Usually with PRA, both eyes are affected to the same degree. Photo courtesy of Noelle La Croix, DVM, DACVO
Progressive retinal atrophy, or PRA, describes a group of inherited degenerative disorders of the retina that occur commonly in dogs and rarely in cats. The retina is like the film in a camera. It contains photoreceptors that convert light into electrical nerve signals. These disorders cause the photoreceptors to die prematurely.
The different forms share similar features but have different genetic causes. Many different gene mutations are known to cause PRA. It is not known what the underlying mutation for many forms of PRA is, but more and more mutations are being found every year. PRA has no current commercially available treatment and no cure. PRA alone is not painful.
In experimental research situations, the first cures of some dogs with PRA have taken place with gene therapy. This was used as a stepping stone to develop treatment for humans with PRA caused by the same genetic mutation. With gene therapy for PRA, a known abnormal gene causing cell death and PRA is surgically replaced with the normal gene. This is an emerging and promising field of medicine, but this treatment is not commercially available for pets and probably will not be any time soon.
Some of the photoreceptor death in PRA is due to oxidative stress, so oral antioxidant therapy may help slow the rate of vision loss, but will not stop a dog from going blind.
Both eyes become blind, but the vision loss occurs slowly, giving dogs time to adjust to the changes. Typically, the first thing caregivers notice is their dog losing ability to see at night; then over a few months to one to two years, the ability to see under bright light conditions during the day is also lost. Caregivers may notice their dog’s reluctance to go outside at night, enter a dark room, or go up or down (especially stairs). Their dog may also have dilated pupils that don’t constrict well in bright lights, as well as an increase in the brightness of the normal eye shine visible within the pupil, called the tapetal reflection.
Dogs often adapt so well and the vision changes are so slow that no one notices anything different until their dog is in a new place or when loss of vision is almost complete.
The onset of PRA depends in part on whether the photoreceptors never form properly in the first place (photoreceptor dysplasias) or whether they form properly but then degenerate (photoreceptor degenerative disorders). In the most common type, the photoreceptors develop and function normally at first but then begin to deteriorate at some point. In the earliest onset form, puppies can have decreased vision by 12 weeks of age and may be blind by 1 to 2 years old. Some degenerative forms begin in early adulthood, some when the dogs are mature, and some as late as 9 to 11 years of age. The type or form differs between breeds of dogs.
PRA is most commonly diagnosed in purebred dogs, but it can be seen in mixed breed animals. Specific genetic mutations are known to cause PRA in specific breeds but some breeds can have the potential to be affected by more than one type of mutation. Breeds disposed toward the more common degenerative form include the miniature poodle, American and English cocker spaniels, Tibetan terriers, Samoyed, Akita, longhaired and wirehaired dachshunds, Labrador retriever, Papillion, Tibetan spaniel, and many more. Breeds disposed toward the less common photoreceptor dysplasia include the Norwegian elkhound, Irish setter, collie, Cardigan Welsh corgi, and miniature schnauzer.
Some dogs with PRA may get secondary cataracts or they may also be prone to inherited cataracts that make the vision loss happen more quickly. Because of the PRA, no affected dog is a good candidate for surgical removal of the cataracts. Usually, dogs with PRA are not expected to have a higher risk of any other eye problems unless they develop secondary cataracts. Cataracts can cause an inflammatory reaction in the eye known as lens-induced uveitis, and this can lead to other problems like glaucoma, where the pressure in the eye is high. Both uveitis and glaucoma can be painful, so dogs with secondary cataracts are usually treated long-term with anti-inflammatory eye drops to control the lens-induced uveitis and to try to reduce the risk of further complications. Dogs with secondary cataracts usually need regular eye exams to monitor for complications.
Diagnosis
Diagnosis is usually made with an eye examination. Your veterinarian may choose to refer your dog to a veterinary ophthalmologist to confirm the diagnosis if PRA is suspected. The eye exam involves evaluating the fundus (retina/back of the eye) with specialized lenses. Electroretinography can be used to look at the electrical responses in the retina, and is a good way to confirm PRA if retinal findings are inconclusive. Electroretinography is usually performed under sedation by a veterinary ophthalmologist. Secondary cataracts are diagnosed with an eye exam, but they may take a while to show up.
Since many genetic mutations that cause PRA have been identified, DNA testing for PRA is available for some breeds; the test can detect if a dog has a specific genetic mutation causing PRA or is a carrier for it. DNA testing is especially useful for screening dogs before they are used for breeding because it can help determine if a dog is going to be affected before they show any signs. Affected dogs should not be bred, as that is the only prevention available. Most forms of PRA need a dog to have two copies of the mutation to be affected -- one inherited from each parent.
DNA tests can also find carrier animals, who have one copy of the mutation and are not affected, but who could pass on the mutation to their offspring. Rarely, some forms of PRA only require a dog to have one copy of the mutation. Many companies offer DNA testing and new tests are regularly becoming available as more underlying mutations are identified. Some examples of companies are Laboklin, UC Davis, and Embark. Their websites can be consulted for further information on which breeds can be tested using the DNA test.
Living with a Blind Dog
Because vision is the third most important sense to the dog (smell is first, hearing is second), most dogs do very well when they are blind. Since PRA develops slowly in most dogs, they have time to memorize their environment, hone the use of their other senses, and adjust to living without vision. They can be so good at doing this that it may be impossible to tell when they finally go completely blind.
Living with a dog who has PRA is not difficult. Your reaction to the loss of vision is likely to be much stronger than the dog’s. Most of these dogs don't need any medication because PRA alone does not cause any pain, and PRA does not affect the rest of the dog as can many other causes of blindness. If your dog develops secondary cataracts, these may require anti-inflammatory medication and monitoring. Keep in mind that it is easiest for dogs with vision loss if the furniture is not moved and children are taught not to leave toys and clothes out in the dog’s usual pathways. Safety gates, like those used for babies, can help keep dogs safe from falling down open stairs and prevent other mishaps. Swimming pools can also be a hazard and may need to fenced off. Keep food and water bowls in the same location. Most blind dogs do well outside in their own fenced yards. They can be taught certain commands like “wait,” “curb,” “down,” or “stop” when being walked outside. If you know in advance that your dog has a condition like PRA that will make them to go blind, it is an opportunity to teach them vocal commands instead of hand signals. Similarly, playing with a ball that has a bell inside to make noise while they can still see helps train them to be still able to play ball after they lose vision. When travelling, leashes and harnesses are valuable tools.
Sometimes other dogs in the household will serve as seeing-eye dogs for their blind companion. The blind dog will follow the sighted dog around and pick up on cues from the other dog.
Be sure to communicate with the blind dog using his other senses, such as smell and sound. Talk to him, avoid startling him when he is asleep by calling him or tapping your foot near him, slap your leg as you walk if you want him to follow you, etc.
As long as you have a good attitude towards your dog’s condition and encourage him to remain an active participant in the family and home, most affected dogs remain happy and otherwise healthy.