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Retinitis Pigmentosa (RP)
Retinitis pigmentosa (RP) is a group of inherited eye diseases that affect the retina. 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 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.
Symptoms
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 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 degenerate causing night blindness and 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.
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 from the cell. 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 resultant 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, Foundation 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.
Related diseases
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 that researchers are studying with funding from The Foundation Fighting Blindness include Bardet-Biedl (Laurence-Moon) syndrome, Best disease, choroideremia, gyrate-atrophy, Leber congenital amaurosis, and Stargardt 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.
SOURCE: The Foundation Fighting Blindness Blindness.org
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