Encyclopedia - Retinitis Pigmentosa

What are the symptoms of this condition?
What recently discovered treatment helps this condition?

Retinitis pigmentosa is a genetic condition that causes progressive loss of vision through the gradual deterioration of the rod and cone cells of the retina. It is one of the most common human inherited eye diseases, affecting between 50,000 and 100,000 persons in the United States and 1.5 million worldwide.

Retinitis pigmentosa encompasses a number of different conditions that are caused by mutations in many different diseases. As recently as the 1990s, none of these genetic abnormalities were known. Now at least 6 different genes, and 100 different mutations, have been found to cause various forms of retinitis pigmentosa.

What are the symptoms of this condition?

The first symptoms generally do not appear until adolescence, sometimes not until middle age, and the progression to blindness can take decades, with great variation from patient to patient. The first symptom usually is a loss of night vision, affecting visual ability in dim light. Testing of the field of vision shows a ring-shaped region of blindness that gradually extends over the visual field. The rods, which are found more in the periphery of the retina, are affected more than the cones. The cones are concentrated in the macula, the centermost part of the retina, so central vision usually is retained longer. An eye examination shows masses of branching black pigment in the areas of vision loss. That pigment gives the disease its name.

What recently discovered treatment helps this condition?

Until recently, there was no treatment for the condition. In 1993, however, a large-scale study sponsored by the National Eye Institute showed that daily doses of 15,000 units of vitamin A could slow the progression of the disease. An average person in the study who started taking the 15,000-unit dose daily at age 32 retained some useful vision until age 70, the study found, while a patient not taking vitamin A would lose useful vision by the age of 63. The study also found that high doses of vitamin E had a negative effect. The disease progressed faster for patients taking 400 units of vitamin E each day. Normal dietary amounts of vitamin E had no effect.

The National Eye Institute recommends that persons with retinitis pigmentosa consult their eye professional about vitamin A supplementation, have blood levels of the vitamin measured before starting supplementation, and eat a balanced diet. In addition, retinitis pigmentosa patients should take vitamin A palmitate, the form used in the study. Vitamin A palmitate is better than beta-carotene, a natural precursor of vitamin A, because each person breaks down beta-carotene differently.

Pregnant women are advised not to take high doses of vitamin A, which have been linked to birth defects. Patients should not take more than 15,000 units daily 6, because such doses can cause side effects such as liver disease.

Identification of some of the genetic mutations has led to experiments on gene therapy, in which a virus is used to introduce a normal version of the affected gene. These experiments are in their early stage and are limited to work with laboratory animals. Some limited success has been reported, but human gene therapy for retinitis pigmentosa is at best a long-term prospect.