What is the function of the retina?
The retina is a light sensitive layer of cells at the back of the eye that contains cells called photoreceptors. When light enters the eyes, it is focused by the lens onto the retina, where it is converted into electrical signals that are sent to the brain for processing and interpretation.
The two main photoreceptor cells of the retina are the rod cells and the cone cells. The dog's eyes contain many more rods than cones. Rod cells are responsible for vision in low light conditions and for detecting and following movement. Cone cells are responsible for detecting color. Cone cells do not work very well in low light.
What is progressive retinal atrophy?
Atrophy means the partial or complete wasting or deterioration of a body part. Progressive retinal atrophy (PRA) is a group of degenerative diseases that affect photoreceptor cells. With this disease, the cells deteriorate over time, eventually leading to blindness in the affected dog.
"Atrophy means the partial or complete wasting or deterioration of a body part."
Are there different forms of PRA?
There are two main forms of PRA recognized in dogs:
- an early onset or inherited form, also called retinal dysplasia, which is typically diagnosed in puppies around two to three months of age, and
- a late onset form that is detected in adult dogs, usually between the ages of three to nine years.
The early onset form is commonly called retinal dysplasia and the late onset form is commonly called PRA.
How does it happen?
With retinal dysplasia, the photoreceptor cells of the retina develop abnormally, leading to an early onset of blindness. Usually, both the rods and the cones are affected.
With late onset PRA, the cells of the retina develop normally, but over time the cells deteriorate.
In most cases, the rod cells degenerate first, affecting night vision before daytime vision, but eventually the cone cells also degenerate, leading to complete blindness.
Are any dog breeds more likely to get PRA?
In general, PRA is an inherited disease that occurs in many dog breeds, including mixed breeds. In most dogs, it appears to be inherited in an “autosomal recessive” pattern, meaning that the affected dog must inherit the defective gene from both parents. Breeds commonly affected include Bedlington Terriers, Cavalier King Charles Spaniels, Labrador and Golden Retrievers, Rottweilers, American Cocker Spaniels, and English Springer Spaniels.
However, in some breeds, such as Old English Mastiffs and Bullmastiffs, it appears to be a dominant gene, meaning that a dog only needs to inherit one copy of the defective gene to be affected by the disease. In the Siberian Husky and the Samoyed, it appears to be sex-linked and found mainly in male dogs.
When a dog develops PRA, it should be removed from breeding programs, because the disease is heritable. The dog’s parents and siblings should also be removed from breeding programs, even if they do not show any signs of the abnormality.
The rationale for removing related dogs from breeding programs is that the typical inheritance pattern is recessive, meaning that affected dogs must have inherited the defective gene from both parents. Unaffected siblings and parents are carriers of the defective gene, but have also inherited a normal gene, so they will not “express” or show the abnormality. With selective breeding, PRA could potentially become increasingly less common. Some genetic testing is available through various companies and may involve a cheek swab or blood samples. Additionally, a board-certified veterinary ophthalmologist can perform certified eye exams to help detect early signs and remove affected dogs from the breeding pool.
What are the signs of PRA?
PRA is not a painful condition, so it is rarely noticed in its earliest stages of development. Usually, the first sign noticed in a dog is night blindness. Affected dogs tend to be nervous at night, may be reluctant to go into dark rooms, or may bump into things when the light is dim. Pet owners with dogs that are developing PRA often observe that their pet's eyes have become very reflective when light shines on them, and that the pupils are more dilated than normal. Both eyes are affected.
"PRA is not a painful condition, so it is rarely noticed in its earliest stages of development."
In some cases, the pet owner may not notice anything abnormal when their dog is at home but may gradually notice that their pet is clumsier when in unfamiliar surroundings. For dogs with the inherited form, the initial sign may only be a loss of day vision or could be complete blindness.
How does PRA affect my dog?
As PRA progresses, your dog's vision gradually worsens until he becomes completely blind. For the average family pet, blindness is not as significant as it would be in a human. Dogs rely more on other senses such as smell and are able to move around well in their home environment, as long as furniture and other objects are not moved around. In the later stages, cataracts can develop.
How fast does PRA develop?
The speed of development depends on the breed and form of disease. In most cases, the dog experiences a complete loss of vision over a period of one to two years.
How is PRA diagnosed?
Your veterinarian may suspect PRA if your dog has dilated pupils and appears to have vision loss, based on a general ophthalmic examination that shows sluggish pupillary light responses. In early stages, it may be difficult to observe any obvious changes to the retina, but as the disease progresses, examination of the back of the eye with an ophthalmoscope will show changes in the optic nerve, changes in the retinal blood vessels, and increased reflectivity of a portion of the retina called the “tapetum lucidum”.
Your veterinarian will usually recommend additional sophisticated testing, such as an electroretinogram (ERG), to confirm the diagnosis and/or rule out other causes of deteriorating vision. In most cases, this means a referral to a veterinary ophthalmologist. The ERG is sensitive enough to diagnose PRA in dogs even before they show obvious symptoms.
In some areas, genetic screening for inherited PRA may be available. This test may help detect carriers of the disease.
Could my dog’s signs be caused by something else?
There is a form of acquired PRA called sudden acquired retinal degeneration (SARD) that has an unknown cause and is characterized by sudden blindness that occurs within days to weeks. This disease typically affects middle-aged to older dogs and is differentiated from inherited PRA because it develops at a later age.
Other causes of slowly developing blindness in dogs include slowly progressive cataracts, glaucoma, uveitis, lens luxation, and optic nerve disease. These diseases tend to be painful.
What is the treatment?
There is currently no effective treatment available for PRA. To date, antioxidant supplements and vitamins have not shown any measurable effect on this disease, although these supplements are not harmful; they may reduce stress on the lens cells and delay cataract formation. If a condition such as cataracts or retinal detachment has been determined to be the cause of your dog's blindness, treatment of this underlying cause may prevent further loss of vision.