Dog Has Different Sized Pupils

Some dogs have pupils that differ noticeably in size for long periods of time. If this is happening to your dog, don't be too worried.


anisocoria in a Maltese

Q: I noticed that the pupils of my Siberian husky’s eyes were uneven. The right one was constricted almost to a pinpoint while the left was fully dilated! I immediately took him to my veterinarian. After the exam, I was told that in some dogs this was somewhat common and not to worry because my dog showed no signs of neurologic upset. After a while the pupils evened out. There was no change in his personality, appetite, or anything else. Your thoughts?

Geoffrey Wiggins
Norwalk, Connecticut

A: The medical term for different-size pupils is known as anisocoria. There are many causes, but, says veterinary ophthalmologist Nancy Bromberg, VMD, MS, DAVO, not knowing the breed, age, or activity of your dog makes it difficult to pinpoint the cause with certainty. Among the most common triggers:

1. Intraocular inflammation, meaning inflammation somewhere within the eye. This happens when there’s irritation occurring at the muscles that make up the pupil (the hole in the iris) along with the muscles of the ciliary body (an area behind the colored iris). It causes the pupil to become smaller (miotic). Muscle irritation due to intraocular inflammation can result from bumping the eye with a toy or against furniture, rough-housing with another dog, or other vigorous contact. Sometimes it’s transient, and sometimes it requires medical treatment with topical or systemic anti-inflammatories. One source of inflammation can be the development of cataracts, especially in young dogs. A cataract acts as a foreign body, causing an inflammatory response. Cataracts can be inherited in the Siberian husky. But since the anisocoria went away on its own in your dog, we assume he had transient intraocular inflammation. If the pupils become uneven again, additional examination by a veterinary ophthalmologist is recommended.

2. Sometimes the muscle of the pupil thins and degenerates as a function of age. This may happen at a different rate in each eye, leading to anisocoria. It does not hurt the dog but may cause the dog to react to bright sunlight. The pupil size controls the amount of light that enters to eye. If the pupil is large due to iris atrophy, the only way the dog can control the light is by squinting.

Glaucoma, or increased intraocular pressure, can cause anisocoria. When the intraocular pressure is elevated, the pupil dilates and is less responsive to light.

3. Problems with the nerve supply to the eye and pupil can also causeanisocoria. One such problem is Horner’s Syndrome, where the nerve supply to the pupil muscles is interfered with, so the pupil is smaller than normal. Two frequent causes of Horner’s Syndrome are the nerve being pinched by a choker collar, or a severe ear infection, since the nerve is just under the skin on the side of the neck, and passes through the inner ear on the way through the brain to the eyes.

Considering Probiotics

Q: My four-year-old mixed breed dog has been diagnosed with gastritis. He is doing well after treatment by his vet. My question is this: would a daily probiotic help him in preventing another attack?

Patty Hughes
Casa Grande, AZ

A: Your question is a timely one. “A lot of dog owners are using probiotic products right now,” says Tuft veterinary nutritionist Cailin Heinze, VMD, DACVN. In fact, she points out, “I would say a third of our clients are currently feeding their pet a probiotic product. But they often don’t work. It’s not as simple as people think.”

Consider your own dog’s illness. Gastritis is a broad term used to describe vomiting that presumably has resulted from irritation of the stomach lining. So without knowing what caused the irritation and the ensuing vomiting, it’s difficult to impossible to know if giving your dog a daily probiotic supplement would be of any benefit.

What is known is that probiotics are beneficial bacteria, that is, the bacteria that are normally found in a healthy gut. The goal of concentrating those bacteria in a supplement is to repopulate the gut with a healthy bacterial balance should something go wrong. But what can go wrong and change the bacterial counts in a dog’s gut is “pretty broad,” says Dr. Heinze. “For example, there are new data that show obesity changes the gut’s flora [bacteria], along with dietary modifications, cancer, and myriad other health and environmental factors.” However, we don’t necessarily know what the “ideal gut flora” is for various health conditions, so trying to get the right amounts of different kinds of bacteria into the gut is something of a moving target. It’s hard to know which bacteria to administer.

“At this point in dogs,” Dr. Heinze says, “the best data to show benefit of probiotics focus on supplements for dogs with diarrhea resulting from stress colitis. They get boarded and develop diarrhea secondary to stress. There has also been a study showing some benefit of a very specific probiotic product for dogs with Inflammatory Bowel Disease (IBD), which is similar to the Irritable Bowel Syndrome (IBS) that some people have. The illness involves inflammation of the intestinal wall, often from unknown causes, and leads to vomiting, diarrhea, and weight loss that were improved in one study.” The hypothesis, Dr. Heinze says, is that good bacteria in the probiotic out-compete the bad bacteria causing or perpetuating the gastrointestinal illnesses.

But there remains so much that we don’t know. “It matters what strain of bacteria you use in the product, the dose, and what dog you use it in,” Dr. Heinze says. “There are so many variables that it can be hard to make extrapolations from research studies to your own pet in your own home. “People generalize,” Dr. Heinze says, “but it doesn’t always work.” Part of it is that if something looks useful in, say, mice, that doesn’t mean that it’s going to have the same impact on dogs.

A second challenge with probiotics, she points out, “is that quality control is a big problem.” Consider that once swallowed by the dog [or person], the bacteria have to survive the harsh acidic environment of the stomach (one of the jobs of the stomach is to help kill bacteria in food) and make their way to the intestine, where they have to colonize — take up residence and multiply like crazy. That’s a tall order for a living microorganism, and one that’s hard to fill. Indeed, when internationally renowned microbiology researcher J. Scott Weese, DVM, DVSc, DACVIM, analyzed some 25 veterinary probiotics available in regular retail outlets, he found that only a couple of them were able to “grow out” the number and types of bacteria stated on the label “and even had an appropriate label,” Dr. Heinze says. Dr. Weese, working out of the Ontario Veterinary College in Canada’s University of Guelph, found that almost a dozen of the products did not even list the bacterial counts on their labels. Some products listed bacteria that don’t even exist — the names were made up or misspelled.

There’s a similar problem with probiotics added to foods themselves — another burgeoning trend. Dr. Weese analyzed about 20 pet foods that mentioned probiotics on their labels and found that they could not “grow out” the types of bacteria reported on the label from a single one of the foods, Dr. Heinze reports. “If the bacteria are dead in the food, they’re not going to then grow in the gut,” she says.

“It’s a cool line of research, and there’s a lot of promise about what probiotics can do for a dog with all sorts of health problems,” she says, “and the preliminary information is really interesting, but it’s not quite ready for prime time.”

Not that Dr. Heinze never suggests probiotics for a dog with a GI problem. There are a few brands that she will recommend depending on the situation. But even they don’t always work, she adds.

And an owner deciding on probiotics for her dog without talking to a vet could potentially prolong the time until a dog is brought in for a GI problem that could be treated with tried-and-true medical techniques, thereby unnecessarily compromising the dog’s health.


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