PRA Expert Helps Reduce Inconsistencies in Diagnosis

          Over the last few years, breeders have contacted me regarding inconsistencies in the diagnosis of progressive retinal atrophy (PRA) during CERF exams. The problem is the individual ophthalmologist’s interpretation of the small retinal vessels often seen in the Tibetan Terrier eye. It appeared that there were different interpretations and a number of different approaches to the Tibetan Terrier retina with small vessels.

            For example, one ophthalmologist might interpret an eye as normal, while another might conclude the dog is suspicious for PRA. He might recognize that for the dog to have “progressive” retinal atrophy, he would need to see further changes before calling it PRA, ask to reexamine the dog in six months, then pass its CERF if there were no further changes. Still another might see the small vessels and agree to pass the dog if it tested normal on an electroretinogram (ERG).

            These inconsistencies in diagnosis have caused problems for breeders. Some may have pulled a good dog from a breeding program, essentially because the ophthalmologist did not understand the difference between the Tibetan Terrier eye and the eyes of many larger breeds. Others may have erroneously marked their pedigrees with PRA, passing down incorrect information.

            Simon Petersen-Jones, DVM, Ph.D., is involved in PRA research on Tibetan Terriers at Michigan State University. I asked him to clarify the differences between normal and PRA eyes. According to Dr. Petersen-Jones, “PRA is the name given to a group of inherited conditions that result in a progressive loss of vision leading to blindness. The disease targets the photoreceptors in the retina.

            “The onset and speed of vision loss varies between the types of PRA. Some forms result in night-blindness in puppies followed by total blindness in the first few years life, whereas other forms have an onset in middle age and result in blindness several years later. In Tibetan Terriers the onset of vision loss is in early middle age,” said Dr. Petersen-Jones.

            “Owners may notice that the pupils of a PRA-affected dog seem more dilated than those of their other dogs. There is also an increased reflection of light from the back of the eye (eye shine), a change that is made more obvious by the more widely dilated pupils. Secondary cataract is common with the later-onset forms of PRA. Indeed, owners sometimes wrongly assume that the loss of vision is due to the formation of cataracts,” he said.

            Dr. Petersen-Jones stressed that regular eye examinations are useful in detecting PRA. “Changes are visible in the eye as the retina dies and becomes thinner. Most dogs have a reflective structure in the top part of the fundus. (The fundus is the name for the back of the eye.) The changes the ophthalmologists can see are an increased reflection from the tapetum of the eye. (The tapetum is a highly reflective structure in the wall of the upper part of the back of the eye).

            “The tapetum underlies the retina and reflects light back through the retina to help increase vision in dim light. It is responsible for the colored reflection seen from the eyes of animals caught in a car’s headlights. When the retina becomes thinned due to PRA, it allows even more reflection of light back from the tapetum. The next change the ophthalmologist looks for is the thinning of the blood vessels that overlie the retina. This thinning can first be seen in the smaller blood vessels. As the disease progresses, the head of the optic nerve, which can be seen exiting the back of the eye starts to look atrophied,” he said.

            According to Dr. Petersen-Jones, there are potential difficulties in making an early diagnosis of PRA. “Some breeds of dog, particularly the smaller breeds, may have a very much reduced size of tapetum and some dogs have no tapetum. This is a perfectly normal variation but can make it more difficult to detect the retinal thinning that develops with PRA. In other dogs the tapetum may be more gray color and less reflective than in dogs with a bright yellow tapetum. This normal variation can also make the early detection of retinal thinning more challenging.

            “The size and extent of retinal blood vessels can vary quite a lot between individual dogs. In general, smaller breeds such as the Tibetan Terrier tend to have less prominent vessels. As a loss of diameter of retinal blood vessels is one of the features that is looked for in the early detection of PRA in eye examinations, this can complicate early diagnosis. To distinguish between a normal dog that has less prominent retinal blood vessels and a dog with blood vessel thinning due to PRA the ophthalmologist looks for the presence of early retinal thinning.”

            When a regular eye exam alone cannot distinguish the normal eye from one with PRA, Dr. Petersen-Jones says an ERG can be useful in assessing the function of the retina. While most ophthalmologists use basic ERGs to assess retinal function prior to cataract surgery, Dr. Petersen-Jones said that for early detection of PRA a more detailed ERG needs to be performed. “It is essential that the normal range of results that can be expected from a breed is known because the response does vary between breeds. Also, the response varies with age. The ERG response matures by about seven weeks of age and then decreases in magnitude slightly as the dog becomes a young adult. As dogs then become older, the response gradually decreases as a normal part of aging.”

            Dr. Petersen-Jones has provided us with several annotated photos that show both examples of PRA and normal eyes. Please check them out on the TTCA website at http://www.ttca-online.org/Petersen-Jones_PRA_article.pdf . They should be helpful to you in your understanding of PRA and to your ophthalmologist in his understanding of the wide range of normal Tibetan Terrier eyes.