What is Progressive Retinal Atrophy?
Progressive Retinal Atrophy (PRA) is a common term used for a group of inherited diseases that cause loss of vision due to degeneration of the retina. It is seen in many breeds of dogs, but most commonly in Cocker Spaniels, Poodles, Australian Cattle Dogs, Silky Terriers and Labradors.
The disease causes gradual blindness, first affecting vision in dim light (night vision) and eventually, resulting in complete blindness many months or years later. Towards the end of the disease the affected eyes develop cataracts.
The first sign noticed by owners of pets is poor vision in dim light. Their dog may be reluctant to go outside in poor light, may seek out brighter light and may actually bump into things at night. Some dogs may show an increased green reflection of their eyes in artificial light.
With time the day vision begins to deteriorate and cataracts start to develop. These are seen as a white opacity in the centre of the pupil. The cataracts exacerbate the loss of vision because they prevent light passing through the eye to the retina.
Since the vision loss is so gradual, most owners don't notice a problem until the blindness has progressed to a severe end stage.
Treatment of PRA
There is no known treatment for PRA at this time, as there is no known treatment for a similar disease, retinitis pigmentosa, in people. There has, however been recent interest in the use of anti-oxidants, supposedly to slow the rate of retinal degeneration. This interest has been extrapolated from some clinical studies in man which show that humans with age related macular degeneration slow the rate of development of their blindness if started on Lutein, a carotenoid supplement, if recognised in the early stages of the disease. At this time, there are no controlled clinical studies in dogs which show that Lutein slows the rate of degeneration in canine PRA. One of the problems is that canine PRA is often first recognised much later in the course of development of the disease.
The cataracts are usually not treated as their removal will not result in an improvement in vision. Eventually the cataracts, visible as white opacities in the centre of each eye, can cause other conditions such as uveitis (inflammation of the iris) and even glaucoma (high pressure in the eye) Usually treatment of the earlier stages of the inflammation with cortisone drops will prevent the development of later glaucoma.
Diagnosis of PRA
At present the only way we can detect the condition at an early stage is by doing an electroretinogram (ERG). The ERG is a test of electrical function within the retina, and is a sensitive indicator of PRA, often revealing abnormal findings long before clinical sign are apparent. The condition is diagnosed in the later stages of development by simply examining the retina to observe the classic signs of degeneration.
Prevention of PRA
Genetics play a vital role in the disease and therefore, individuals related to an affected dog should also be examined for PRA with an eye examination. PRA is generally but not always inherited as an auto somal recessive trait. This means that the condition is passed on from both parents.
Since the mid 1990s DNA blood tests have become available to detect affected dogs and parents who may be genetic carriers of the condition.