Progressive Retinal Atrophy (PRA) is the term used to describe a group of inherited genetic eye diseases in dogs (and also, although rarely, in cats) that result in the degeneration of the retina. This degeneration leads to impaired vision and ultimately, blindness. PRA affects both eyes but is not painful and because it develops gradually most dogs adjust well to their loss of sight, using their other senses such as smell and hearing to compensate.
The retina is the light sensitive part of the eye that captures and transmits images to the brain; in effect it acts as a camera for the brain. The retina contains photoreceptor cells, which capture light and convert it to chemical signals that the brain interprets into images. The retina has two types of photoreceptor cells: rod cells and cone cells.
- Rod cells are responsible for black and white vision, night or dim light vision and the perception of shape and movement
- Cone cells are responsible for colour vision, day or bright light vision, definition and acute focal vision.
Progressive Retinal Atrophy is an inherited condition which is passed on through a defective gene from parent to offspring. There are two main inheritance patterns:
- Autosomal Recessive (the most common PRA inheritance pattern):
If the defective gene is inherited from only one parent the offspring won’t develop PRA but will be a carrier and will pass the defective gene on to their own offspring. If both parents have the defective gene, the offspring will develop PRA.
- Autosomal Dominant (is a less common PRA inheritance pattern and effects only a small number of breeds, such as Bullmastiffs):
Offspring that inherit the defective gene from only one parent will develop PRA and will also pass the defective gene on to their own offspring.
There is also an inheritance pattern known as X-Linked (i.e. X-Chromosome linked) where the defective gene is inherited via the X-Chromosome. This inheritance pattern is rare and primarily affects male dogs, usually from the Siberian Husky and Samoyed breeds.
There are two distinct types of Progressive Retinal Atrophy:
1. Generalized PRA (GPRA): (the most common type of PRA) in which the structure of the retina gradually degenerates, causing impaired vision and eventually leading to total blindness. General Progressive Retinal Atrophy is further broken down into two forms:
- Dysplastic or “Early Onset” GPRA:
The photoreceptor cells develop abnormally, which causes vision problems within the first few months of life
- Degenerative or “Late Onset” GPRA:
The photoreceptor cells develop normally but then degenerate later in life, usually between around 3 to 5 years of age
Certain breeds are predisposed to Generalized PRA including, but not limited to:
Akita, Miniature longhaired Dachshund, Papillon, Tibetan Spaniel, Tibetan Terrier, Samoyed, Irish Setter, Sloughi, Collie, Cardigan Welsh Corgi, Norwegian Elkhound, Miniature Schnauzer, Belgian Shepherd Dog, Alaskan Malamute, Glen of Imaal Terrier, Poodle, English Cocker Spaniel, American Cocker Spaniel, Labrador Retriever, Portuguese Water Dog, Chesapeake Bay Retriever, Australian Cattle Dog, American Eskimo Dog, Nova Scotia Duck Tolling Retriever
2. Central PRA (CPRA): also known as Retinal Pigment Epithelial Dystrophy (RPED). This is a rare form of PRA which affects the Retinal Pigmented Epithelium (the superficial coating of the retina which shields the retina from excess incoming light). CPRA is a condition which causes the Pigmented Epithelium to lose its ability to process light, and this consequentially impairs vision. CPRA occurs in later-life and generally progresses slowly. Because this condition affects older dogs the symptoms will often be dismissed, in the early stages, as age-related weakening of the eyesight. As the condition progresses dogs will generally see better in low light, and vision of moving or distant objects will be less impaired than vision of static or near objects. Peripheral vision is usually retained for a long time.
Note: Not all dogs with CPRA will lose their sight, however secondary cataracts are common. Dogs with CPRA should be monitored for any signs of cataracts developing, as these can lead to other, more serious conditions.
Note: CPRA has been associated with vitamin E deficiency.
Certain breeds are predisposed to Central PRA including, but not limited to:
Labrador Retriever, Golden Retriever, Border Collie, Collie, Shetland Sheepdog, English Cocker Spaniel, English Springer Spaniel, Chesapeake Bay Retriever, Cavalier King Charles Spaniel, Briard (particularly susceptible), Poodle
The first sign of GPRA is usually the loss of night or dim-light vision. This eventually progresses to a loss of vision in bright light and eventually to total blindness.
