Unequal Pupil Size (Anisocoria) in Cats — Symptom Decision Guide

A practical guide to unequal pupil size (anisocoria) in cats, signs to watch for, likely causes, an action decision tree, home assessment steps, red flags, and what to tell your vet.

Quick Assessment

• Is this an emergency? Yes if the change is sudden, the affected eye is painful (squinting, tearing), your cat is disoriented, or there are neurologic signs (head tilt, falling, collapse). No if the change is mild, gradual over days with no other signs — but still schedule a vet visit.
• Most common cause: Horner's syndrome and age-related iris atrophy are common causes of anisocoria in cats. Uveitis and glaucoma are common causes that require prompt attention.
• When to see a vet: Immediately for sudden onset, pain, or neurologic signs; within 24–72 hours for a stable, mild change without pain.

Note: This guide helps you assess symptoms and decide how urgently to seek professional care. It does not replace veterinary diagnosis.

What unequal pupil size looks like

Anisocoria means one pupil is larger or smaller than the other. Owners may notice:

• One eye’s black center (pupil) is noticeably wider or narrower than the other when the cat is relaxed and both eyes are open.
• The difference may be visible in room light or only when the cat is in bright or dim conditions.
• Other signs that can accompany anisocoria include squinting, redness, tearing, cloudiness of the eye, eyelid droop on one side, a constricted or dilated third eyelid, head tilt, walking in circles, or changes in vision/behavior.

• Look at your cat in both bright and dim light; some problems are only obvious in one lighting condition.
• Take a quick video — movements and lighting changes can help your vet.

Possible causes (ranked by likelihood)

Decision tree — If [symptom] + [other sign] → likely [cause] → [action]

• If unequal pupils + droopy eyelid + protruding third eyelid, but NO squinting or obvious pain → likely Horner’s syndrome → schedule vet exam within 24–72 hours to search for underlying cause; not always emergency but needs diagnosis.

• If unequal pupils + squinting + red eye + tearing + light sensitivity → likely uveitis (or severe corneal disease) → urgent vet exam same day; pain control and diagnostics (ocular exam, tonometry, infectious disease testing).

• If unequal pupils + cloudy cornea + eye appears enlarged + signs of pain (reduced appetite, hiding) → likely glaucoma → emergency vet visit today; glaucoma can cause rapid irreversible vision loss.

• If unequal pupils + neurologic signs (head tilt, ataxia, circling, behavior change) → likely intracranial disease (stroke, tumor, inflammatory) → emergency vet or specialty neurology referral immediately.

• If unequal pupils + chronic intermittent eye inflammation + systemic illness (weight loss, lethargy) or known FeLV-positive status → consider ocular lymphoma or infectious uveitis → schedule prompt veterinary evaluation with systemic testing for FeLV/FIV and imaging/biopsy as indicated.

• If unequal pupils + recent trauma to head/eye → possible traumatic injury, iris damage, or nerve injury → emergency assessment recommended same day.

• If small, gradual difference noticed with no other signs in an older cat → likely iris atrophy → non-urgent vet visit within 1–2 weeks for confirmation and baseline exam.

Home assessment steps (what to check and how)

Important: do NOT attempt to measure intraocular pressure at home. Tonometry is a veterinary procedure.

When it’s an emergency — red flags

Seek immediate veterinary care (same day) if your cat has any of the following:

• Sudden onset anisocoria and/or the eye looks painful (squinting, vocalizing, pawing).
• Cloudy cornea, visibly enlarged eye, or suspected glaucoma signs.
• Trauma to the head or eye with bleeding or a visible wound.
• Neurologic signs: seizures, collapse, circling, severe disorientation, head tilt, loss of balance.
• High fever (>104°F / >40°C), severe lethargy, or breathing difficulty.

When to schedule a vet visit (non‑urgent but necessary)

• Mild anisocoria discovered without pain, trauma, or neurologic signs: within 24–72 hours.
• Persistent or progressive unequal pupils noticed over days to weeks.
• Recurrent episodes of eye redness or tearing.
• Any cat with known FeLV/FIV, systemic illness, or history of ear/neck problems and new anisocoria.

Home care — safe steps while monitoring

• Keep your cat quiet and in a familiar room with soft lighting — avoid bright sunlight or sudden movements if the eye is painful.
• Prevent rubbing: consider an e‑collar if the cat is pawing the eye; contact your vet for guidance.
• Do NOT apply over‑the‑counter eye drops or human medications unless specifically directed by your veterinarian.
• If your vet prescribes pain relief or anti‑inflammatories, follow dosing instructions precisely — never give dog meds or human meds without approval.
• Collect photos/videos and note changes to share with your vet.

What to tell your vet — essential information

Prepare the following details before your appointment or during a phone triage call:

• When the change was first noticed and whether it was sudden or gradual.
• Which eye is affected (left/right) and whether the pupil is larger or smaller.
• Any signs of eye pain (squinting, pawing), discharge, cloudiness, or swelling.
• Any recent trauma, falls, fights, or exposure to chemicals/medications.
• Neurologic signs (head tilt, stumbling, seizures) or other systemic signs (fever, weight loss, reduced appetite).
• Indoor/outdoor status, vaccination history, and FeLV/FIV test status (and dates/results).
• Any current medications (topical or systemic) and recent changes in health.
• Photos or short videos showing the eyes in different lights.

Common tests your vet may perform

• Ophthalmic exam with pupillary light reflex testing
• Intraocular pressure measurement (tonometry)
• Slit‑lamp and fundic exam (may require pupil dilation)
• Bloodwork, FeLV/FIV testing, and infectious disease screening (Toxoplasma, Bartonella as indicated)
• Imaging (chest/neck X‑rays, CT, MRI) or referral for neurologic evaluation
• Cytology/biopsy if an intraocular mass is suspected

Key thresholds and timelines to remember

• Fever: >102.5°F (39.2°C) is considered a fever in cats; >104°F (40°C) is high and concerning.
• Glaucoma concern: intraocular pressure (IOP) >25 mmHg is worrisome and requires prompt treatment.
• Urgency: sudden onset, eye pain, or neurologic signs → same‑day emergency visit. Mild, stable anisocoria without other signs → vet visit within 24–72 hours.

Sources and further reading

Primary source: Merck Veterinary Manual — Anisocoria in Dogs and Cats: https://www.merckvetmanual.com/eye/eye-disorders-in-small-animals/anisocoria-in-dogs-and-cats

Other reputable resources: veterinary ophthalmology chapters and specialist guidance (Cornell University College of Veterinary Medicine, American College of Veterinary Ophthalmologists).

Remember: this guide is for assessment only. Only a veterinarian can perform the exams and tests needed to determine the cause and appropriate treatment. Early evaluation often preserves vision and comfort.

Frequently Asked Questions