Coloboma is the term used to describe a gap or cleft in one of the structures of the eye. It occurs in the early stages of pregnancy when a part of the eye fails to complete its growth and fuse together leaving a gap. It does not mean that there is a hole in the eye. Different parts of the eye may be affected, including the lens, choroid, retina, optic nerve, and most commonly the iris, resulting in a ‘keyhole-shaped’ pupil.
Medical text written December 2011 by Professor N Ragge, Professor in Medical Genetics, Oxford Brookes University, Oxford, UK.
Vision may or may not be affected depending on the part of the eye involved. Coloboma may be unilateral (on one side) or bilateral (on both sides) and if bilateral, can be quite asymmetrical. It may be an isolated ocular finding or may be associated with other eye defects, such as microphthalmia (small eye, see entry Anophthalmia) in the same or opposite eye and, extremely rarely, Anophthalmia (absent eye) in the opposite eye.
Coloboma may occur on its own (be isolated) or be associated with other problems in the baby, sometimes as part of a specific genetic disorder. Several genes, including PAX2, MAF and SHH, can be associated with isolated ocular colobomas. New genes are being continually discovered. Several genes are implicated in recognised syndromes, such as the CHD7 gene in CHARGE syndrome.
At present, there is no treatment for the coloboma. A child with coloboma will require specialist checks in hospital to assess the impact of the condition on their vision. They may need glasses to assist with their vision and to correct refractive errors (for example astimatism or long or short-sightedness). The coloboma may be associated other eye defects, such as cataract, which need treatment. Sometimes the eye with a coloboma may become ‘lazy’, this occurs when the child’s brain chooses to ignore the image either because it is out of focus or poorer quality than the image from the other eye. It is important that this is detected early, since glasses and patching might help to improve this. Children with iris coloboma may be sensitive to bright light as the pupil does not constrict properly, this can be helped by using large brimmed floppy hats or sunglasses. Cosmetic contact lenses may be considered at when the child is older to provide the appearance of a round, rather than keyhole-shaped pupil and to help with sensitivity to bright light.
The occurrence of coloboma may be sporadic or may be inherited. The mode of inheritance may be dominant, recessive or X-linked. Genetic counselling is important as there may be a recurrence risk. It is available for familial (those that occur in families) or isolated cases of coloboma.
High-resolution ultrasound is unlikely to detect eye anomalies, such as coloboma prenatally. If the gene change causing coloboma in a family is known, prenatal genetic diagnosis could be offered.
Information and support in the UK for coloboma is provided by M.A.C.S. (see entry Anophthalmia).