When a cat’s pupils are noticeably different sizes, the condition is called anisocoria, and it almost always signals that something is affecting the eye or the nerve pathways that control pupil size. Unlike in humans, where a slight difference between pupils can be a harmless quirk, anisocoria in cats is considered a medical concern that warrants prompt veterinary attention. The causes range from eye infections and inflammation to neurological problems and, in some cases, tumors.
How a Cat’s Pupils Normally Work
A cat’s pupil size is controlled by the nervous system. Nerve signals travel from the brain along specific pathways to the muscles in the iris, telling them to constrict in bright light or dilate in dim light. Both eyes normally receive the same signals and respond in sync. When those nerve pathways are disrupted, one pupil may dilate while the other constricts, creating a visible mismatch.
The key diagnostic question your vet will work through is whether the problem is in the eye itself or somewhere along the nerve pathway between the brain and the eye. That distinction determines what tests come next and how urgent the situation is.
Eye Inflammation (Uveitis)
One of the most common reasons for unequal pupils in cats is uveitis, which is inflammation inside the eye. The affected eye typically has a smaller, constricted pupil. You may also notice redness, squinting, a cloudy appearance to the eye, or your cat pawing at its face. Uveitis is painful, and cats often become quieter or less interested in food when it flares up.
Uveitis has many possible triggers, including infections, immune system disorders, and injuries. Left untreated, it can lead to secondary glaucoma, where pressure builds inside the eye. Glaucoma itself can cause anisocoria, but in the opposite direction: the affected pupil becomes abnormally large and dilated. Your vet will measure the pressure inside each eye (a painless, quick test) to distinguish between these two conditions, since they require very different treatment approaches.
Horner’s Syndrome
Horner’s syndrome is a neurological condition that produces a distinctive cluster of signs, all on the same side of the face. The affected eye has a noticeably smaller pupil, a slightly droopy eyelid, and a visible third eyelid (the pinkish membrane that slides partway across the eye). The eyeball may also appear slightly sunken into the head.
The syndrome happens when the sympathetic nerve pathway, which runs from the brain down through the spinal cord, along the chest, up through the neck, and into the eye, is damaged at some point along that route. Common culprits include:
- Middle ear infections: These are the most frequent cause because the nerve passes very close to the middle ear. Cats with ear infections may also show signs of dizziness or head tilting.
- Trauma to the neck or chest: Injuries that stretch or damage the nerves, including foreleg injuries where the leg is pulled forcefully.
- Tumors: Masses in the chest or brain can compress the nerve at different points along its path.
- Spinal cord problems: Disk issues or blood flow disruptions in the neck region of the spinal cord.
The good news is that Horner’s syndrome from an ear infection often resolves once the infection is treated. When the underlying cause isn’t obvious, your vet may recommend imaging to look for masses or other structural problems along the nerve pathway.
Iris Melanoma
Cats, especially older ones, can develop a type of tumor called feline diffuse iris melanoma. It often starts as a dark spot or patch of increased pigmentation on the iris that slowly spreads over months or years. As the tumor progresses, it can thicken the iris and distort the shape and size of the pupil. In one large study, pupil distortion was documented in roughly 20% of confirmed cases at initial presentation, though in cases that had progressed further, that number rose to over 76%.
Not every dark spot on a cat’s iris is melanoma. Benign freckles (iris melanosis) are common and harmless. The concerning signs are when pigmented areas become raised, change the texture of the iris, or cause visible distortion of the pupil. Your vet can monitor subtle changes over time with close-up photographs and slit lamp exams. If melanoma is confirmed and has spread significantly through the iris, removal of the eye is sometimes recommended to prevent the cancer from spreading to other organs.
Infections That Affect the Nervous System
Certain viral infections can cause anisocoria by directly damaging the nerves responsible for pupil control. Feline leukemia virus (FeLV) is specifically linked to a condition called spastic pupil syndrome, where the virus damages the nerve cluster and tiny nerve fibers that control the iris muscles. This can cause one or both pupils to behave erratically, constricting and dilating unpredictably or settling at different sizes.
If your cat hasn’t been tested for FeLV recently and develops unexplained anisocoria, your vet will likely recommend a blood test. Cats that go outdoors or have unknown histories are at higher risk.
Head Trauma
A blow to the head, a fall, or being hit by a car can cause unequal pupils by damaging the brain or the specific cranial nerves that control pupil response. The third cranial nerve, which originates in the midbrain, is particularly vulnerable. Damage to this nerve or compression from brain swelling can cause one pupil to become fixed and dilated, meaning it no longer responds to light.
Unequal pupils after any kind of head injury are a red flag for serious brain involvement. If your cat has experienced trauma and you notice pupil changes along with disorientation, difficulty walking, or loss of consciousness, this is a genuine emergency.
What Your Vet Will Do
A veterinary exam for anisocoria typically starts with observing the eyes from a distance to check for symmetry, discharge, and obvious abnormalities. Your vet will then shine a light into each eye to test the pupillary light reflex, checking whether both pupils respond normally and whether the response in one eye triggers the expected reaction in the other. This simple test reveals a lot about where the problem might be.
From there, the workup may include measuring eye pressure to check for glaucoma or uveitis, staining the cornea with fluorescein dye to look for ulcers or surface damage, and a detailed examination of the internal structures of the eye with magnification. If the cause appears neurological rather than eye-related, imaging of the head, neck, or chest may follow.
Signs That Need Immediate Attention
Any noticeable difference in your cat’s pupil sizes warrants a vet visit, but certain accompanying symptoms make the situation more urgent:
- Eye redness or cloudiness
- Squinting or pawing at the eye
- Discharge from the eye
- Droopy eyelid or visible third eyelid
- Lethargy or loss of appetite
- Difficulty walking, head tilting, or disorientation
A cat that is acting normally otherwise still needs to be seen, but these additional signs suggest the underlying cause is actively progressing. Because anisocoria can stem from conditions as treatable as an ear infection or as serious as a brain tumor, getting a diagnosis early gives your cat the best range of treatment options.