Retinitis Pigmentosa and Allied Disorders

Retinitis pigmentosa (RP) is a generic term for a group of disorders characterized by hereditary diffuse usually bilaterally symmetrical progressive dysfunction, cell loss and eventual atrophy of retina. Initially photoreceptors are involved and subsequently inner retina is damaged. Although both rods and cones are involved, damage to the rods is predominant. RP may be seen in isolation (Typical RP) or in association with systemic diseases. The reported prevalence of typical RP is approximately 1: 50000 worldwide. Most commonly 46% of the cases are sporadic with only one affected member in a given family. Xlinked recessive inheritance is least common, amounting to 8%. Autosomal dominant inheritance is found in 19% and recessive in 19%. The age of onset and the natural history of the disease depend on the inheritance. Genetic heterogeneity exists within an inheritance pattern, for instance nine genetic loci exist on seven chromosomes for dominantly inherited RP and three genetic loci exist for x-linked RP. Therefore, RP is a group of diseases caused by abnormal genes at various loci within the human genome (1). Typical Retinitis Pigmentosa Clinical Features :Night blindness (Nyctalopia):People with RP have constriction of the visual field in dark and visual disorientation in dim lit environments. Becoming accident prone especially at night is a highly suggestive symptom. Onset of night blindness occurs at median age of 10.7 years in autosomal recessive disease and 23.4 years in autosomal dominant disease. Nyctalopia can also be a feature of high myopia, congenital stationary night blindness and age related macular degeneration (2). Visual field loss is insidious, progressive, peripheral and symmetric between two eyes (except x-linked RP which can have bizarre and asymmetric patterns). In the majority of patients the earliest defects are relative scotomas in the periphery between 30 and 50 degrees from fixation, which enlarge, deepen and coalesce to form a ring of visual field loss. As ring scotomas enlarge toward the far periphery, islands of relatively normal vision remain usually temporal but occasionally inferiorly. In typical RP the progression of visual loss is slow and relentless. Berson et al found that overall about 4.6% of remaining visual field was lost per year (3).