What Is Retinitis Pigmentosa (RP)?
Retinitis Pigmentosa (RP) is the name given to a group of hereditary retinal diseases characterized by progressive loss of visual field, night blindness and reduced or absent electroretinogram (ERG test) recording, which indicates that a large portion of the retina is damaged.
RP causes the degeneration of photoreceptor cells in the retina. Photoreceptor cells capture and process light helping us to see. As these cells degenerate and die, patients experience progressive vision loss.
There are two types of photoreceptor cells: rod cells and cone cells. Rod cells are concentrated along the outer perimeter of the retina. Rod cells help us to see images that come into our peripheral or side vision. They also help us to see in dark and dimly lit environments. Cone cells are concentrated in the macula, the center of the retina, and allow us to see fine visual detail in the center of our vision. Cone cells also allow us to perceive color. Together, rods and cones are the cells responsible for converting light into electrical impulses that are transmitted to the brain where "seeing" actually occurs.
What Are the Different Types of Retinitis Pigmentosa?
Genetics of RP
Within the nucleus of every human cell reside a host of genes. Genes are the fundamental building blocks of life. Inherited from our parents, genes carry family traits like eye and hair color, the shape of our face and even diseases like RP.
Genes are like computer programs containing sets of coded instructions. Each gene instructs the cell to create a specialized protein that performs a specific task for the cell. In retinal cells, some genes encode proteins that allow the cell to process light. Other genes encode proteins that uptake nutrients and eliminate waste. Still other genes encode proteins that form the cell walls and other structures within the cell.
Sometimes, the coded instructions within a gene become altered. These alterations, known as mutations, can confer a benefit, allowing the organism to better adapt to its environment. However, mutations can also interfere with the proper encoding of a protein. The resulting protein cannot perform its job within the cell, thereby hampering the cell's well being and leading to disease.
Retinal cells are among the most specialized cells in the human body and depend on a number of unique genes to create vision. A disease-causing mutation in any one of these genes can lead to vision loss. To date, researchers have discovered over 100 genes that can contain mutations leading to RP.
RP can be passed to succeeding generations by one of three genetic inheritance patterns - autosomal dominant, autosomal recessive, or X-linked inheritance.
Each type of inheritance causes a different pattern of affected and unaffected family members. For example, in families with autosomal recessive RP, unaffected parents can have both affected and unaffected children. In recessive RP, there is often no prior family history. In families with the autosomal dominant RP, an affected parent can have both affected and unaffected children. In families with the X-linked type, only males are affected, while females carry the genetic trait but do not experience serious vision loss.
It is very important to remember that because RP is an inherited disorder, it can potentially affect another member of the family. If one member of a family is diagnosed with a hereditary retinal degeneration, it is strongly advised that all members of that family contact an ophthalmologist.
Other inherited diseases share some of the clinical symptoms of RP. Some of these conditions are complicated by other symptoms besides loss of vision. The most common of these is Usher syndrome, which causes both hearing and vision loss. Other rare syndromes include Bardet-Biedl (Laurence-Moon) syndrome, rod-cone dystrophy, choroideremia, gyrate-atrophy, Leber congenital amaurosis, and Stargardt's disease.
RP and related diseases are rare and difficult to accurately diagnose. Only a specialist can properly distinguish between the subtle clinical features of these diseases. Therefore, it is important that patients who are symptomatic see an ophthalmologist who specializes in retinal degenerative diseases.
- Normal visual acuity in early stages, possibly -- but not usually -- progressing to no light perception
- Donut shaped visual field loss progressing to severe constriction (loss of peripheral vision)
- Night blindness
- Decreased response to magnification
- Need for more light
The most common feature of all forms of RP is a gradual degeneration of the rods and cones. Most forms of RP first cause the degeneration of rod cells. These forms of RP, sometimes called rod-cone dystrophy, usually begin with night blindness. Night blindness is somewhat like the experience normally sighted individuals encounter when entering a dark movie theatre on a bright, sunny day. However, patients with RP cannot adjust well to dark and dimly lit environments.
As the disease progresses and more rod cells degenerate, patients lose their peripheral vision. Patients with RP often experience a ring of vision loss in their mid-periphery with small islands of vision in their very far periphery. Others report the sensation of tunnel vision, as though they see the world through the opening of a straw. Many patients with RP retain a small degree of central vision throughout their life.
Other forms of RP, sometimes called cone-rod dystrophy, first affect central vision. Patients first experience a loss of central vision that cannot be corrected with glasses or contact lenses. With the loss of cone cells also comes disturbances in color perception. As the disease progresses, rod cells also degenerate causing night blindness and loss of peripheral vision.
Symptoms of RP are most often recognized in children, adolescents and young adults, with progression of the disease continuing throughout the individual's life. The pattern and degree of visual loss are variable.
These special tests can be used to help diagnose RP:
These tests measure the accuracy of your central vision at specific distances in specific lighting situations.
This can help determine the status of your cone cells, the retinal cells that interpret color.
Visual Field Test
This test uses a machine to measure how much peripheral vision you have.
Dark Adaptation Test
This test will measure how well your eyes adjust to changes in lighting and can help the doctor better understand the current function of your rod cells, which are the retinal cells responsible for night vision.
The ERG (electroretinogram test) records the electrical currents produced by the retina due to a light stimulus. The intensity and speed of the electrical signal becomes reduced as the photoreceptor cells degenerate.
Recent research findings suggest that in some forms of RP, prolonged, unprotected exposure to sunlight may accelerate vision loss.
Some women feel that their vision loss progressed more rapidly during pregnancy. However, the effect of pregnancy on RP has not been clinically studied.
RP is an inherited, genetic disease. It is caused by mutations in genes that are active in retinal cells. Gene mutations are programmed into your cells at the time of conception. RP is not caused by injury, infection or exposure to any toxic substance.
What You Can Do to Reduce Risk
Reducing your exposure to sunlight is important for keeping the eye protected. However, since RP is an inherited disorder and it runs in families, the disease is not preventable. If someone in your family is diagnosed with a retinal degeneration, it is strongly advised that all members of the family contact an eye care professional.
As yet, there is no known cure for RP. However, intensive research is currently under way to discover the cause, prevention, and treatment of RP.
Researchers have identified some of the genes that cause RP. It is now possible, in some families with X-linked RP or autosomal dominant RP, to perform a test on genetic material from blood and other cells to determine if members of an affected family have one of several RP genes.