Genetic Eye Disorders

Reviewed:
Last Updated: 3/01/89
Created: 3/01/89
Revised:

A genetic (or inherited) disorder is a condition that may be passed on from parents to their children through their genes. Genes contain coded information that determines the traits an individual will express. Genetic disorders differ from other medical problems in that they tend to recur within families. Just as common traits such as facial features are passed from parent to child through the genes, an alteration in a gene that could lead to an eye disorder may be inherited. Often, the risk of occurrence for other family members can be predicted.

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Artist's rendition of a DNA double helix.

For any eye condition in which a genetic origin is known or suspected, a thorough ophthalmologic examination will establish the diagnosis of the eye disorder, and a genetic workup will clarify any genetic involvement. A genetic workup consists of constructing a diagram called a pedigree to show the genetic relationships and medical history in a family. From this pedigree, the patterns of transmission of familial disorders may emerge. This, in turn, will enable the diagnosis of a genetic disorder and allow risk assessment for family members.

Genetic eye disorders are classified according to the type of genetic abnormality. Often the abnormality occurs on only one gene. These single-gene defects are the easiest type of genetic abnormality to identify by pedigree analysis. They are subdivided into autosomal dominant, autosomal recessive, and X-linked (sex-linked) conditions. Each person has two genes for any trait and randomly passes on one of them to each child, who then has received one from each parent. In the autosomal dominant type, a person who has inherited one abnormal gene will express symptoms of the disorder. Thus, there is a 50910 chance that the children of an affected person will be affected. In the autosomal recessive type, a person who has inherited two abnormal genes will express the symptoms whereas a person with one abnormal and one normal gene will be a carrier. The carrier is often clinically asymptomatic. Unless a carrier has children with another carrier of the same genetic disorder, this individual's children are not at risk for the disease. If both parents are carriers, their offspring have a 25% chance of being affected. X-linked conditions differ from autosomal types in that only females are carriers and affected persons are usually male.

A vast array of hereditary eye disorders has been identified. These include conditions limited to the eye as well as ocular manifestations of other heritable disorders and complex syndromes. Congenital cataracts (those present at birth) and retinal degenerations rank high among the many genetic causes of blindness. Approximately one out of 250 infants is born with a cataract; others develop cataracts later, often in association with another underlying genetic disorder. Among the heritable retinal degenerations is a group of eye disorders called retinitis pigmentosa (RP), which affects one in 5,000 in the United States. Onset of symptoms in RP is during the first two decades of life, with progressive deterioration that leads to severe vision loss usually by the fourth and fifth decades. The severity of the disorder varies according to subtype, which can be transmitted by either of the two autosomal modes or in the X-linked recessive fashion.

Many common eye conditions are observed to recur within families although the patterns of transmission may not be as clear-cut as those described above for single gene defects. In such cases, the risk of recurrence is estimated statistically. For example, the most common form of glaucoma (increased pressure in the eye), primary open angle glaucoma, affects about Do of individuals over age 40. The risk for first-degree relatives of affected persons to develop this glaucoma ranges from 5% to 16%. Strabismus, or crossed eyes, is another commonly inherited condition. Recurrence risks for this condition have been estimated at 15% for the brother or sister of an affected child. If a parent of a child with strabismus is also affected, the recurrence risk for other children is reported to be about 40%.

Due to the medical implications of genetic disorders for family members as well as the affected individual, counseling is an important part of managing genetic disorders. Advice of recurrence risks is particularly crucial for those couples wishing to have more children. Periodic screening of at-risk individuals may lead to prevention of blindness through early diagnosis and treatment. Determination of carrier status, usually by laboratory testing when available, may provide assurance that the couple is not at risk for affected children or it may alert to the need for prenatal diagnostic evaluation and follow-up. Genetic counseling is also important because genetic disorders often put emotional stress on the family. Assurance that the parents of children diagnosed with genetic disorders are not at fault could lighten their emotional burden.

On the whole, the most effective way to combat genetic disorders is through medical research. Currently, research in the molecular and cell biology of the eye holds promise for more effective management of heritable eye disorders and provides hope for therapy and, ultimately, cure for some genetic eye diseases. Due to the technological breakthroughs in these scientific disciplines, major strides have already been made, across the nation, in gaining an understanding of several eye diseases. It is expected that advancements in carrier detection and presymptomatic diagnosis are forthcoming for many genetic eye disorders.

The Clinical Genetic Handbook, published in 1987 by the ideational Genetics Foundation, Inc., was used as a reference.

"Eye Facts" is intended as an informational series and should not be used as a substitute for medical advice.

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