Generality/Definition
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Retinitis Pigmentosa (RP) can be described as a progressive cause of visual loss, which is attributed to the loss of viable photoreceptors. RP usually is associated with pigmentary changes in the retinal pigment epithelium (RPE), which may be primary or secondary to the photoreceptor loss. The photoreceptors that predominantly are affected may be rods or cones, and the RPE mostly may be affected centrally or peripherally. Given the number and distribution of rods and cones in the retina, prognostic information about the patient's visual loss depends on whether the process is primarily a rod-cone or cone-rod dystrophy. Patients with rod-cone dystrophies present with ring scotoma and night vision problems, which progress to a slow loss of all peripheral vision; central vision is spared the longest. Patients with cone-rod or pure-cone dystrophies present with visual acuity loss, color discrimination loss, and day vision problems.
Source: Retinitis Pigmentosa (emedicine.com)
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Retinitis pigmentosa is a term that refers to group of hereditary disorders that affect the retina's ability to respond to light. It primarily affects rod cells, the photoreceptor cell that is responsible for night vision, seeing in dim light, and peripheral vision. Cone cells, which are responsible for color vision and seeing in bright light, may also be affected as the disease progresses. Retinitis pigmentosa is caused by a genetic defect. Retinitis pigmentosa may be caused by mutations in any one of at least ten different genes.
Source: Retinitis Pigmentosa (visionchannel.net)
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Epidemiology
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In the US: The incidence of primary RP is approximately 1 in 4000. To date, more than 70 different genetic defects have been identified, including the following: X-linked (9%), autosomal recessive (16%), and autosomal dominant (22%); the remaining cases are primary RP or RP simplex. Internationally: Worldwide incidence is same as in the United States. Sex: Usually, no sexual predilection exists. However, because of X-linked varieties, men may be affected slightly more than women. Age: The age of onset varies depending on the disorder. RP usually is diagnosed in young adulthood, although it can occur anywhere from infancy to mid 30s to 50s.
Source: Retinitis Pigmentosa (emedicine.com)
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Retinal cells are among the most specialized cells in the human body and depend on a number of unique genes to create vision. A disease-causing mutation in any one of these genes can lead to vision loss. To date, Foundation researchers have discovered over 100 genes that can contain mutations leading to Retinitis Pigmentosa.
Source: Retinitis Pigmentosa (blindness.org)
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Symptoms
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Ocular signs start with the breakdown of rod cells. Rods are present both within and outside the macula (center of the retina). The peripheral retina, responsible for side vision and vision in low light conditions, is predominantly rods. Symptoms of RP usually manifest between the ages of 10 and 30. At first, there is a decrease in night vision and the inability to see in dimly lit places such as movie theaters. The progressive loss of peripheral sight leads to what is called tunnel vision. The gradual reduction in the ability to see peripherally may cause tripping over objects or a motor vehicle accident. This occurs when rod cells and outer cone cells are affected.
Source: Retinitis Pigmentosa (visionchannel.net)
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Most forms of RP first cause the degeneration of rod cells. These forms of Retinitis pigmentosa, sometimes called rod-cone dystrophy, usually begin with night blindness. Night blindness is somewhat like the experience normally sighted individuals encounter when entering a dark movie theatre on a bright, sunny day. However, patients with Retinitis pigmentosa cannot adjust well to dark and dimly lit environments. As the disease progresses and more rod cells degenerate, patients lose their peripheral vision. Patients with Retinitis Pigmentosa often experience a ring of vision loss in their mid-periphery with small islands of vision in their very far periphery. Others report the sensation of tunnel vision, as though they see the world through a straw. Many patients with Retinitis Pigmentosa retain a small degree of central vision throughout their life. Other forms of Retinitis Pigmentosa, sometimes called cone-rod dystrophy, first affect central vision. Patients first experience a loss of central vision that cannot be corrected with glasses or contact lenses. With the loss of cone cells also comes disturbances in color perception. As the disease progresses, rod cells degenerate causing night blindness and peripheral vision.
