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My cat is probably around 25 years old. She still has decent vision, hearing, jumps up onto my chair, no apparent arthritis or any other health issues. The vet said she's fine. The only odd thing is here eyes are always dilated. My apartment tends to be dark, but even if I take her outside during the day, her pupils do not make the familiar cat-slit shapes but stay wide open round. She still seems to see well enough to not bump into walls or plants, but I imagine her view of the world is like an overexposed photograph. She'e mostly indoors anyway.

The vet said this sometimes happens for older cats. I am curious to know more.

Why is this? Is there a treatment, and if so how helpful would it be for this almost indoors-only cat?

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Cats have vertical pupils as it helps them to hunt.

As given in the site Why cats have vertival pupils

Ambush predators, like many cats and snakes, were most likely to sport vertical-slit pupils, particularly when those animals were active at night. The reason for this correlation most likely has to do with the mechanics of the eye, Banks told Live Science. Ambush hunters need to be very good at gauging depth so they can effectively leap out at their prey.

There are two ways to gauge depth without moving. In one method, stereopsis, the brain compares the distance between the two images returned by each eye to gauge depth. (Hold out your finger, focus on it and close each eye in succession. That "jump" you see is the distance used in stereopsis.)

The other method, blur, takes advantage of the fuzziness of objects behind and in front of the spot on which you're focusing.

As it turns out, the side-to-side displacement used in stereopsis is easier to gauge using vertical lines and contours than it is with horizontal ones. Thus, Banks said, the vertical pupil provides the best view for stereopsis.

To judge horizontal distances, though, cats and other slit-pupil predators likely use blur, Banks said. To maximize blur, the pupil must open wide. And to maximize blur for horizontal lines, the pupil must open wide from top to bottom. In other words, the ideal shape is narrow horizontally and wide vertically — precisely the arrangement of a cat's eye.

"This is the right arrangement to maximize stereo and blur as cues to distance simultaneously," Banks said.

I guess that since your cat is mostly an indoor cat, who is hand feeded by you, and never had any experience of hunting, with time gradually her pupils have become more like human pupils.

Predators who are active at night have vertical pupils.

Predators who are tall and active at both night and day (eg. humans) have horizontal pupils.

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    Nocturnal apex predators eyes dilate upon arousal. What I mean by that is they get moon eyes when they get excited. This helps them hunt in near total darkness. A relaxed cat doesn’t have moon eyes. Sleepy relaxed cats have slit pupils. Given the age of your cat I would evaluate her for pain, and treat appropriately. Pain is one of that things that gets predators excited. – fmorales Aug 13 '17 at 8:46

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