Overview of Retinitis Pigmentosa
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 photoreceptor cells.
Incidence & Prevalence of Retinitis Pigmentosa
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 for Retinitis Pigmentosa
Retinitis pigmentosa is caused by a genetic defect. Patterns may be of three types:
- 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 & Symptoms of Retinitis Pigmentosa
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.