Source: Retinitis Pigmentosa (blindness.org)
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Diagnosis
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Electroretinogram ERG is the most critical diagnostic test for RP because it provides an objective measure of rod and cone function across the retina. The full-field ERG in RP typically shows a marked reduction of both rod and cone signals, although rod loss generally predominates. A and b waves are reduced since the primary site of disease is at the photoreceptors or RPE. The ERG is usually abnormal in infancy or early childhood, except for some of the very mild and regional forms of RP. By contrast, the diagnosis for cone dystrophies is aided in part by these clinical findings but more definitively by the ERG. Severe and selective loss of cone function occurs with varying degrees of rod abnormality. In fundus albipunctatus, ERG recordings have absent rod function; after 3-4 hours of dark adaptation, ERG findings may be normal. CSNB displays a negative waveform on ERG. Electro-oculogram Electro-oculogram (EOG) findings are always abnormal when ERG findings are abnormal; therefore, EOG is not helpful to the clinician in diagnosing RP. Central macular changes, normal ERG findings, and abnormal EOG findings suggest Bests vitelliform macular dystrophy. Visually evoked cortical potentials (VECPs) rarely provide additional information to the clinician when diagnosing RP. Formal visual field This test is the most useful measure for ongoing follow-up care of patients with RP because a progressive loss of side vision is often the major symptom along with visual acuity changes. Goldmann (kinetic) perimetry is recommended to reach the far periphery. Color testing: Mild blue-yellow axis color defects are common, although most patients with RP do not clinically complain of major difficulty with color perception. Dark adaptation Contrast sensitivity often is reduced out of proportion to visual acuity in patients with RP. Patients are usually sensitive to bright light. Patients with fundus albipunctatus have poor dark adaptation but may have normal results after 3-4 hours of adaptation. Genetic subtyping : Because of the wide variety of subtypes of so-called RP or related pigmentary retinopathies, the definitive test for diagnosis is identifying the particular defect. Genetic subtyping remains primarily a research tool because of the slow process and lack of financial reimbursement
Source: Retinitis Pigmentosa (emedicine.com)
Treatment
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Medical Care: Vitamin A/beta-carotene Antioxidants may be useful in treating patients with RP, but no evidence in favor of vitamin supplementation exists; slight evidence to the contrary may even exist. A recent comprehensive epidemiologic study concluded that very high daily doses of vitamin A palmitate (15,000 U/d) slow the progress of RP by about 2% per year. The effects are modest; therefore, this treatment must be weighed against the uncertain risk of long-term adverse effects from large chronic doses of vitamin A. Annually check liver enzymes and vitamin A levels. Beta-carotene doses of 25,000 IU have been recommended. Acetazolamide In a small percentage of patients with RP, cystoid edema may respond to oral carbonic anhydrase inhibitors, such as acetazolamide, with some subjective improvement in visual function. In these patients, the macular RPE is relatively uninvolved by disease because carbonic anhydrase inhibitors must act upon functional RPE to enhance water transport. High doses of vitamin E (400 U/d) were modestly deleterious, but doses as high as 800 IU/d have been recommended. Although doses of 1000 mg/d ascorbic acid have been recommended, no evidence exists that ascorbic acid is helpful. Diltiazem A recent study in Nature Medicine showed decreased degeneration of the retina in rd mutant mice. Homologous mutations in humans represent about 4% of patients with RP. No current recommendations exist regarding the use of diltiazem (a calcium channel blocker that commonly is used in cardiac disease) in any patients with RP, including those with the homologous mutation. Lutein Lutein apparently may slow retinal degeneration, but the benefits of this substance in human diseases are uncertain. Doses of 20 mg/d have been recommended. Although bilberry is recommended by some practitioners of alternative medicine in doses of 80 mg, no controlled studies exist that document its safety or efficacy in treating patients with RP. In patients who present with antiretinal antibodies, immunosuppressive agents (including steroids) have been used with anecdotal success.
Source: Retinitis Pigmentosa (emedicine.com)
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