Retinitis Pigmentosa Defined
Retinitis Pigmentosa (RP) is a group of rare, genetic disorders that involve a breakdown and loss of cells in the retina—which is the light sensitive tissue that lines the back of the eye. Common symptoms include difficulty seeing at night and a loss of side (peripheral) vision. 
Image courtesy of Robert N. Fariss, Ph.D., chief of the NEI Biological Imaging Core, and Ann H. Milam, Ph.D., former professor in the Department of Ophthalmology at the University of Washington.
Symptoms of RP
The first sign is usually increasing difficulty in night vision, followed by difficulty seeing in the periphery. Slowly progressive constriction of the visual field leads to tunnel vision. A small area of central vision in both eyes usually persists for years. Generally, night blindness precedes tunnel vision by years or even decades. Total blindness eventually ensues in most cases. The age of appearance of legal blindness ranges from as early as childhood to as late as the 40s. 
 “Facts About Retinits Pigmentosa.” National Eye Institute, National Eye Institute, Dec. 2015, nei.nih.gov/health/pigmentosa/pigmentosa_facts.
 Weinstock, Frank J, and Andrew A Dahl. “Retinitis Pigmentosa.” MedicineNet, www.medicinenet.com/retinitis_pigmentosa/article.htm#retinitis_pigmentosa_facts