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Overview
Retinitis pigmentosa is a term that refers to
group of hereditary disorders that affect the
retina’s ability to respond to light. It primarily
affects rod cells, the photoreceptor cell that is
responsible for night vision, seeing in dim light,
and peripheral vision. Cone cells, which are
responsible for color vision and seeing in bright
light, may also be affected as the disease
progresses.
Retinitis pigmentosa may be caused by mutations in
any one of at least ten different genes, resulting
in a malfunction in the retinal pigment epithelial
(RPE) cells and a breakdown of a portion of the
outer segment disc membrane of photoreceptor
cells. When cells are destroyed at an abnormal
rate, the build-up of waste products interferes
with normal retinal function. The result is the
occlusion (blockage) of small blood vessels, an
abnormal increase in the number of RPE cells
(hyperplasia), and the loss of photoreptor cells.
Incidence and Prevalence
Retinitis pigmentosa is relatively rare. It
affects 50,000 to 100,000 people in the United
States. Worldwide, approximately 1.5 million
people are afflicted.
Risk Factors
Retinitis pigmentosa is caused by a genetic
defect. Patterns may be of three types:
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Autosomal dominant inheritance—The
patient has one gene for retinitis pigmentosa
paired with one normal gene and has a 50% chance
of passing the disease to their child, even if
their partner is unaffected.
-
Autosomal recessive inheritance—There
may not be a known family history of the
disorder. Both parents have normal retinas and
carry a defective gene. There is a 25% chance
that their child will be afflicted.
-
X-linked inheritance—Only
men develop the disease, but women can carry the
gene and may develop a mild form of the
disorder.
Signs and Symptoms
Ocular signs start with the breakdown of rod
cells. Rods are present both within and outside
the macula (center of the retina). The peripheral
retina, responsible for side vision and vision in
low light conditions, is predominantly rods.
Symptoms of RP usually manifest between the ages
of 10 and 30. At first, there is a decrease in
night vision and the inability to see in dimly lit
places such as movie theaters. The progressive
loss of peripheral sight leads to what is called
tunnel vision. The gradual reduction in the
ability to see peripherally may cause tripping
over objects or a motor vehicle accident. This
occurs when rod cells and outer cone cells are
affected.
The rate of progression of the disease varies
among patients and the type. Most patients are
legally blind by around age 40
Diagnosis
A number of tests are used to diagnosis retinitis
pigmentosa. Since most RP patients have no known
family history of RP, a family history is often of
no help in making the diagnosis. A medical history
is taken to rule out systemic conditions that may
affect the eyes.
An electroretinogram (ERG) measures the
response of the retina to a light stimulus and can
be helpful in confirming the diagnosis of RP. A
corneal electrode is gently placed behind the
lower eyelid, lightly touching the cornea, and
neutral electrodes are placed on the skin around
the eye. A light is shone in the eye and the
electroretinogram records electrical changes in
the retina. This provides information about the
performance of rods and cones.
An exam of the retina is done using an indirect
ophthalmoscope, an instrument that enables the
doctor to examine the different parts of the eye
through a dilated pupil. A healthy retina has an
orange-red appearance when viewed during an eye
examination. The retinal layer itself is actually
clear and the color that is seen is due to the
underlying tissue, the choroid, which is composed
mostly of blood vessels. When an eye with
retinitis pigmentosa is examined, the orange area
appears broken up with black or dark brown spots
of pigment, caused by the breakdown of the pigment
epithelial cells. These spots are called bone
spicules. Other findings are a pale optic nerve
and narrowed retinal blood vessels.
Visual acuity, refraction, color testing, and
peripheral vision testing are all part of a
complete eye examination in a patient who is
suspected of having a retinal degeneration.
Visual acuity
is tested using a standard eye chart, which
features black letters on a white background. The
chart measures how well the patient sees at
various distances and may detect vision loss.
A refraction test determines visual acuity
and the prescription for eyeglasses or contact
lenses. While corrective lenses do not improve
vision loss from retinitis pigmentosa, many
patients have accompanying eye disorders such as
nearsightedness that can be corrected. The patient
looks through a device called a phoropter or
refractor, which resembles a large set of
binoculars with multiple lenses and reads an eye
chart approximately 20 feet away. The lenses are
adjusted until the chart is as clear as possible.
Loss of color vision is tested by using a
number of simple screening tests. The Hardy-Rand-Rittler
(H-R-R) and Ishihara tests evaluate the type and
the degree of color blindness. In these tests,
colored triangles, squares, and other shapes lie
within a jumble of dots that vary in color and
intensity. As the patient identifies the colored
shapes, the eye care professional can determine
the ability to differentiate colors. The Holmgren
yarn-matching test and the Farnsworth-Munsell
100-hue disk-matching test evaluate the patient’s
ability to match up colors.
Treatment
There is no standard treatment for retinitis
pigmentosa. Recent studies report that vitamin A
may slow progression of the disease.
Prevention
Retinitis pigmentosa cannot be prevented. Genetic
screening can tell families who has the gene and
who does not. This can aid in family planning.
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