Unequal pupil sizes anisocoria can be harmless, but sometimes indicate serious eye or brain problems.
Common and less serious causes
Many people have a small, stable difference between pupil sizes that they were born with. Certain eye drops or past eye surgery can also affect pupil size.
- Physiological anisocoria where the difference is usually under one millimetre
- Old trauma to the eye or iris
- Medications or eye drops that dilate or constrict one eye
- Previous inflammation affecting the iris muscle
Serious causes needing urgent review
Sudden anisocoria with other symptoms can signal nerve damage, brain problems, or acute glaucoma. Prompt assessment is crucial.
- New unequal pupils with drooping eyelid, double vision, or headache
- Large fixed pupil after head injury or severe eye pain
- Associated weakness, speech difficulty, or confusion
- Any rapid change should be treated as an emergency until checked
