Moderate severity

Anisocoria (Unequal Pupil Sizes)

Anisocoria is not a disease in itself but a sign — the two pupils are different sizes, one larger or smaller than the other. It can be harmless in rare cases but often points to an eye or nervous-system problem that needs investigating.

⚠️ Common symptoms

One pupil noticeably larger or smaller than the other, either constantly or coming and goingThe affected eye may look red, cloudy, painful or waterySquinting, pawing at the eye, or sensitivity to lightChanges in vision — bumping into things or hesitancy in dim lightSometimes other signs such as a drooping eyelid, a raised third eyelid, or a wobbly, disoriented cat if the cause is neurological

Anisocoria simply means that a cat's two pupils are different sizes — one wide and dark while the other stays small, or the two failing to match as the light changes. It looks striking, and it is important to understand that it is a sign rather than a diagnosis: the pupils are windows onto the health of the eye and the nervous system, and when they stop matching, something upstream is usually amiss.

The list of possible causes is long and spans two broad areas. Within the eye itself, inflammation (uveitis), glaucoma, injuries, corneal ulcers and retinal disease can all alter how a pupil opens and closes. Beyond the eye, the size of a pupil is controlled by a delicate chain of nerves, and damage anywhere along that pathway — from head trauma, a tumour, or the nerve disorder known as Horner's syndrome — can leave one pupil stuck. High blood pressure and some viral infections, such as feline leukaemia virus, are other well-recognised triggers, particularly in older cats.

What makes anisocoria worth taking seriously is that a few of its causes can steal a cat's sight quickly. Glaucoma, for example, can do lasting damage within hours, so a sudden difference in pupil size — especially with a red, cloudy or obviously painful eye, or with any clumsiness, disorientation or a head tilt — is a same-day veterinary matter, not something to watch and hope about.

Because the pupil difference is only the clue, a vet's job is detective work: examining both eyes, checking the pressure inside them, measuring blood pressure, and looking for neurological signs to find the real problem. Treatment then follows the cause, whether that is drops for glaucoma or uveitis, treating an infection or managing hypertension. Occasionally no serious cause is found and the anisocoria turns out to be harmless — but that is a conclusion for the vet to reach after looking, never an assumption to make at home.

🔍 Causes

Many things can make the pupils unequal: inflammation inside the eye (uveitis), glaucoma, injury to the eye or head, corneal ulcers, retinal disease, high blood pressure, certain infections such as feline leukaemia virus, tumours, and neurological conditions affecting the nerves that control pupil size (including a specific pattern called Horner's syndrome). Some medications and, rarely, contact with certain plants or chemicals can also affect one pupil.

🛡 Prevention

There is no single way to prevent anisocoria because it has so many causes, but you can reduce the risk of some by keeping vaccinations and viral testing up to date, protecting cats from fights and trauma, and having older cats' blood pressure checked, since hypertension is a common and treatable cause of sudden eye changes. Prompt attention to any eye injury or infection also helps stop small problems becoming sight-threatening.

💊 Treatment

Because anisocoria is a symptom, treatment depends entirely on the underlying cause, which the vet must first identify. Options range from medicated drops for glaucoma or uveitis (inflammation inside the eye), to treatment of an infection, to investigation of high blood pressure, trauma or a neurological problem. Some causes, such as damage to the nerves controlling the pupil, may be managed rather than cured. The key point is that the pupil difference itself is a clue, and chasing the cause is what matters.

When to see a vet: Treat unequal pupils as an urgent sign and see your vet promptly, especially if it comes on suddenly or is accompanied by pain, redness, cloudiness, obvious vision loss, or any neurological signs such as stumbling or a head tilt. Some causes, like glaucoma, can permanently damage vision within hours, so this is not a 'wait and see' symptom. Note when it started and whether it changes with light to help your vet's diagnosis.

⚕️ Medical disclaimer

  • This information is educational only and is not a substitute for veterinary advice.
  • Symptoms overlap between conditions — only a vet can diagnose accurately.
  • Never withhold treatment or self-medicate based on information found online.
  • When in doubt, call your vet. Early intervention almost always leads to better outcomes.

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