Retinitis pigmentosa is the name given to a set of heritable degenerations of the retina. Patients with retinitis pigmentosa typically experience night blindness and loss of the midperipheral visual field early in the disease. As the disease progresses, the visual field is reduced to a shrinking island of central vision (called tunnel vision) and scattered patches of far peripheral vision. In many cases, all useful vision is lost during middle age. In advanced cases and sometimes even in early cases, the fundus oculi exhibits the following features: thin retinal vessels, a pale optic nerve head, and clumps of intraretinal pigment. Related diseases that feature progressive degeneration of the retina include cone-rod degeneration, cone degeneration, and macular degeneration. In cone-rod degeneration, there is early loss of visual acuity due to a progressive panretinal degeneration that in early stages affects cone photoreceptors more severely than rod photoreceptors. In cone degeneration, cones but not rods degenerate. In macular degeneration (see Chap. 293), cones and rods degenerate in the macula but not elsewhere in the retina. Most forms of retinitis pigmentosa and related retinal degenerations affect only the eye, although in a minority of cases the retinal degeneration is one feature of a syndrome that includes other systemic abnormalities, such as retinitis pigmentosa and deafness in Usher syndrome. In young patients with the symptom of night blindness, it is important to differentiate retinitis pigmentosa from stationary night blindness. Both diagnostic categories feature defective vision in dim light as an early symptom, but in stationary night blindness, cone photoreceptors and daytime vision do not deteriorate substantially during the patient's lifetime.
Measuring the retina's electrical response to flashes of light is the best way to distinguish the early stages of progressive retinal degeneration such as retinitis pigmentosa from nonprogressive retinal diseases such as stationary night blindness. This noninvasive measurement, called the electroretinogram (ERG), can be used to diagnose young patients with retinitis pigmentosa and many other forms of retinal degeneration even before visual symptoms or funduscopic abnormalities are apparent.
The visual loss in retinitis pigmentosa and related retinal degenerations corresponds to the degeneration of photoreceptor cells of the retina. Molecular genetic and biochemical studies have revealed that the defects causing these cells to die can be placed into three categories depending on the mutant gene in each case: (a) a primary biochemical defect inherent in the photoreceptor cells (the rods, the cones, or both cell types), (b) a primary biochemical defect in neighboring retinal cells such as the retinal pigment epithelium, or (c) a peculiar sensitivity of the photoreceptors or the retinal pigment epithelium to a generalized metabolic defect. In stationary night blindness, rod photoreceptors malfunction because of defects inherent in them or perhaps in the cells they synapse with; however, the rod or cone photoreceptors do not degenerate at a rate symptomatically faster than that which accompanies normal aging.
Families with retinitis pigmentosa, cone-rod degeneration, cone degeneration, or stationary night blindness can demonstrate any of ...