PRA (Progressive Retinal Atrophy)
Brady Beale, VMD, DACVO
What is Progressive Retinal Atrophy?
Progressive Retinal Atrophy (PRA) is a degenerative disease of the retina that ultimately leads to loss of vision. The retina is the neurosensory structure in the back of the eye that transmits images to the brain. In one of the most common form of PRA called prcd-PRA (progressive rod-cone degeneration), the rods degenerate first leading to a loss in night vision followed by the cones with a loss of day vision.
What causes PRA?
PRA is an inherited disease observed in purebred dogs but may be seen in mixed breeds. In most breeds, the disease is inherited by simple autosomal recessive mode. Autosomal recessive inheritance requires one bad gene from both parents. Neither parent may be affected by the disease but both have to be carriers of the trait. The age of onset varies with the breed affected with some breeds demonstrating clinical signs within the first years of life while others may be elderly before vision loss is observed.
How is PRA diagnosed?
Most cases of PRA are recognized by a gradual decrease in vision generally with the initial vision loss seen in dim light (early in the morning or late evening). Other animals may exhibit no signs that are easily recognized by the owner. Animals have an amazing ability to adapt to vision loss and some animals are not diagnosed with PRA until they present to the ophthalmologist for cataract surgery or furniture is rearranged in the home.
The ophthalmic exam reveals evidence of retinal degeneration including a decrease in retinal blood vessels and hyper-reflectivity of the tapetum (increase “eye shine”). Often the retinal exam alone provides sufficient evidence of the disease but if it is difficult to make a definitive diagnosis, further testing including an electroretinogram (ERG) or genetic testing is available.
An ERG is a test to demonstrate a decrease in retinal function in response to flashing bright light. DNA-based genetic tests are available for many affected breeds to confirm a diagnosis or help screen carriers from breeding programs.
How is PRA treated?
Generally, there is no treatment for PRA. However, veterinary ophthalmologists are conducting research to find treatments for this disease. Fortunately, the condition is not painful. Animals adapt well to vision loss especially with help from their owners. Training techniques have been described. A veterinary ophthalmologist can discuss how you can help your animal adjust and even thrive with blindness.
Are there complications associated with PRA?
Cataracts (an opacification or cloudiness of the lens) are the most common secondary complication seen with PRA. Cataract removal is not typically recommended since vision may not be improved and could deteriorates following surgery. Animals with PRA should not be used in breeding programs since the condition may be passed on to offspring.
Figure 1: Normal retina Sample
Figure 2: Detached retina sample