By Frank J. Weinstock, M.D.
Edited by Maureen A. Duffy, M.S., CVRT
What Is Retinitis Pigmentosa?
Retinitis pigmentosa (RP) is the most common of a large group of progressive retinal degenerations or dystrophies [i.e., degenerative disorders].
There is considerable overlap among the various types. It usually refers to a group of hereditary conditions involving one or several retina layers, causing progressive degeneration.
Some Facts about the Retina
The retina is the light-sensitive tissue that lines the inside surface of the eye.
The retina contains photoreceptor cells that convert (or process) incoming light into electrical impulses. These electrical impulses are carried by the optic nerve (which resembles your television cable) to the brain, which finally interprets them as visual images.
There are two types of photoreceptors: rods and cones, which are the light-processing cells responsible for peripheral (side) and central (straight-ahead) vision.
- The specialized, highly light-sensitive retinal processing cells are able to function in low light levels. They provide peripheral (or side) vision, are responsible for dark adaptation, and are most sensitive to movement/motion. They are less sensitive to color perception.
- A normal retina contains approximately 120-150 million rods, primarily in the peripheral, or outer, retina.
- Rods provide scotopic vision which refers to eyesight in low-light conditions.
- The specialized retinal processing cells function in bright light levels and provide central (or straight-ahead) vision, along with sharp visual acuity, detail, and color vision. They require bright light to function and are not sensitive to lower light levels.
- A normal retina contains approximately 6-7 million cones, primarily in the macula, the small area in the center of the retina that provides clear central vision. Cones are the most concentrated in the fovea, which is located in the center of the macula and provides the sharpest detail vision.
- Cones provide photopic vision, which refers to eyesight in daylight conditions.
In most retinitis pigmentosa (RP) and RP-like diseases, the rods deteriorate first, followed by the cones, although there may also be some involvement of the cones.
With RP, central vision is usually retained until late in the disease as the peripheral retina slowly constricts.
What Causes Retinitis Pigmentosa?
Retinitis pigmentosa is primarily an inherited condition that involves both eyes and is generally diagnosed later in life. If it starts in one eye, the other eye usually develops the same condition in several years.
If you have been diagnosed with retinitis pigmentosa, you may wish to undergo genetic testing of your family history and family tree (called a “genetic pedigree”) with the aid of a genetic counselor. Usually, this is managed by a retinal specialist, who may also enlist the help of “super-specialists.” A super-specialist is a physician with additional, highly specific training in a specialty area of medicine.
Retinitis pigmentosa occurs in 1 in 4,000 people in the United States. A related condition, Usher syndrome, may occur early in life. Usher syndrome includes retinitis pigmentosa accompanied by significant hearing loss. Here is a first-hand account of what it is like to live with Usher syndrome.
What Are the Symptoms and Signs of Retinitis Pigmentosa?
Since retinitis pigmentosa is a rod dystrophy, starting in the periphery of the visual field, you will start to notice an increasing difficulty in night vision, with progressive constriction, or closing in, of the visual field [i.e., the total area an individual can see without moving the eyes from side to side] to 20 degrees or less in the better-seeing eye (“tunnel vision”).
Rarely, total blindness may eventually occur. In some younger patients, in addition to the visual findings, poor hearing or deafness may occur. Fortunately, significant visual loss may not occur until later in life.
Portions of this article were published originally at www.medicinenet.com/retinitis_pigmentosa/article.htm