One pupil larger than the other anisocoria can be normal or a sign of disease. Context and associated symptoms matter.
Benign causes
A constant, small difference present for years is often harmless. Some medications also dilate a single pupil inadvertently.
- Physiological anisocoria with stable appearance in old photos
- Accidental contact with dilating drops or patches
- Past eye trauma affecting the iris muscles
- Long-standing nerve palsy already investigated
Serious causes of a large pupil
Sudden onset of one enlarged pupil can indicate nerve compression or eye injury. This always needs urgent assessment.
- Third nerve palsy often with double vision and drooping eyelid
- Acute glaucoma causing a mid-dilated fixed pupil and pain
- Head injury with raised intracranial pressure
- Immediate hospital evaluation is essential in new cases
