The Inheritance of Retinal Degenerations

Page 7

Click for Table of Contents
Isolated Cases

When only one member of a family has a retinal degeneration, as we see in about 40 percent of families affected by RP, it is impossible to determine how the disease is inherited in that family. Nevertheless, to assess the risk of retinal degeneration occurring in other family members, it must be assumed that the disease is following one of the known inheritance patterns.

Isolated cases of retinal degeneration most often represent autosomal recessive disease. Even though carrier parents have a 25 percent chance of having an affected child, there is still a 75 percent chance of having an unaffected child. So, it is often the case that there is only one child with a retinal degeneraUon in the family.

Rarely, isolated cases of retinal degenerations represent new gene mutations. Retinal degenerations are caused by mutated genes that can pass through generations of a family. Occasionally, even when neither parent is affected nor carries an altered gene, one of the genes, usually in the egg or sperm, changes spontaneously from normal and becomes an altered gene, causing the disease in the child. New gene mutations most often represent the start of an autosomal dominant retinal degeneration, or when a ma is affected, either an autosomal dominant or X-linked retinal degeneration. Because a X-linked carrier female often shows mild effects of the altered gene, any mother of a male with an isolated case of retinal degeneration should be examined by an ophthalmologist to determine if she is a carrier.


RETINAL DEGENERATIVE DISEASES

Retinitis pigmentosa is just one of a number of inherited diseases that cause the retina to degenerate. There are many other retinal degenerations similar to RP but with their own distinct characteristics. Some of these diseases are described below. Most of these diseases are genetic and follow either an autosomal dominant, an autosomal recessive, or an X-linked inheritance pattern. The risk of any one of these diseases occurring in other family members is based on its inheritance pattern and is the same described in the "Inheritance Patterns" section.

Bardet-Biedl (Laurence-Moon) Syndrome

Multiple physical problems are usually found in a person with Bardet-Biedl syndrome. The most common are RP, extra fingers and/or toes, obesity, mental retardation and kidney disease. Not all of these occur in every person with this disease. Bardet-Biedi syndrome is an autosomal recessive disease.

Bassen-Kornzweig Syndrome (Abetalipoproteinemia)

RP and progressive neurologic problems are symptoms of this disease. Patients also have oddly shaped red blood cells. Bassen-Kornzweig syndrome is an autosomal recessive disease.

Best Disease (Vitelliform Dystrophy)

This disease is characterized by a lesion in the macula, which leads to impaired central vision in one or both eyes. It is an autosomal dominant disease.

Choroideremia

Choroideremia has symptoms similar to RP, including night blindness followed by loss of peripheral vision. It is characterized by degeneration of the retina and of the choroid. It has an X-linked inheritance pattern and affects males. Female carriers may experience mild symptoms of the disease.

Gyrate Atrophy

This retinal degenerative disease is associated with a deficiency in the enzyme ornithine aminotransferase. Myopia, night blindness, reduction in peripheral vision and cataracts are characteristic of this syndrome. Gyrate atrophy is an autosomal recessive disease.

Leber Congenital Amaurosis

Leber congenital amaurosis is characterized by severe visual impairment from birth or very early childhood. It is an autosomal recessive disease. Leber congenital amaurosis causes visual problems that are different from those of Leber optic neuropathy, a condition that is not caused by primary degeneration of the retina.

Macular Degeneration

Macular degeneration is divided into two broad categories: early onset and age-related. Early onset forms are inherited macular degenerations that include Best disease, Stargardt disease, fundus flavimaculatus and other rare macular dystrophies like Sorsby's macular dystrophy and North Carolina macular dystrophy. Age-related macular degeneration is the leading cause of central vision loss in people over the age of 60. The first symptom is usually a blank spot in the center of the visual field or a distortion of normal central vision. Although it can be found in more than one member of a family, its inheritance pattern is usually unknown. It is sometimes seen as an autosomal dominant disease in some families

Refsum Syndrome

Refsum syndrome is a complex disease. It is believed to be due to the absence of phytanic acid hydroxylase, an enzyme found normally in circulating blood. In addition to having RP, patients may have hearing loss, neurologic problems and dry and/or flaky skin. It is an autosomal recessive disease.

Retinoschisis (Juvenile)

Juvenile retinoschisis is characterized by vision loss that is usually diagnosed in childhood. In this disease, the layers of the retina separate, and the macula may also be affected. Juvenile retinoschisis has the X-linked inheritance pattern and affects males. A non-inherited form of retinoschisis may occur in some individuals as a part of aging and may not affect vision.

Stargardt Disease/Fundus Flavimaculatus

This form of macular degeneration usually appears before the age of 20. It is characterized by a reduction of central vision with a preservation of peripheral vision. The symptoms and progression of fundus flavimaculatus are very similar to that of Stargardt disease. Many researchers believe that these two conditions may be the same. In most affected families, Stargardt and fundus flavimaculatus are autosomal recessive diseases, although autosomal dominant families have been identified.

Usher Syndrome

The combination of RP and congenital hearing impairment in a person is known as Usher syndrome. Usher syndrome is divided into three types. Type I is characterized by profound hearing impairment, problems with balance and typical RP. Type II is characterized by moderate hearing impairment and typical RP. Type Ill is characterized by progressive hearing impairment and typical of RP. All types of Usher syndrome are autosomal recessive diseases.


[Click for Previous Page] [Go to Top of Page] [Click for Next Page]



EMail
Send comments to rpmail@jwen.com

Date last modified December 4, 1998