Progressive retinal degeneration or atrophy (PRD/PRA) represents a group of inherited eye diseases characterized by abnormal development (dysplasia) or premature degeneration (deterioration) of the retina. In the United Kingdom a type of retinal degeneration is occasionally seen that affects the retinal layer behind the photoreceptors, and this disease is called central PRA. However, in the United States, PRA is primarily a disease of the light detecting cells of the retina (photoreceptors) and is called generalized PRA.
There are two types of photoreceptors in the retina and these are the light-sensitive rods and cones. They are responsible for detecting light and converting it into an electrical signal that travels to the brain. When the photoreceptor cells deteriorate, vision is lost because the animal has no way to generate an image from the light reaching the retina.
The degenerative form of PRA/PRD in dogs initially affects the rods. The rods are responsible for dim light vision; therefore, the dog loses its nighttime vision first. The disorder is progressive and eventually the cones are affected. Over time, the dog slowly goes completely blind. The disease affects both eyes at the same time.
In the dysplastic forms of the disease the rods of both eyes never develop properly, so the dog is born with poor dim light vision, and the cones rapidly degenerate as the puppy
ages. The onset of blindness is much more rapid than with the degenerative forms, and the puppies are usually blind before one year of age.
Over 30 different breeds of dogs are known to have inherited forms of PRA, including the Akita, American cocker spaniel, collie, English cocker spaniel, Irish setter, Labrador retriever, miniature dachshund
, miniature schnauzer, all three types of poodle
, Norwegian elkhound, papillion, Portuguese water dog and Tibetan terrier. The age of onset varies among the breeds. The onset of clinical signs can be as young as six weeks in dogs with the dysplastic form and as late as six to eight years in dogs with the degenerative form. Most forms of PRD/PRA are inherited as a recessive gene, although a sex-linked form occurs in the Samoyed and Siberian husky, and a dominant form is found in the Mastiff. The disease is uncommon in mixed-breed dogs.
Many dogs are not seen until the late stages of disease and have advanced changes in their retinas because they compensate very well as their vision slowly deteriorates. Sometimes the blindness can appear to be sudden in onset, even though it has been developing for months, because the dog may show almost no clinical signs until the last bit of vision has been lost.