Coloboma Health Article

Definition

Coloboma, also known as keyhole defect of the iris, is a congenital genetic disorder that affects the iris of the eye. Present at birth, coloboma implies the absence of tissue.

Description

A coloboma describes a condition wherein a portion of a structure of the eye is absent, usually the iris, retina, or the optic nerve. The disorder is often referred to as a keyhole defect of the iris because the shape of the coloboma appears as the shape of a keyhole or an upside-down pear. There are many different types of colobomas, as described below.

Types of colobomas:

• Optic disc coloboma. This disorder occurs when the coloboma covers the optic nerve and may involve the macula, a structure in the eye that is responsible for visual acuity.
• Iris coloboma. This type of coloboma may be in one eye (unilateral) or in both eyes (bilateral). The pupil is often described as an upside-down pear shape when an individual has an iris coloboma.
• Retinal coloboma. In this disorder, a notch or cleft of the retina or part of the retina is missing. For example, 35% or more of the retina may be missing.
• Choroidal coloboma. This condition is similar to a retinal coloboma. The choroid is a structure in the eye that lies between the sclera and the retina.
• Morning glory syndrome. This condition, a type of optic nerve coloboma, affects the shape of the optic nerve. The syndrome is aptly named because it describes the appearance of the optic nerve, which looks like the inside of a morning glory flower.

Genetic profile

Colobomas may be isolated abnormalities in otherwise normal individuals or they may occur as part of a syndrome. As isolated findings, they are generally sporadic (not inherited). Some families, however, have shown an autosomal dominant inheritance pattern, meaning only one copy of the abnormal gene needs to be present for the disorder to occur. Some of the genetic disorders thought to contribute to coloboma include cateye syndrome, trisomy 13, trisomy 18, Sturge-Weber syndrome, and basal cell nevus syndrome.

Demographics

The condition occurs in about one in 10,000 births. Coloboma may be associated with hereditary or genetic conditions, trauma to the eye, or eye surgery.