Progressive retinal degeneration can slowly cause blindness in any dog of any breed, but learning about the condition is the key to minimising it in breeding, writes Dr Peter Higgins.
Progressive retinal degeneration (PRD) is also known as progressive retinal atrophy (PRA) and refers to retinal diseases that cause blindness.
Some breeds are affected by blindness because of abnormal development of the retina; this is called dysplasia. Other breeds have a slowly progressive degeneration or death of the retinal tissue; this is degeneration.
PRD has been seen in almost every registered breed as well as in mixed breed dogs, and also occurs in humans – it is known as retinitis pigmentosa. In general, it is thought to be inherited, but inherited differently in each breed, and can be passed to offspring by parents that have normal eyes. Therefore, identification of breeding animals with PRD is essential to prevent spread of this condition.
In the retina
To better understand PRD, a basic understanding of the function of the retina is needed. The retina is a highly complicated tissue located in the back of the eye. Light strikes the retina and starts a series of chemical reactions that causes a nerve impulse.
The impulse passes through the layers of the retina to the optic nerve and, from there, to the brain, where vision takes place. In the retina, cells called ‘rods’ are involved with black and white or night vision, and cells called ‘cones’ are involved with colour or day vision. Progressive retinal degeneration may affect either the rods alone, the cones alone or both the rods and cones together.
PRD is not a painful condition, so dogs affected by it do not have a reddened eye, increased blinking or squinting. For this reason, most owners do not notice the early stages of the condition.
Some owners notice an abnormal shine coming from their dogs’ eyes. This abnormal shine is because the pupils are dilated and don't respond as quickly to light as the pupils of normal dogs.
The earliest signs of PRD include night vision difficulties that, in most cases, progresses to day blindness. After diagnosis, many owners comment that their dogs have seemed disoriented when going out to the yard at night, or that they had started leaving lights on for them.
Night blindness may also manifest as fear of dark rooms, and occasionally dogs with PRD in early stages will get lost in their own home after the lights have been turned off.
To diagnose PRD, the veterinary ophthalmologist examines the retina with an instrument called an indirect ophthalmoscope. Changes in the retinal blood vessel pattern, the optic nerve head and the reflective substance within the dog's eye (called the tapetum) are classic signs of PRD.
However, PRD will progress at different rates in different breeds. In some breeds PRD causes little or no early changes, and the eyes of these dogs may appear normal until they are in the later stages of the disease. This variation makes it difficult for the vet to determine just how long any particular dog will continue seeing.
There is no possible treatment for PRD, although a number of vitamin therapies have been suggested. At this time, none of the vitamin treatments have been scientifically proven to be effective, so use of vitamins must be deemed a naturopathic remedy rather than a medical treatment.
Cataracts may occur in some patients with PRD, and generally occur later in the disease. Formation of cataracts may interfere with the ophthalmologist's direct examination of the retina and make other tests such as an electroretinogram (ERG) essential for diagnosis.
The ERG test involves sophisticated instrumentation used to measure the response of the retina to flashes of light.
The dog is anaesthetised for this test. It is then placed into a darkened area, a special contact lens with a gold ribbon is placed on the cornea, and two tiny needles are placed under the skin around the eye.
A light flash that has been dimmed with filters stimulates the retina and this procedure is repeated intermittently for 20 minutes. Finally, bright red, blue, and white flashes are used for analysis.
A healthy retina will produce a characteristic waveform on the ERG, which builds from the time the lights are turned out. The ERG is sensitive enough to diagnose dogs with PRD before they begin to demonstrate signs of the disease.
Testing breeding stock
PRD refers to a broad group of inherited retinal diseases which result in the blindness of dogs. Because of the nature of the condition and the sometimes-late onset, breeding stock should be examined annually by a veterinary ophthalmologist.
Genetic testing can determine a dog’s genetic predisposition, says Dr Amanda Chin from Animal Network, so breeders can find out whether their stock is likely to be clear, carrying the condition or affected.
Any dogs found to be affected by or carrying the condition should be removed from breeding programs, and certification of the eye health of both sire and dam should be available to puppy-purchasers.
Ask the experts
Dr Peter Higgins is a veterinary surgeon and teaches in the Faculty of Veterinary Science at the University of Sydney.