Retinitis pigmentosa

Retinitis pigmentosa (RP) is a heterogeneous group of inherited retinal disorders characterized by progressive bilateral degeneration of the rod and cone photoreceptors that leads to night blindness and progressive visual field defects.

RP belongs to the group of pigmentary retinopathies. Historically, classifications of RP have been based on either distribution of the retinal involvement (central, pericentral, sector or peripheral subtypes) or age of onset or modes of inheritance or predominant photoreceptors involved. At present, there is no uniformly accepted classification system of RP. Many authors divide RP either into two main groups: isolated (when the pathology is confined to the eye alone) and syndromic (when the ocular degeneration is associated with abnormalities in one or more organ systems) or three main groups: nonsyndromic, or "simple" (affecting the eye alone), syndromic (affecting other systems, such as hearing) or systemic (affecting multiple tissues and organs). It is generally considered that 70-80% of all RP cases fall in the category nonsyndromic rod-cone dystrophy.

The retina is the light-sensitive tissue lining the inner surface of the eye. It is a complex, layered structure with several layers of neurons interconnected by synapses (Figure 1).

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Figure 1. Structure of the retina - Origin:

The only neurons directly sensitive to light are the photoreceptor cells that through a cascade of biochemical events convert the light into nerve impulses, which are then processed by the retina and sent through nerve fibers to the brain. There are mainly two photoreceptor types: the rods and cones. Rod photoreceptors are responsible for motion detection and vision in scotopic conditions (night vision). Cone photoreceptors provide trichromatic color and high contrast vision and operate in photopic conditions (i.e. normal daylight conditions). A third, much rarer type of photoreceptor, the photosensitive ganglion cell, is important for reflexive responses to bright daylight (nyctemeral cycle).

In RP, light-sensitive rods and cones are damaged and visual symptoms indicate the gradual loss of the photoreceptor function leading to irreversible visual loss. The outer nuclear layer of the retina (consisting of rod and cone photoreceptor nuclei) is severely attenuated in patients with RP. The inner nuclear layer (composed of nuclei from the amacrine cell, bipolar cell and horizontal cell neurons) and the ganglion-cell layer are fairly well preserved until late in the disease course. In most forms of typical RP (rod-cone RP), rods are affected more severely than the cones. In other RP types, rod and cone decline is similar. Occasionally, the deficit of cones far exceeds that of rods. In this case, the condition is termed cone-rod degeneration, a form of RP in which loss of visual acuity and defective color vision are the prominent early symptoms.