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Progressive retinal atrophyProgressive retinal atrophy (PRA) is a group of genetic diseases characterized by degeneration of the retina that occurs bilaterally. It is seen in certain breeds of dogs and more rarely, cats. It causes progressive vision loss culminating in blindness. The condition in nearly all breeds is inherited as an autosomal recessive trait, with the exception of the Siberian Husky (inherited as an X chromosome linked trait) and the Bullmastiff (inherited as an autosomal dominant trait).[1] There is no treatment. PRA is similar to retinitis pigmentosa in humans.[2] Additional recommended knowledge
DiagnosisProgressive vision loss in any dog in the absence of glaucoma or cataracts can be an indication of PRA. It usually starts with decreased vision at night, or nyctalopia. Other symptoms include dilated pupils and decreased pupillary light reflex. Fundoscopy to examine the retina will show shrinking of the blood vessels, decreased pigmentation of the nontapetal fundus, increased reflection from the tapetum due to thinning of the retina, and later in the disease a darkened, atrophied optic disc. Secondary cataract formation in the posterior portion of the lens can occur late in the disease. In these cases diagnosis of PRA may require electroretinography (ERG). For many breeds there are specific genetic tests of blood or buccal mucosa for PRA.[1] Types of PRAGeneralized PRA is the most common type and causes atrophy of all the neural retinal structures. Central progressive retinal atrophy (CPRA) is a different disease from PRA involving the retinal pigment epithelium (RPE), and is also known as retinal pigment epithelial dystrophy (RPED). Generalized PRA can be divided into either dysplastic disease, where the cells develop abnormally, and degenerative, where the cells develop normally but then undergo a damaging change. PRA can be further divided into affecting either rod or cone cells. Rod cells detect shape and motion, and function in dim light. Cone cells detect color and definition, and function in bright light. Generalized PRACommonly affected breeds:[3]
Rod-cone dysplasiaThis type of PRA has an early onset of severe vision loss. It is caused by a defect in the gene for cGMP-phosphodiesterase, which leads to retinal levels of cyclic guanosine monophosphate ten times normal.[4] Rod-cone dysplasia type 1
Rod-cone dysplasia type 2
Rod-cone dysplasia type 3
Rod dysplasia
Early retinal degeneration
Photoreceptor dysplasiaThis is caused by an abnormal development of both rod and cone cells. Dogs are initially night blind and then progress to day blindness.
Cone degeneration
Progressive rod-cone degeneration (PRCD)This is a disease with normal rod and cone cell development but late onset degeneration of the rod cells that progresses to the cone cells. It is inherited as an autosomal recessive trait and has been linked to the ninth canine chromosome.[2]
X-linked PRAThis condition is linked to the X chromosome.
Dominant PRA
Feline PRA
Central progressive retinal atrophy (CPRA)CPRA is also known as retinal pigment epithelial dystrophy (RPED). The cause of this condition is the loss of the retinal pigment epithelium's ability to effectively process the photoreceptor outer segment (POS) and subsequent accumulation of POS material in the RPE and loss of function. The loss of function of the RPE leads to photoreceptor degeneration.[4] Vitamin E deficency may play a role in the development of CPRA.[7] It is characterized by accumulation of pigment spots in the retina surrounded by retinal atrophy and a mottled appearance of the pigmented nontapetal fundus. The pigmented spots eventually coalesce and fade as the atrophy of the retina increases. It is an inherited condition (in the Labrador Retriever it is inherited as an autosomal dominant trait with variable penetrance).[1] CPRA occurs in older dogs. Peripheral vision is retained for a long time. Vision is better in low light and better for moving or distant objects. Not all affected dogs go blind. Secondary cataracts are common. Commonly affected breeds
Hereditary retinal dysplasiaThere is another retinal disease in Briards known as hereditary retinal dysplasia. These dogs are night blind from birth, and day vision varies. Puppies affected often have nystagmus. It is also known as lipid retinopathy.[3] See also
References
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Progressive_retinal_atrophy". A list of authors is available in Wikipedia. |