What this means
A cat's pupils keep changing size: in the dark, when frightened or while playing they open wide, and in bright light they shrink to a narrow slit. That is normal. What should worry you is pupils that stay wide and do not shrink in bright light, two pupils of different sizes, or a pupil that changed suddenly while the cat was calm.
Sometimes the way the eyes reflect light changes too. As the retina thins, the eyes can glow more brightly in a photograph or in torchlight than they used to. The pupil itself can look wide because fewer light signals reach the brain. Such a disease is usually painless, and the eyes can look healthy from the outside, so often you are the first to notice.
Changed pupils are not a disease of their own: they show that something is wrong in the eye, the retina, the nervous system or the body. So do not put anything in the eyes and do not use home remedies. Cats adapt to failing sight so quietly that bumping into things or avoiding heights can be the only sign that a cat sees less well.
Possible causes
These are things that can cause this sign. It is not a diagnosis, and they are not equally likely for your cat.
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An inherited disease of the retina
When the light-sensing cells of the retina slowly die off, the pupils can look wider and the eyes can reflect light more brightly. Sight fades gradually, first in dim light.
About the condition: Progressive retinal atrophy -
Inflammation inside the eye
An inflamed eye can have a smaller or uneven pupil and be sensitive to light. The eye can look hazy or red.
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Raised pressure in the eye
When pressure rises in the eye, the pupil can widen and stop reacting to light. The eye hurts and sight can fade quickly.
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Damage to the retina from illness in the body
Some illnesses, such as high blood pressure in older cats, damage the retina. The pupils may then be wide and a cat can go blind suddenly.
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An injury to the eye or head
A blow to the eye or head can change the size of a pupil. One pupil may become larger than the other.
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A disorder of the nervous system
When nerves or the brain are affected, the pupils can be different sizes or fail to react to light. Your cat's behaviour or movement often changes too.
What else to watch for
These observations help your vet build the picture faster. Note down what you saw and when.
- Whether the pupils are the same size in both eyes
- Whether the pupils shrink in bright light
- Whether the eyes glow more brightly than before when light falls on them
- Whether your cat moves hesitantly at dusk or bumps into things
- Whether your cat avoids jumping up
- Whether the eye looks cloudy, squinted or red
When to book a routine visit
Book a visit if the pupils stay wide, the eyes reflect light more brightly or your cat has started to see less well. If the pupils are different sizes or an eye changed suddenly, see a vet the same day.
Related conditions
Conditions whose entries mention this sign. The list is somewhere to read on, not a way to reach a diagnosis.
- Progressive retinal atrophy Eyes An inherited eye disease in which the light-sensing cells of the retina slowly die off. Sight fades gradually, first in dim light, until in time the cat may go completely blind.
- Toxoplasmosis Parasites A disease caused by a single-celled parasite, Toxoplasma. A cat is infected by eating infected prey or raw meat but may show no signs at all, and the parasite matters for people too, especially pregnant women.