Retinitis Pigmentosa (RP) is an inherited retinal dystrophy that is characterized by retinal degeneration and progressive vision loss in affected individuals.  The disorder is caused by the gradual atrophy of the photoreceptor cells in the retina.  Patients initially lose their rod photoreceptors, resulting in night vision loss (nyctalopia) and subsequently peripheral vision loss (tunnel vision).  As the disease progresses, cone photoreceptors are affected, leading to problems with color vision, visual acuity and loss of the central visual field.