Jtvetinitis pigmentosa (RP) has a prevalence of about 1 in 4000.'~ 7 An estimated 1.5 million people are affected around the world. In the United States, based on a survey in Maine, the frequency of families by genetic type has been estimated to be 19% dominant, 19% recessive, 8% X-linked, 46% isolates, and 8% undetermined. 6 Molecular genetic techniques have revealed dominant forms on chromosomes 3, 6, and 8; a recessive form on 3; a recessive form with partial deafness called Usher's syndrome, type 2 on 1; and at least two X-linked forms on the short arm of the X-chromosome. 8'18 Thus, retinitis pigmentosa is a group of diseases caused by gene abnormalities on several chromosomes.

The symptoms and signs of retinitis pigmentosa are well known. 19" 31 Affected patients typically report impaired adaptation, night blindness, and difficulty with mid peripheral visual field in adolescence. As the condition progresses, they develop a tendency to blue blindness, lose far peripheral field, and eventually lose central vision as well. Patients can have a normal fundus appearance in the early stages. In more advanced stages, signs include attenuated retinal vessels, intraretinal pigment, and waxy pallor of the optic discs. The intraretinal pigment is distributed circumferentially around the mid periphery in the zone where rods normally are at maximum concentration. Cataracts develop in most cases, 32 and some have cystoid macular edema. 3334 Refractive errors, including myopia and