In Dysplastic or “Early Onset” GPRA this loss of night vision usually occurs before six months of age, with complete blindness following before the age of two.
In Degenerative or “Late Onset” GPRA loss of night vision occurs between three and five years of age, with total blindness by the age of six to nine years.
If your dog seems reluctant to go outside or remains close to the lights from the house after dark, has difficulty in dim-light conditions with stairs, jumping onto or off furniture, and generally negotiating objects, walls and furniture that present no problems during daylight hours, it is possibly the first signs of PRA and you should consult your vet immediately.
Other symptoms to look out for:
- Dilated pupils and/or slow response to changes in the amount of light
- Cloudy or greyish discolouration to the surface of the eye
- Greenish sheen to the eye
- Formation of Cataracts
CPRA is more difficult to spot because night and dim-light vision may actually be better, as well as vision of distant and moving objects. Peripheral vision may remain for a long time, allowing the dog to continue to function apparently normally for a considerable time. Not all dogs with CPRA will go completely blind, although secondary Cataracts are common and should be monitored regularly for signs of other conditions or complications.
Diagnosis of PRA:
The vet will examine the dog’s eye using an instrument called and indirect ophthalmoscope. This instrument allows the vet to see any changes in the blood vessel pattern of the retina, the optic nerve and the Tapetum Lucidum (the reflective portion at the back of the eye in many vertebrate animals, which helps their night vision).
Some breeds, however, may exhibit little or no visible changes to the eye in the early stages of the disease. Also if cataracts have formed they may obscure visual examination of the retina, making this diagnostic method difficult.
Once your vet is reasonably certain that your dog has PRA, or if indirect opthalmoscapy is not possible, you may be referred to a specialist opthalmic vet for further tests.
The Specialist will carry out further tests:
An electroretinogram (ERG) measures the electrical impulses produced by the retina. This test is performed under General Anaesthetic and involves the placing of a special contact lens onto the cornea plus two tiny needles (electrodes) placed under the skin around the eye. Flashing lights are then used to stimulate the retina and the electrical response of the retina is recorded.
- Genetic Testing:
Because PRA is an inherited genetic disease, it is possible to carry out genetic testing, in the form of a blood test, to confirm the presence of the defective gene. This test identifies dogs with PRA as well as those who are carriers, who may not have PRA themselves but will pass the defective gene to their offspring.
Treatment of PRA:
Unfortunately, there is currently no treatment available for PRA. Gene therapy is continually developing and advancing and it is entirely possible that an effective treatment may become available in the future, but for the present PRA cannot be cured or reversed.
At present, the only way to deal with PRA is by a process of eliminating the defective gene from the breeding population. Any dog with the defective gene, as well as any offspring of dogs with the defective gene (even if the offspring don’t have the condition) should be neutered and removed from the breeding programme.
Prognosis (Long Term Outcome):
PRA is a permanent condition which, in the case of Generalized PRA ultimately results in total blindness, and in the case of Central PRA results in severe impairment of vision and possible eventual total blindness.
PRA is not, however, painful and dogs with this condition can continue to live long and happy lives, so long as certain measures are taken to help them navigate their surroundings safely. See Living with a Blind Dog for advice on how to help your dog adjust to life without eyesight.
Further information and recommended reading:
For more detailed information on this condition the following websites are well worth a visit:
- Wikipedia.org – Progressive Retinal Atrophy
An excellent, although quite technical article on PRA in dogs. Contains detailed information on the causes, symptoms and diagnosis.
- PetWave – Progressive Retinal Atrophy in Dogs
A good, easy to read article on PRA in dogs. Probably the best article to read for those who don’t want too much technical jargon but who need to understand exactly what PRA is.
- VetStreet – Progressive Retinal Atrophy in Dogs
A basic but nonetheless very informative article. It considers the various types and forms of PRA rather than keeping the information general.
- EyeCare for Animals – Progressive Retinal Atrophy
A straightforward, easy to follow article on PRA in dogs. Strikes a good balance between detail and technical jargon. Plenty of information, but not too technical for those of us who want to understand PRA but don’t have a technical background.
- PetMD – Degeneration of the Image Forming Part of the Eye in Dogs
Informative but easy to follow article on PRA in Dogs. Not too technical yet packed with information regarding this condition.