Progressive retinal atrophy (PRA) is a group of inherited eye diseases that cause degeneration of the retina in dogs which results in permanent blindness. The retina is a thin layer of nervous tissue that lines the back of the eye and is responsible for converting light into electrical impulses. The electrical impulses are then transmitted along the optic nerve to the brain, where the impulses are interpreted as an image. The cells within the retina that are directly responsible for the conversion of light to an electrical impulse are called photoreceptors. There are 2 types of photoreceptors called rods and cones. The rods are responsible for dim light vision and the cones are responsible for bright light and color vision. PRA begins with degeneration of the rod photoreceptors.
The onset time of PRA depends in part on whether the photoreceptors were never properly formed, called “dysplasia”, or whether they form properly but then degenerate over time. Most commonly, the photoreceptors develop and function normally at first but then begin to deteriorate at some point. It can occur in puppies as early as 12 weeks of age or it can begin in early to late adulthood. The particular form of PRA differs between the breeds of dogs that are most likely to be affected by it. The different PRA forms share similar features but have different biochemical causes. PRA has no treatment and no cure, but it is not a painful condition in dogs.
The disease occurs in both eyes, but vision loss occurs very slowly which gives dogs time to adjust to the changes. PRA may be initially noticed by pet owners as night blindness or decreased “confidence” in dimly lit areas. Eventually, as the PRA slowly progresses, dogs will lose their ability to navigate in an even brightly lit environment. Most dogs will learn to accommodate for their visual deficits through their keen senses of hearing and smell. Because of this ability, it is not uncommon for some pet owners to notice the visual deficits only after the PRA is advanced or the animal must navigate a new and unfamiliar environment.
Currently, the most performed diagnostic test for PRA is a complete ophthalmic (eye) examination. A board-certified veterinary ophthalmologist can detect the later stages of PRA by examining the dog’s retina with indirect ophthalmoscopy (visualizing the retina with special optical lenses). Because the earliest stages of PRA are not detected by such an exam, an annual examination and eye registration of breeding stock by a veterinary ophthalmologist through the Canine Eye Registration Foundation (CERF/OFA) is recommended. A more advanced diagnostic for PRA is called an electroretinogram or ERG. An ERG measures the electrical response of the retina that occurs to known amounts of triggered light stimulation. An ERG is capable of detecting PRA much earlier than it can be diagnosed through a visual exam of the retina. Although it has increased sensitivity for early PRA, an ERG requires specialized instrumentation, experience, and general anesthesia. For these reasons, an ERG is not commonly used as a screening tool for PRA but rather used to confirm a suspected diagnosis or in research settings when early detection of PRA is required.
As we know, our canine friends are very resourceful. In fact, vision is the third most important sense to the dog behind their sense of smell and sense of hearing. Most dogs do very well even if completely blind. It is likely to bother the owners more than the dog. Affected dogs may require any medication and their eyes are not painful. PRA does not affect the rest of the dog or negatively impact health. Blind dogs can easily learn their way around the home environment. It is important to keep their routine unchanged so that they will remain confident and happy. Fenced-in yards and baby gates can be very useful to safeguard dogs from dangerous situations. Often, another household dog will act as a “seeing eye dog” for the affected dog. With the help of an owner’s positive attitude and encouragement, blind dogs can live a happy and fulfilled life.
Dr. Cory Mosunic
Board-certified Veterinary Ophthalmologist
Portland Veterinary Emergency & Specialty Care