Eye conditions are a common in dogs, but there's a huge spectrum between conditions that are a normal progression based on age and diseases that need treatment throughout your dog's lifetime. Educating yourself on the signs and symptoms of eye conditions is the second best thing you can do to help your dog have good vision for as long as possible, but the absolute best thing you can do for your dog is to have an annual eye exam by either a vet or a veterinary ophthalmologist (eye specialist) -- this will help ensure if your dog does have a progressive disease or condition that you can begin treating it as early as possible. We recently created a comprehensive guide to pannus, an autoimmune disease that affects the cornea and can lead to blindness if left untreated. If you haven't had a chance to read it yet, be sure to check it out.
Today we'll look at both Progressive Retinal Atrophy (PRA) and iris atrophy as well as the key differences. Above picture from Instagram: @leagueoftheunsane
What is iris atrophy, and can it be treated?
Iris atrophy is common in older dogs and is usually noticed in dogs around 8-years-old. It refers to atrophy of the iris muscle in the eye over time. The iris is the colored part of the eye, and it's actually made up of two muscles that work in tandem to control how much light reaches the pupil. Depending on the amount of light available, the iris then controls the size of the pupil. For example, if it's dark out, the iris will let more light in, enlarging the pupil (this uses the iris dilator muscle). If it's really bright out, the iris will restrict more light from getting in by shrinking the size of the pupil (this uses the iris sphincter muscle).
As a dog gets older, it's common for the iris muscle to begin to weaken. When this occurs, it makes it harder for the iris sphincter muscle to constrict the pupil, letting too much light in. This may make your dog more sensitive to bright sunlight, so you'll probably see them squinting a lot if you're out on a bluebird day. Another thing you might notice is that your dog's eyes seem shinier at night -- this is a light reflection in the back of the eye and is often referred to as "eye shine."
Other symptoms that are sometimes noticeable with iris atrophy include holes that appear in the iris itself, visible rough edges around the pupil and unequal pupil sizes (or sometimes what looks like a double pupil).
Because this is a common condition associated with aging, iris atrophy on its own is not something that is treated (but if your dog is squinting severely in the sun you might want to consider helping them out by giving them goggles that protect them from the sun and act like sunglasses). However, the same signs and symptoms can also exhibit in more serious conditions such as cranial nerve abnormalities, Horner's syndrome, glaucoma and PRA, among others where treatment may be available and needed. If you notice any of these signs, be sure to make an appointment with a professional for a full diagnosis, and as recommended above, annual eye exams are the best tool available to make sure you catch an eye condition as early as possible.
Progressive Retinal Atrophy
Unlike iris atrophy, Progressive Retinal Atrophy (PRA) is inherited and progresses over time to cause either partial or complete blindness. It is created by a defective enzyme that "attacks" the retina by killing off healthy cells. Critical to eyesight, the retina is the part of the eye that creates a visual image that is then sent to the brain through the optic nerve.
To understand PRA, let's first get a better understanding of how the retina works. The retina is a layer of tissue that is light-sensitive, so it's often compared to the film in a camera. In the retina, the ability to have vision comes from photoreceptor cells. There are two types of these cells in dogs: "rods," which are responsible for vision in dim light; and "cones," which are responsible for vision in bright light and for seeing color (yes, dogs can see color, just not as well as us humans, but that's for another article).
Signs and Symptoms
With PRA, the "rod" cells are the first to be attacked, so often the first sign of PRA will be your dog having trouble seeing at night or having complete night-blindness. If you read about iris atrophy above, you might guess (correctly) that this will cause the iris to attempt to let more and more light in, so you may also notice your dog has very dilated pupils or even the same "eye shine" symptom you see with iris atrophy. However, iris atrophy most commonly affects the muscle that controls the constriction of the pupil (keeping light out) rather than over-dilating. In other words, if your dog is bumping into things at night or seems completely blind at night, get to a veterinary ophthalmologist ASAP.
PRA may begin by attacking the "rods," but it will progressively attack the "cones" as well, causing progressive loss of all vision and usually complete blindness.
