The Inheritance of Retinal Degenerations
Page 7
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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.
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Date last modified January 23, 2000