The cells of the retina receive light stimuli from the external environment and transmit the information to the brain where it is interpreted to become vision. In progressive retinal atrophy (PRA), deterioration of the retinal cells causes blindness.
The retina lines the back of the eye. The inner layer is the neural retina (called simply the retina) which has 9 layers, the outermost of which consists of the photoreceptor cells - the rods and cones. The outer layer of the retina is the retinal pigmented epithelium (RPE). In dogs the retina is not mature until 6 or 7 weeks of age.
The term progressive retinal atrophy covers several types of inherited degeneration (deterioration) of the retina. Sub-classifications of PRA are based on the age at which dogs show signs of the disease and the type of retinal cell which is affected.
Generalized PRA:These diseases affect primarily the photoreceptor cells. Both eyes are similarly affected and dogs eventually become totally blind.
i) Early onset photoreceptor dysplasias: In these conditions, the photoreceptor cells develop abnormally in the first few weeks after birth, and then degenerate along with the inner layers of the retina.
ii) Later onset photoreceptor degeneration (progressive rod-cone degeneration): Here the retina matures and functions apparently normally for varying periods of time before degenerating. Dogs are not usually clinically affected until 1 year of age or more, although abnormalities can be seen in the eye and on the electroretinogram (ERG) long before owners notice signs of visually impairment.
Progressive rod-cone degeneration has similarities to retinitis pigmentosa in people.
Central PRA:(also called RPE dystrophy) The abnormality is in the retinal pigmented epithelium (RPE). The photoreceptor cells will also degenerate eventually. The rate of vision loss is much slower than with generalized PRA, and not all dogs become totally blind.
Many breeds are affected by one or sometimes more than one form of PRA.
Generalized PRA - early onset: Cardigan Welsh corgi, collie (rod-cone dysplasia type II), Cairn terrier, Gordon setter, Great Dane, Irish setter (rod-cone dysplasia type I), miniature schnauzer (photoreceptor dysplasia), Norwegian elkhound (rod dysplasia, also early retinal degeneration), Tibetan terrier (progressive rod degeneration causing night blindness only)
Alaskan malamute - progressive cone degeneration causing hemeralopia (day blindness) - this condition occurs rarely.
Generalized PRA (progressive rod-cone degeneration) - later onset (usually older than 1 year): Akita, Australian cattle dog, Australian shepherd, American and English cocker spaniel, Basenji, beagle, Belgian sheepdog, Briard, Brittany spaniel, Chesapeake Bay retriever, collie (rough and smooth), dachshund, English springer spaniel, German shepherd, German short-haired pointer, golden retriever, greyhound (without typical initial night blindness), Irish setter, Labrador retriever, mastiff, Nova Scotia duck tolling retriever, old English sheepdog, papillon, pekingese, poodle (miniature and toy), Portuguese water dog, Rottweiler, samoyed, Shetland sheepdog, Shih tzu, Siberian husky, Tibetan spaniel, Tibetan terrier, Welsh springer spaniel, Yorkshire terrier
Central PRA - retinal pigment epithelial dystrophy (RPED): This disease occurs mostly in dogs in the United Kingdom, of the following breeds: border collie, Cardigan Welsh corgi, English cocker spaniel, English springer spaniel, golden retriever, Labrador retriever, rough and smooth collie, Shetland sheepdog
Retinal degeneration in the Borzoi: Unlike other forms of PRA, the eyes are affected asymmetrically and the retinal lesions appear inflammatory. Males are affected more often than females. Ultimately, the ophthalmoscopic lesions are similar to those of PRA.
For many breeds and many disorders, the studies to determine the mode of inheritance or the frequency in the breed have not been carried out, or are inconclusive. We have listed breeds for which there is a consensus among those investigating in this field and among veterinary practitioners, that the condition is significant in this breed.
