Is it normal to have one pupil bigger than the other?

Often, yes. Around one in five people has a small, natural difference called physiological anisocoria, which is harmless when it is long-standing and comes with no other symptoms. A new or changing difference, or one with a drooping lid, double vision, pain or a red eye, should be assessed.

How do doctors work out the cause of uneven pupils?

By deciding which pupil is abnormal, using whether the difference is greater in dim or bright light, and then, where needed, pharmacological tests such as apraclonidine for Horner's syndrome, dilute pilocarpine for an Adie's tonic pupil, and pilocarpine 1% to separate a third nerve palsy from a pharmacological cause.

Free interactive tool

Uneven pupils, explained.

A difference in pupil size is called anisocoria. Change what the eyes are doing and choose a scenario to see how the pupil, the eyelid, and the response to light and a near target point to the cause. Then use the short self-check, or open the clinician’s algorithm.

The eyes are

Scenario

A short self-check

This does not diagnose anything. It helps you judge whether a pupil difference is likely harmless or worth having looked at.

For health professionals: a practical approach to anisocoria

An educational aid, not a substitute for examination and imaging where indicated.

Step 1. Which pupil is abnormal?

Compare the anisocoria in dim and in bright light.

Greater in dim light → the smaller pupil is failing to dilate (sympathetic problem). Follow the Horner’s pathway.
Greater in bright light → the larger pupil is failing to constrict (parasympathetic or iris problem).
Equal in light and dark, both react briskly → physiological anisocoria.

Smaller pupil, greater in the dark (Horner’s pathway)

Confirm: apraclonidine 0.5–1% (reversal of the anisocoria confirms Horner’s, from denervation supersensitivity; avoid in infants), or cocaine 4–10% (the Horner’s pupil fails to dilate).
Localise: hydroxyamphetamine 1% dilates a third-order (postganglionic) intact neuron, so dilation suggests a central or preganglionic lesion, and failure to dilate suggests a postganglionic lesion.
Act: a new Horner’s warrants imaging of the oculosympathetic pathway. Acute, painful Horner’s raises carotid dissection and needs urgent assessment.

Larger pupil, greater in bright light

Light-near dissociation and dilute pilocarpine 0.1%: constriction of the affected pupil confirms an Adie’s tonic pupil (cholinergic supersensitivity).
Pilocarpine 1%: constricts a third nerve palsy pupil and a normal pupil, but a pharmacologically blocked (atropinic) pupil will not constrict.
Third nerve palsy: look for ptosis and a down-and-out eye. A pupil-involving, painful third nerve palsy is treated as a neurosurgical emergency (posterior communicating artery aneurysm) and needs urgent imaging.
Also consider iris sphincter trauma and posterior synechiae.

Doses and agents are for teaching and vary with local practice and availability. Always correlate with the full examination.

Important: This tool is for educational purposes only. It cannot diagnose anisocoria or any other condition and does not replace an eye examination. A pupil difference that is new, changing, or comes with a drooping eyelid, double vision, eye pain, a severe headache, or a red eye should be assessed promptly, and some combinations are emergencies. This tool does not store or transmit any data you enter.