Progressive Retinal Atrophy (also known as "PRA") is a group of inherited diseases that cause degeneration of the retina in dogs and results in permanent blindness. The retina is a thin layer of nervous tissue that lies in the back of the eye and is reponsible for converting light into electrical impulses. The electrical impulse is then transmitted along the optic nerve to the brain, where the electrical impulses are interpretted as an image. The cells within the retina that are directly responsible for the conversion of light to an electrical impulse are called photoreceptors. There are two types of photoreceptors: rods and cones. The rod rods are responsible for dim light vision, and the cones are responsible for bright light and color vision. Progressive Retinal Atrophy begins with degeneration of the rod photoreceptors. This may be noticed by pet owners as night blindness or decreased "confidence" in dimly lit areas. As the degeneration of rods progresses, the cones will begin to degnerate. Therefore, loss of vision in brightly lit environments will occur later in the progression of the disease. As PRA progresses slowly, many dogs will learn to partially accomodate for their visual deficits through their senses of hearing and smell. Because of this accommodation, it is not uncommon for some pet owners notice the visual deficits only after significant degneration of both rods and cones has occured.
At present, researchers at the University of Missouri are studying PRA in English Springer Spaniels. This research is likely to be expanded to other breeds in the near future - please check back for details breeds to be looked at next.
Currently, the most commonly performed diagnositic test for PRA is a complete ophthalmic examination. A veterinary ophthalmologist can detect the later stages of PRA by examining the retina with indirect ophthalmoscopy. Unfortunately, the earliest stages of PRA are not detected in such an exam. As the age of onset of PRA can vary widely from animal to animal, annual examination and eye registration of breeding stock by a board-certified veterinary ophthalmologist through the Canine Eye Registration Foundation (CERF) is recommended.
Another test that can be performed to diagnose PRA is an electroretinogram (ERG). Electroretinography measures the electrical response of the retina that occurs to known amounts of triggered light stimulation. An electroretinogram is capable of detecting PRA much earlier than it can be diagnosed through ophthalmoscopy. Although it has increased sensitivity for early PRA, electroretinography also requires more specialized instrumentation, experience, and general anesthesia for early PRA detection. For these reasons electroretinography is not commonly used as a "screening" tool for PRA, but is instead used to confirm a suspected diagnosis or in research settings where early detection of PRA is required.
Unfortunately, blindness from PRA is permanent and it has no cure.