Progressive retinal atrophy (PRA) is a collection of inherited diseases affecting the retina that cause blindness. Each breed exhibits a specific age of onset and pattern of inheritance, and the actual mechanism by which the retina loses function can vary. The result of almost all types of PRA is similar - generally an initial night blindness, with a slow deterioration of vision until the dog is completely blind. The age at which the dog becomes fully blind also varies, depending on the genetic disruption present and the breed. Affected eyes are not painful, unless complicated by a secondary problem, such as cataract or uveitis (inflammation due to a leaking cataract).
Progressive retinal atrophy (PRA) has been classified in several different ways. The simplest of these is by age of onset. Early onset PRA occurs when the affected dog is night blind from birth, and generally is completely blind between 1 - 5 years of age. Late onset PRA is where the dog is night blind at some time over 1 year of age, and full blindness will occur at a somewhat later stage in life.
Another is by the type of genetic abnormality causing the PRA. PRA may be inherited by recessive, dominant or sex-linked mechanisms in dogs. For many types of PRA in many breeds a DNA test is now available to allow for easy screening for the disease. Despite the complexity of the disease and its many forms, ultimately all forms have one thing in common – degeneration of the retina causing progressive loss of vision.
DNA tests are not yet available for all affected breeds. And because breeds may also be prone to several forms of PRA (and not all may have a genetic test available) examination of the retina by a veterinary ophthalmologist remains a mainstay of the diagnostic testing regimen. In some breeds with a late onset PRA, serial eye examinations may be required before the signs of retinal degeneration become apparent. Electroretinography (ERG) is a diagnostic test that the veterinary ophthalmologist may choose to use in some cases and is a very sensitive method of detecting loss of photoreceptor function. The ERG can be a very good screening test for puppies that may have an early onset form of PRA.
In progressive rod cone degeneration (known as prcd-PRA) photoreceptors of the retina appear to develop normally, then develop irregularities and progressively lose function. A mutation has been discovered on a gene called PRCD, and this mutation seems to be responsible for this condition in at least 18 breeds, when a dog possesses two copies of the mutation. This is therefore an autosomal recessive mutation, and a DNA test is available.
The age of onset of retinal changes varies depending on the breed. Clinical signs may be seen from as early as 2 years of age in the golden retriever or may not be clinically apparent until 3-5 years of age, as in miniature and toy poodles. (Hence it is a late onset form of PRA.) Some cocker spaniels are even older than this when clinical signs are first seen.
Initially the disease will manifest as night blindness, but will slowly progress to blindness in bright light. Serial eye exams are required to detect the early signs of PRA. Often cataracts can develop concurrently, and this may lead to uveitis or glaucoma, which can certainly be painful and needs to be treated appropriately.
Dogs generally adapt quite well to blindness - especially when it develops gradually - as long as their surroundings remain familiar (e.g. furniture does not get rearranged, they do not move house etc). They should always be kept on a lead outside the yard, and care should be taken not to startle them. Balls containing bells (as an example) can be used as toys for mental stimulation.