Progressive retinal atrophy (degeneration)

The back surface of the inside of eye is called the retina. Light rays entering the eye are focused on to the retina which converts these images to small electric currents. These currents travel via nerves to the brain where they can be interpreted as vision. Retinal atrophy is degeneration (deterioration) of the retina, causing the brain to receive less information resulting in impaired vision. The term progressive retinal atrophy refers to a group of inherited conditions which result in retinal degeneration. PRA is generally classified as types I, II, and III

Any breed of dog may be affected by PRA type I which is inherited as a simple autosomal recessive trait. PRA type I also occurs occasionally in cats with Abyssinians and Persians more commonly affected. Clinical signs start with early loss of night vision followed by loss of day vision and difficulty seeing moving objects. These signs can appear as early as 12 months of age with no upper age limit at which PRA can become apparent. It is difficult to estimate how long it will take for the dog to become completely blind after initial diagnosis however younger dogs appear to deteriorate at a faster rate.
Other signs which may be noted by owners include dilated pupils with back of eyes appearing bright yellow / green due to increased reflectivity from the tapetum (pigmented area of the retina) as a consequence of the degeneration. Some dogs will develop secondary cataracts (cloudiness of the lens).
Examination of the eye with an opthalmoscope may identify other changes indicative of PRA which include attenuation (reduction in size and number) of the retinal vessels, paleness of the optic disc(attachment of the optic nerve to the retina), increased tapetal reflectivity and loss of pigment on the rest of the retina (nontapetal fudus).

Diagnosis will require a thorough examination of the back of the eye to identify these changes or performing an electroretinogram (using a small electric current to stimulate the retina and then measuring how the retina responds).
There is no effective treatment for PRA. Currently the recommendation is to not breed with affected stock. Unfortunately an examination of the eye in young dogs, prior to breeding, is not able to identify all the affected animals. Some dogs may be affected but the signs are not apparent at the time of examination, whilst other dogs may be carriers (animals that carry a gene and pass it on to future offspring, yet are not affected themselves).

PRA type II affects mainly hunting and working dogs. The effects are generally less severe than type I and usually do not occur until middle age. Animals initially lose central vision with peripheral vision remaining until later in the course of the disease. This results in good distance vision and ability to see moving objects but inability to see well up close. Night and day vision are affected equally. Complete blindness is not common.
The hyperreflective changes to the retina are also less severe occurring in patches. The nontapetal fundus is not affected and the optic disc remains normal until late in the disease. No changes are detected using electroretinography.

PRA type III is a rare inherited disorder of Alaskan malamute and miniature poodles. These dogs become photophobic, a condition where they have poor tolerance to light showing pain and watering in bright light. In the Alaskan malamute the change is recognized between 6-10 weeks of age, though some may not be apparent until 6 months. In the Miniature poodle the change is usually recognized around 3 months of age.

A similar condition to PRA exists in male greyhounds that lose the lure, especially under artificial light in night racing. Generally this is noticed around 12 months of age is termed Greyhound Retinal Degeneration.