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Latest health and behavior news and advice from the veterinarians at Tufts University.

Expert Advice February 2018 Issue

Dear Doctor: The Dog has been Squinting

canine iris atrophy

© J. Star/Flickr

What canine iris atrophy looks like close up. (The right iris is normal.)

canine iris atrophy

© J. Star/Flickr

Q I think my cattle dog, Shorty, who is about 9 years old, has been squinting when it is very sunny out. I never used to notice this. Could he really have started squinting at this point in life, or is it something that has always gone on but I just never noticed it?

John Danielle

Shreveport, Louisiana

Dear Mr. Danielle,

A It is very possible that a dog who wasn’t a squinter has started reacting to sunlight that way. Sometimes, as a normal part of aging, older dogs develop what is known as iris atrophy.

The iris is the part of the eye that gives it color, and it’s made up of two muscles. They each have a job in determining how much light reaches the pupil — that dark circle in the middle of the iris. If it’s dark out, one of the iris’s two muscles (the iris dilator muscle) will enlarge the pupil to let in more light. But if it’s very sunny out, the iris’s other muscle (the sphincter muscle) will take down pupil size to restrict the amount of light that can enter.

The thing is, as a dog ages, the sphincter muscle loses some of its ability to shrink pupil size, letting in too much light in certain situations. Ergo, the squinting; because of the condition, a dog will shut his eyes to keep out some of the light. You’ve no doubt experienced squinting for the same reason.

There can also be other signs of iris atrophy, such as an irregular margin around the pupil. Anisocoria (different sized pupils) is common, because the pupil sphincter muscle does not atrophy at the same rate in both eyes. The stroma (body) of the iris can also atrophy with age, giving a lacey appearance, or looking like there is more than one pupil in the eye.

Iris atrophy does not need to be treated. It’s just part of the wear and tear experienced by an aging body. That said, if you see Shorty squinting really hard on a cloudless day and can tell that he’s uncomfortable, you might want to consider getting him sunglasses in the form of protective goggles.

If your dog has started squinting noticeably, it’s probably no big deal, but just to be sure, take him to the vet, who might recommend a visit to a veterinary ophthalmologist.

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