Ask a Vet: Why Are Your Cat’s Pupils Always Dilated?

2026-06-03 09:54:45

Q:


My elderly cat’s pupils seem to stay dilated, even in bright light. Why?

A:

In old cats, dilated pupils are often just a sign of a harmless condition called iris atrophy. However, if your cat has dilated pupils and seems sleepy, is not walking normally, stops eating or bumps into things, you should call your veterinarian immediately, or take your cat to an emergency clinic. These signs can indicate serious underlying diseases that need immediate treatment. And if your cat is squinting or appears visually impaired — or if their eyes appear cloudy or red — talk to your veterinarian about whether your cat could benefit from seeing a board-certified veterinary ophthalmologist.

How do cats’ eyes work?

Just like in humans, a cat’s pupil is the black hole in the center of the colored part of the eye (the iris). The pupil narrows and widens to regulate the amount of light that the eye takes in, optimizing image perception. One tiny sphincter muscle encircles the pupil and acts like the drawstring of a bag to narrow the pupil in response to strong light. Another muscle dilates the pupil when it’s dark.

Enlargement of the pupil is a critical component of the fight-or-flight response. When an animal is being pursued by a predator, visual acuity can mean the difference between life and death, so the pupils dilate. The fight-or-flight response also happens when cats are tracking prey; if you’ve ever watched a cat just before it pounces, you’ve seen its pupils enlarge in anticipation of its catch. And cats experiencing the terror of transport to a veterinarian often have pupils as big as dinner plates.

Why do pupils get wider with age?

But big pupils don’t always mean fear or excitement. With age, the part of the iris that surrounds the pupil can become frayed, so its edges take on a scalloped appearance (we refer to this as iris atrophy). Age-related degeneration of the sphincter muscle also weakens the pupil’s ability to constrict. Sometimes the entire iris thins and looks like a fine lace veil. When that happens, light reflected by the iridescent part of the retina (the

tapetum lucidum

) filters back through the iris, creating a “stained-glass window” effect.

Iris atrophy usually does not impair sight, but it can increase sensitivity to light. “Iris atrophy is usually a happenstance finding in senior cats that we examine for other problems — and almost every senior dog has some degree of iris atrophy,” says
Sara Thomasy
, a diplomate of the American College of Veterinary Ophthalmology and professor of surgical and radiological sciences at the University of California at Davis. (
Senior has been defined
as more than 10 years of age for cats and the last 25 percent of life for dogs).

What are some concerning causes of pupil dilation?

While pupil dilation may be no cause for worry, it can also be a sign of serious illness. Extensive damage to the retina or the optic nerve leads to blindness and failure of the pupils to narrow in response to light. That damage can be caused by certain infections, very high blood pressure, some medications, a dietary deficiency of the amino acid taurine, inherited diseases that cause the retina to degenerate and glaucoma.

Glaucoma, which has many causes, refers to increased pressure within the eyeball. The rise in pressure first damages the iris sphincter muscle, then the injury extends to the retina, leading to blindness and dilated pupils. According to Thomasy, glaucoma in cats most often results from inflammation in the eye, or uveitis. Although uveitis can cause the eye to appear cloudy, “often dilated pupils is the only sign of glaucoma in cats,” she added.

Thomasy also described rare reports of blindness with pupil dilation in cats that underwent lengthy dental procedures under anesthesia, including a
2025 case report from the UK
. In these cats, having their jaws held open for a long time (using spring-loaded instruments) cut off blood flow to the retina, causing the retinal cells to die. To prevent this, veterinarians have been advised to avoid prolonged jaw opening during surgical procedures in cats.

Dilated pupils may also result from brain damage, which is usually accompanied by other signs of brain disease, such as unresponsiveness or seizures. Infections, certain cancers, head trauma, toxins
such as snail bait
and dietary
deficiency of thiamine
(vitamin B1) may also cause brain disease with pupil dilation.

What should I do if I am worried about my cat’s eyes?

If your cat is less interactive, not walking properly, eating less or bumping into things, see your veterinarian as soon as possible to rule out more serious problems of the eye and brain. Go to an emergency clinic if it is after hours or you can’t get a routine appointment within 24 hours. Veterinarians can do a neurological examination to assess the function of the brain and the nerves that control pupil size. They can also use special instruments that illuminate and magnify each part of your cat’s eye to distinguish among causes for dilated pupils. In some cases, referral to a board-certified veterinary ophthalmologist might be needed. But there’s no such cause for such concern if your cat is behaving normally, especially if the iris just looks a little ragged around the edges.

Jane Sykes is a professor of veterinary medicine at the University of California-Davis. She is a board-certified specialist in small animal (dog and cat) internal medicine with a special interest in infectious diseases. In addition to her veterinary degree, she has a PhD from the University of Melbourne, an MBA from the University of Georgia and an MPH from Kansas State University, and is a distinguished fellow of the National Academies of Practice. She is the proud owner of two Labrador retrievers (Flora and Bodhi) and an orange tabby cat (Freckles).