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.
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.
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.
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.
Signs and symptoms
Chorioretinal colobomas are those that affect the choriod (light impermeable lining consisting primarily of blood vessels) and the retina (the photosensitive lining inside the eye). The extent to which vision would be impaired depends on the size of the coloboma, and its impact on the optic nerve and macula. A coloboma can appear as a black indentation of varying depth at the edge of the pupil, and gives the pupil an odd or irregular shape. It may also appear as a split in the iris from the pupil to the edge of the iris.
Symptoms usually present as blurred or decreased vision, and an appearance of a hole or odd-shaped pupil in the individual's eye. A smaller colboma, especially if it is not attached to the pupil, often causes a secondary image to focus on the back of the eye, producing blurred vision or decreased visual sharpness.
A diagnosis is made by a physical exam and includes a detailed eye examination by an opthalmologist. The ophthalmologist will also ask the individual when the symptoms were first noticed, determine what part of the eye is affected, the size and shape of the dark area in the eye, and ask for reports of any changes in the individual's vision.
Certain diagnostic tests are often used to diagnose coloboma. These include a visual acuity test, refraction test, and an in-depth history of symptoms.
Treatment and management
Colobomas may be accompanied by other problems that may be neurological or chromosomal in nature. In addition, some genetic syndromes also include coloboma as part of the disorder's potential findings. More importantly, a specific combination of abnormalities identified by the acronym CHARGE must also be considered when a diagnosis of coloboma is made.
The medical condition known as CHARGE association is a very rare and serious condition. Individuals that have the condition will require attention from several specialists and treatment from an early age. Colobomas are usually one of the findings in individuals with CHARGE. The disorder includes these problems:
- (H)eart defects
- (A)tresia of the choanae, which is a blockage of the nasal passages
- (R)etarded growth and development
- (G)enital hypoplasia, which occurs when the testes do not descend properly
- (E)ar abnormalities
While there is no specific treatment for coloboma, some treatments are available that can manage vision problems associated with the disorder. For example, physicians often recommend cosmetic contact lenses and sunglasses for individuals whose eyesight is adversely affected. Additional optical aids are often helpful such as eye patching. Since many indivduals with coloboma are highly sensitive to light, opthalmologists often recommend special lights or other personalized visual aids.
The effects of coloboma can be mild or severe, depending upon the extent and location of the gap or cleft. The gap itself is usually located at the bottom of the eye, but it may occur in the iris, choroid, macula or optic nerve.
A coloboma of the lens, particularly if it is large, may also include abnormalities of the iris and choroids, which increases the risk of retinal tearing. In severe cases of coloboma, the eye may be reduced in size. This condition is called microphthalmous, a disorder that can arise with or without coloboma.
The specific gene or genes responsible for coloboma have not yet been identified, but research continues throughout the United States, Scotland, and England.
Royal National Institute for the Blind. PO Box 173, Peterborough PE2 6WS. <http://www.rnib.org.uk>.