Developmental defects of the iris result in a pupil that is misshapen. A complete coloboma (iris defect) results in a pupil that resembles a keyhole while a partial defect may cause the pupil to appear oval. Colobomas may be associated with complex syndromes involving other parts of the body but often, as in isolated colobomas described here, only the eye is involved.
Simple colobomas involve only the iris. When the developmental defect involves only the eye it is called an isolated coloboma and may be associated with an abnormally small eye (microphthalmia) or small cornea. Microphthalmic eyes are often associated with cysts. The missing segment of the iris may be associated with defects in other structures inside the eye including the retina, lens and optic nerve. A coloboma may only affect one eye in which case the coloration of the affected iris is darker than the other eye.
Colobomas are part of many syndromes, both ocular and systemic, resulting from gene mutations. The pattern of inheritance in isolated coloboma is usually autosomal dominant. The eye defect is passed directly from parent to each child with a 50% probability.
The diagnosis is usually made by an ophthalmologist. It is important to have a complete eye and physical examination to determine if other anomalies or conditions are present. The visual prognosis is highly variable depending on what other eye structures are involved. No medical or surgical treatment is available. In bilateral cases and where partial vision is present, low vision aids can be helpful. Tinted contact lenses can hide the iris defect.