A genetic disorder of the retina that causes gradual vision loss. With retinitis pigmentosa (sometimes called RP) the photoreceptors (rods and cones of the retina) or their supporting cells gradually stop working, affecting vision. Some people develop visual loss in infancy while others have problems later in life. Often the rods (the photoreceptors responsible for peripheral and night vision) are involved such that the vision constricts inward making it hard to see in the periphery. Night vision is often severely diminished. There are over 80 different types of retinitis pigmentosa that have been discovered. Some are inherited from family, some occur by recessive mutations that only occur when two parents with the same gene defect have a child. Some kinds of RP are sporadic with no family history at all. There have not been found to be any effective treatments for retinitis pigmentosa, though some retina doctors have tried high levels of vitamin A in the past. There is vigorous ongoing research looking for a cure for this disorder.