Here's the thing about dogs: they have an incredible ability to adapt to their environment by depending more heavily on their sense of smell and hearing. Since PRA is progressive, and depending on how quickly it moves, their ability to adapt can often make it hard for an owner to even notice the vision loss. We recently wrote up a post on the amazing agility duo, Dianne and Zero (who has PRA), and it took Dianne years to realize Zero's vision was deteriorating because of his amazing ability to adapt as his vision decreased. This isn't unusual for dogs with PRA -- unfortunately, many dogs are already completely blind by the time a veterinary ophthalmologist first examines them. In many other cases, the dog may be 6- or 7-years-old before the owner notices symptoms (again because of that ability to adapt). However, if take your dog for an annual eye exam, an ophthalmologist has specialty equipment that allows them to diagnose PRA at an earlier age. For a searchable database of veterinary ophthalmologists by location, visit http://member.acvo.org/search_results.
There is also genetic testing available for PRA that can be done at a very early age. If you choose to do this, there are a variety of services available that you can find through a quick online search or by talking to your vet. However, keep in mind that there are many mutations of PRA, some that have not yet been found, so it's possible your pup could test as normal or as a carrier, but later develop a different strain of PRA that hasn't yet been mapped. For this reason, even DNA testing companies recommend you begin annual eye exams by age two so that an experienced ophthalmologist can detect changes in your dog's vision as early as possible.
There is currently no cure for PRA, although some ophthalmologists have begun to utilize antioxidant support and vision supplements in an attempt to help retinal health and protect some daytime vision. Although this approach is not proven to change the outcome, there are many success stories of dogs with PRA that have retained some daytime vision through the end of their life.
Even though there's no cure for PRA, it's still important to have it properly diagnosed as it can lead to other eye conditions. PRA on its own is not painful, but it can lead to advanced cataracts (which can cause inflammation and become painful) and glaucoma (also painful).
Most importantly, if your dog has PRA, it doesn't mean they can't live a happy and healthy life. Again, PRA is a progressive disease, so your dog likely has a strong sense of her world and how to navigate it. It doesn't mean you automatically have to keep your dog on a leash at all times. But it does mean your dog is at a higher risk for other kinds of eye injuries, such as getting a stick in their eye or running into other objects. Providing your dog with eye protection is a great way to keep their eyes safe from injury and still let them explore the world and join in on fun activities.
How the Genetics of PRA Work
Since PRA is not curable, it's especially important for both dog owners and breeders to understand the importance of identifying carriers of the gene to keep it from spreading. Here's a quick breakdown:
A dog with PRA comes from parents who both carry the gene for the abnormal enzyme defect. How does this happen? Let's look at the possible scenarios.
1. Both parents have one dominant gene for normal enzyme production but also have one recessive gene for abnormal enzyme production. In this case, neither parent has PRA, but they are both considered carriers. In the subsequent litter, 25% of the puppies will end up with two dominant/correct enzyme genes (meaning they will not develop PRA nor will they be carriers), 50% will be carriers and inherit the same genetic makeup as their parents, and 25% will inherit PRA.
2. If one parent has two normal genes and the second parent is a carrier, 50% of the offspring will have two normal genes while 50% of the offspring will be carriers.
3. If a dog that has PRA is bred to a normal dog (does not have PRA nor is a carrier for it), 100% of the offspring will be carriers of the abnormal gene.
4. If a dog that has PRA is bred to a dog that is a carrier of the gene for PRA, 50% of the offspring will have PRA and 50% of the offspring will be carriers.
5. If two dogs are bred who both have PRA, 100% of the offspring will develop PRA.
Why Does this Matter?
As previously mentioned, dogs with PRA can live a happy and healthy life. But because this genetic disease affects every breed of dog, if you're a breeder, you can help less dogs end up with PRA by using responsible breeding practices. And in the case of a person who has a dog with PRA that came from a breeder (even mixed breeds), you can help by informing the breeder so they're aware that one or more of the parents is a carrier.
If your dog has PRA or if you're interested in learning more, get in touch with a veterinary ophthalmologist (or more than one) to find out the most up-to-date resources and gene tests available. You can search for an ophthalmologist here.