A coloboma is a gap in one or more of the eye’s structures that occurs during prenatal development and remains throughout life. It may be caused when a gap in the choroidal fissure fails to close completely before birth. Colobomas usually affect only the iris but can also affect the retina, choroid or optic disc. In the case where the iris is affected, the classic coloboma looks like a keyhole as a wedge of iris tissue is missing next to the round pupil.  The gap is usually inferiorly placed. It effects less than 1 in 10,000 births. Coloboma can occur in one eye or both eyes.  It may be associated with several chromosomal abnormalities and syndromes.

Symptoms and signs

The classic keyhole appearance of the iris may be present and clearly visible with the naked eye. Smaller gaps may only be visible through an eye examination. Vision problems can range from none at all to complete blindness in the effected eye(s), depending on what part of the eye is affected.  Small gaps in the iris may be asymptomatic, while larger gaps affecting the choroid or optic nerve head may cause large visual field defects.

Treatment

While there is no treatment available for the vision loss associated with coloboma of the choroid and optic nerve, specialised contact lenses can improve both the cosmetic appearance and the visual quality of those with iris coloboma.