Generalized PRA - early onset: The first sign is generally failing night vision, as early as 6 weeks of age, and this progresses to complete loss of vision by about 1 - 2 years of age. Collies may retain some vision until the age of 2 - 3 years. In miniature schnauzers, poor night vision usually develops later (6 months to a year) and there is advanced loss of vision by 3 to 4 years. Affected Alaskan malamutes are day-blind (hemeralopia) at 8 to 10 weeks of age; night vision is never affected.
Generalized PRA (progressive rod-cone degeneration) - late onset: Generally night blindness is noticed between 2 and 5 years of age (depending on the breed) progressing to total blindness within a year or so. Peripheral vision is lost first.
Central PRA (CPRA) - retinal pigment epithelial dystrophy (RPED): Loss of vision occurs much more slowly than in generalized PRA, without initial night blindness. Affected dogs may not lose vision completely. Because the changes are in the centre of the retina, affected dogs initially have trouble locating still objects in bright light.
There are no obvious external changes to the eyes. You may notice that your dog has difficulty getting around when the lights are turned off, or when outside at night. If you suspect that your dog has impaired vision, your veterinarian will look for abnormalities with an ophthalmoscope. PRA may also be detected by electroretinogram (ERG) before your dog has any apparent visual difficulties. Electroretinography, which measures electrical patterns in the retina, is usually only available in specialty veterinary centres. (See CERF website listed in references below.)
Genetic testing is quickly becoming available for different forms of PRA in different breeds. The advantage of such testing is that it can identify dogs whose sight is unaffected, but who are carriers of the disorder (heterozygotes).
FOR THE VETERINARIAN:
Generalized PRA
1. Ophthalmoscopic exam: retinal thinning is seen as hyper-reflectivity of the tapetal fundus, attenuation of the retinal vessels, and shrinking and pallor of the optical disc; cataracts and/or retinal detachment may occur late in the disease.
2. Electroretinogram: Generalized PRA can be detected by ERG long before it is apparent clinically.
3. DNA testing: Rod-cone dysplasia or rcd1 can be detected in Irish Setters by polymerase chain reaction. DNA testing for PRA is also available for Chesapeake Bay retrievers, Labrador retrievers and Portuguese water dogs. For more information, see resources below.
CPRA
1. Ophthalmoscopic exam: initially central multiple light to dark brown spots within tapetal fundus, varying in size, shape and density, due to accumulation of lipopigment within the photoreceptor layer. You will also see hyper-reflectivity and retinal vessel attenuation as the disease progresses.
2. Electroretinogram: The ERG has not been found useful in the early diagnosis of CPRA because the photoreceptor cells are only affected later in the course of the disease.
There is no treatment for PRA. The degree of visual impairment varies with the breed and specific type of retinal degeneration as described above, but most affected dogs will ultimately be completely blind. With their acute senses of smell and hearing, dogs can compensate very well, particularly in familiar surroundings, to the point where owners may be unaware of the extent of vision loss.
You can help your dog by developing regular routes for exercise, maintaining consistent surroundings, introducing any necessary changes gradually, and being patient.
Breeding is not advised for any dog with PRA, or for the parents (assumed to be carriers). Siblings should be carefully screened by electroretinogram if they are considered for breeding. Generalized PRA can often be detected by electroretinography at least a year before clinical signs are apparent.
For some disorders (eg. rod-cone dysplasia type I in Irish setters), blood-based DNA tests are available which can distinguish normal animals from those who are clinically normal but are carriers, and from those that are affected but are not yet showing any signs. Test results are registered through the Canine DNA Registry administered by the Canine Eye Registration Foundation (CERF). DNA testing for PRA is also available for Chesapeake Bay retrievers, Labrador retrievers and Portuguese water dogs. For more information see the resources below.
FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.
American College of Veterinary Ophthalmologists. 1995. Ocular Disorders Presumed to be Inherited in Purebred Dogs. This reference is helpful in differentiating disorders specific to different breeds.
Ackerman, L. 1999. The Genetic Connection. p. 162-167. AAHA Press. Lakewood, Colorado. This reference contains good information on inheritance, age of onset and form of PRA in different breeds.
