Progressive Retinal Atrophy (PRA)
Progressive Retinal Atrophy is a group of eye diseases of the retina, which lead to blindness. It has actually become a bit of a catch-all term for a variety of conditions, each with different specific mechanisms, affecting different breeds at different ages, but the common factor is the blindness that ultimately results from the diseased retina.
A brief discussion of the function of the retina will help in understanding PRA and its various forms. The retina is the innermost layer of the back of the eye, and it contains the cells that are sensitive to light ("photoreceptors") and the nerves structures that run from those cells through the optic nerve and, ultimately, to the brain. There are two types of photoreceptors: rods, which function in dim light, detect shape and motion, do not differentiate color, and are more concentrated around the perimeter of the retina; and cones, which function in brighter light, detect color, and are more concentrated in the central area of the retina.
The two major categories of retinal diseases typically grouped under Progressive Retinal Atrophy are (1) retinal dysplasia, where the key cells of the retina do not develop properly in the first eight weeks of life, and (2) retinal degeneration, where the cells do develop normally in the fetus and early puppyhood, but degenerate later in life. Additionally, some forms of PRA may affect only the rod cells or only the cone cells of the retina.
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