Anisocoria, where one pupil is noticeably larger or smaller than the other, is not a disease itself but a sign that something is affecting one of your cat’s eyes or the nerves that control pupil size. The causes range from mild and self-resolving to serious and vision-threatening. If the difference in pupil size appears suddenly, it warrants prompt veterinary attention.
How Your Cat’s Pupils Normally Work
Each of your cat’s pupils is controlled by two small muscles in the iris, managed by separate branches of the nervous system. The parasympathetic pathway constricts the pupil by activating a circular muscle called the sphincter. The sympathetic pathway dilates the pupil by activating a radial muscle that pulls the iris open. These two systems work together constantly, adjusting pupil size in response to light, arousal, fear, and pain.
Anisocoria happens when something disrupts this balance in one eye but not the other. That disruption can occur at any point along a long chain: the eye itself, the nerves running from the brainstem to the face, or even the spinal cord and chest. Figuring out where the problem lies is the key to finding the cause.
Eye Diseases That Cause Unequal Pupils
Problems inside the eye are among the most common reasons for anisocoria in cats. The affected pupil may be either smaller (if the eye is inflamed) or larger (if pressure is elevated).
Uveitis
Uveitis is inflammation of the inner structures of the eye, particularly the iris and the tissue behind it. It causes the pupil on the affected side to constrict and stay small, often with visible redness, cloudiness, or squinting. Several feline infections are notorious for triggering uveitis. Feline immunodeficiency virus (FIV) is a primary cause of chronic anterior uveitis and can directly produce anisocoria and delayed pupil reflexes. Feline infectious peritonitis (FIP) causes a distinctive pattern of inflammation with fibrin and cellular deposits on the inside of the cornea. Feline leukemia virus (FeLV) can lead to lymphoma within the eye, which initially looks like uveitis. Toxoplasmosis and fungal infections are other possible culprits.
Glaucoma
Glaucoma is elevated pressure inside the eye. In cats, normal intraocular pressure averages around 12 to 18 mmHg. A reading of 25 mmHg or higher, or a difference of 12 mmHg or more between the two eyes, should prompt a thorough evaluation. In glaucoma, the affected pupil is typically dilated and may respond poorly to light. The eye can look hazy or slightly bulging. Feline glaucoma is less common than in dogs but tends to be secondary to other eye diseases, especially chronic uveitis.
Corneal Injury or Ulceration
A scratch, ulcer, or foreign body on the surface of the eye triggers pain and reflex constriction of the pupil on that side. Feline herpesvirus is a frequent cause of corneal disease in cats, producing characteristic branching ulcers along with redness and tearing. Corneal injuries from fights or rough play can cause the same effect. You may notice your cat squinting, pawing at the eye, or avoiding bright light.
Horner’s Syndrome
Horner’s syndrome is one of the most recognizable neurological causes of anisocoria in cats. It results from disruption of the sympathetic nerve pathway that dilates the pupil. The affected eye shows four classic signs: a smaller pupil, a drooping upper eyelid, a sunken appearance of the eye, and protrusion of the third eyelid (the pink membrane that slides across from the inner corner).
The sympathetic pathway is long, running from the brain through the spinal cord, out through the chest, up alongside the neck, and into the skull to reach the eye. Damage anywhere along this route can trigger Horner’s syndrome. Causes include middle ear infections (especially common in cats), chest tumors, neck trauma, and nerve damage during surgery. However, roughly 40 to 55% of feline Horner’s syndrome cases are idiopathic, meaning no underlying cause is ever found. These idiopathic cases tend to resolve on their own, with an average time to resolution of about eight weeks.
Spastic Pupil Syndrome
This unusual condition involves pupils that change size unpredictably, sometimes shifting which eye is larger over a matter of hours. It has been associated with feline leukemia virus (FeLV) and is thought to result from the virus affecting the nerves that control the iris muscles. The anisocoria may come and go, which can be confusing for owners who notice it intermittently. Cats with spastic pupil syndrome may eventually develop fixed dilation in one or both eyes.
Trauma and Toxic Exposure
A blow to the head from a fall, car accident, or animal attack can cause anisocoria through several mechanisms. Direct trauma to the eye can damage the iris muscles, leaving one pupil unable to constrict or dilate properly. Head injuries that cause brain swelling may compress the nerves controlling the pupil, producing a fixed, dilated pupil on the injured side. This is one of the most urgent causes: a suddenly dilated, unresponsive pupil after trauma can indicate increased pressure inside the skull.
Accidental exposure to certain substances can also affect pupil size. If your cat rubs against or walks through topical flea products meant for dogs (particularly those containing certain insecticides), or comes into contact with eye drops left on a counter, one pupil may dilate or constrict depending on the substance. Atropine-containing eye drops, for instance, will dilate whichever eye they contact.
How Your Vet Identifies the Cause
A veterinarian will start by examining both eyes in a darkened room to observe how each pupil responds to light. This simple test reveals a lot: if the smaller pupil fails to dilate in darkness, the sympathetic pathway is likely impaired. If the larger pupil fails to constrict in bright light, the problem is either in the parasympathetic pathway or inside the eye itself.
From there, the exam typically includes measuring intraocular pressure with a handheld device called a tonometer, which takes only seconds and helps distinguish glaucoma from uveitis. A fluorescein stain, an orange dye applied to the eye surface, highlights corneal ulcers or scratches under blue light. Looking inside the eye with an ophthalmoscope lets the vet check for inflammation, masses, or retinal changes that might point to a systemic infection.
If Horner’s syndrome is suspected, the vet may use special eye drops to help localize where along the nerve pathway the problem sits. Blood tests for FIV, FeLV, and FIP may follow if an infectious cause seems likely, especially in younger cats or those with outdoor access. In some cases, imaging of the chest, neck, or skull is needed to look for tumors or middle ear disease.
What Treatment Looks Like
Because anisocoria is a symptom rather than a diagnosis, treatment depends entirely on the underlying cause. Uveitis from an infection requires addressing that infection alongside anti-inflammatory eye drops to reduce pain and prevent scarring inside the eye. Glaucoma needs pressure-lowering medications, and in advanced cases, surgery. Corneal ulcers are treated with antibiotic drops and sometimes a protective contact lens or collar to keep the cat from rubbing the eye.
For Horner’s syndrome, if a cause like an ear infection is found, treating it usually resolves the eye signs over weeks. Idiopathic cases generally improve without specific treatment, though it can take two months or longer. Spastic pupil syndrome linked to FeLV has no direct fix for the pupil changes, but managing the underlying viral disease is the priority.
Traumatic anisocoria depends on severity. Minor iris damage may heal with anti-inflammatory support, while serious head injuries require stabilization and monitoring in a veterinary hospital. Toxic exposures typically reverse once the substance is removed, though the timeline varies from hours to days.