Retinitis pigmentosa (RP) is a group of inherited disorders characterized by progressive peripheral vision loss and night vision difficulties (nyctalopia) that can lead to central vision loss. With advances in molecular research, it is now known that RP constitutes many retinal dystrophies and retinal pigment epithelium (RPE) dystrophies caused by molecular defects in more than 100 different genes.RP is typically thought of as a rod-cone dystrophy in which the genetic defects cause cell death (apoptosis), predominantly in the rod photoreceptors; less commonly, the genetic defects affect the RPE and cone photoreceptors. The phenotypic variation is very significant because over 100 genes can cause RP.e prevalence of typical RP is reported to be approximately 1 in 4000 in the United States. The carrier state is believed to be approximately 1 in 100. The highest reported frequency of occurrence for RP is among the Navajo Indians at 1 in 1878.Worldwide prevalence of RP is approximately 1 in 5000. The frequency of occurrence for RP has been reported to be as low as 1 in 7000 in Switzerland.A multicenter population study by Grover et al of patients with RP who were at least 45 years or older found the following findings: 52% had 20/40 or better vision in at least one eye, 25% had 20/200 or worse vision, and 0.5% had no light perception.Usually, no sexual predilection exists. X-linked RP is expressed only in males; therefore, because of these X-linked varieties, men may be affected slightly more than women.Usually, no sexual predilection exists. X-linked RP is expressed only in males; therefore, because of these X-linked varieties, men may be affected slightly more than women.

http://emedicine.medscape.com/article/1227488-overview