures that
form the eye. They may appear as notches or gaps in one of
several parts of the eye, including the colored part of the eye
called the iris; the retina, which is the specialized light-
sensitive tissue that lines the back of the eye; the blood
vessel layer under the retina called the choroid; or the optic
nerves, which carry information from the eyes to the brain.
Colobomas may be present in one or both eyes and, depending on
their size and location, can affect a person's vision. Colobomas
affecting the iris, which result in a "keyhole" appearance of
the pupil, generally do not lead to vision loss. Colobomas
involving the retina result in vision loss in specific parts of
the visual field, generally the upper part. Large retinal
colobomas or those affecting the optic