[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Ophthalmology](https://durgical.com/en/ophthalmology/) › [Approach by symptom](https://durgical.com/en/ophthalmology/areas/approach/) › Anisocoria: which pupil is abnormal

# Anisocoria: which pupil is abnormal

- Clinical approach

Unequal pupils are often normal. Comparing them in bright light and in the dark shows which is abnormal; a new or painful difference can be an emergency.

Updated Nov 5, 2026

**See it in 3D:** [Iris](https://durgical.com/en/anatomy/iris/) · [Oculomotor nerve (III)](https://durgical.com/en/anatomy/oculomotor-nerve/)

## Key points

- Bigger difference in the dark = small pupil abnormal (Horner).
- Bigger difference in the light = large pupil abnormal (third nerve, Adie, drops).
- Painful new Horner = carotid dissection.
- Dilated pupil + ptosis + ophthalmoplegia = aneurysm.

## What it is

Anisocoria is a difference in pupil size. A small difference that stays the same in light and dark is physiological and common. If the difference is greater in the dark, the small pupil fails to dilate: Horner syndrome (sympathetic). If it is greater in bright light, the large pupil fails to constrict: third nerve palsy, Adie tonic pupil, pharmacological dilation or iris damage.

## Red flags

- New Horner syndrome with neck, face or head pain: carotid artery dissection.
- A dilated pupil with ptosis or eye-movement palsy: third nerve palsy from an aneurysm.
- A dilated pupil in a patient with reduced consciousness: brain herniation.
- Horner syndrome with arm pain or weight loss in a smoker: apical lung tumour.
- Horner syndrome in a young child: neuroblastoma must be excluded.

## Step-by-step approach

- Measure both pupils in bright light and in dim light.
- Decide which pupil is abnormal: bigger difference in the dark means the small pupil; in the light, the large pupil.
- Look for ptosis (mild in Horner, marked in a third nerve palsy) and test eye movements.
- Check the light and near reactions: an Adie pupil reacts poorly to light but better and slowly to near.
- Ask about eye drops, plants, patches and inhalers, and look at old photographs to date the difference.

## Causes of anisocoria

| Cause | Abnormal pupil | Clue | Urgency |
| --- | --- | --- | --- |
| Physiological | Neither | Same difference in light and dark, under 1 mm | None |
| [Horner syndrome](https://durgical.com/en/conditions/horner-syndrome/) | Small | Mild ptosis, slow dilation in the dark | Emergency if painful or new |
| [Third nerve palsy](https://durgical.com/en/conditions/oculomotor-nerve-palsy/) | Large | Ptosis, eye down and out | Emergency |
| Adie tonic pupil | Large | Slow, better response to near; young woman | Routine |
| Pharmacological | Large | Contact with drops, scopolamine patch or plants; no other signs | Routine |
| [Iris trauma or surgery](https://durgical.com/en/conditions/ocular-trauma-hyphema-and-open-globe-injury/) | Large or irregular | History, irregular edge | Routine |

## Keys to the differential

- Horner syndrome is ptosis, miosis and, depending on the level, loss of sweating on that side of the face.
- Apraclonidine drops dilate a Horner pupil and reverse the anisocoria, confirming the diagnosis.
- A pharmacologically blocked pupil does not constrict with standard pilocarpine, whereas a third nerve palsy pupil does.

## Next step / Initial management

- Acute painful Horner syndrome: same-day CT or MR angiography of the head and neck for carotid dissection.
- Dilated pupil with ptosis or eye-movement palsy: emergency CT or MR angiography for aneurysm.
- Horner syndrome without pain: imaging along the sympathetic pathway (brain, neck, lung apex); in children, look for neuroblastoma.
- Isolated Adie pupil or longstanding anisocoria seen in old photographs: reassurance and routine ophthalmology.

## Test yourself

Think of the answer, then tap the question to see it.

## The anisocoria is greater in the dark. Which pupil is abnormal?

The smaller one, which fails to dilate, as in Horner syndrome.

## What must be excluded in a new painful Horner syndrome?

Carotid artery dissection, with same-day CT or MR angiography of the head and neck.

## What are the features of an Adie tonic pupil?

A large pupil with a poor light reaction and a slow, better reaction to near, usually in a young woman.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/ophthalmology/anisocoria-which-pupil-is-abnormal/)

## Elsewhere in DurgiPedia

- [Pupils: light reflex and abnormal pupils (Marcus Gunn, Horner, Argyll Robertson)](https://durgical.com/en/clinical-exam/pupils-light-reflex-and-abnormal-pupils/)
- [The visual pathway, pupillary reflexes and eye movements](https://durgical.com/en/physiology/the-visual-pathway-pupillary-reflexes-and-eye-movements/)

## Connections

## See it in 3D 2

The structures of Durgical's 3D anatomy where it is found.

- [Iris](https://durgical.com/en/anatomy/iris/)
- [Oculomotor nerve (III)](https://durgical.com/en/anatomy/oculomotor-nerve/)

## Related 3

- [Drooping eyelid (ptosis)](https://durgical.com/en/ophthalmology/drooping-eyelid/)
- [Double vision: monocular or binocular](https://durgical.com/en/ophthalmology/double-vision-monocular-or-binocular/)
- [Neurological disease seen in the eye](https://durgical.com/en/ophthalmology/neurological-disease-seen-in-the-eye/)

## Related conditions 3

- [Horner syndrome](https://durgical.com/en/conditions/horner-syndrome/)
- [Oculomotor (third) nerve palsy](https://durgical.com/en/conditions/oculomotor-nerve-palsy/)
- [Raised intracranial pressure and brain herniation](https://durgical.com/en/conditions/raised-intracranial-pressure-and-brain-herniation/)

## In Pharmacology 2

- [Apraclonidine](https://durgical.com/en/pharmacology/apraclonidine/)
- [Pilocarpine](https://durgical.com/en/pharmacology/pilocarpine/)

[← All of ophthalmology](https://durgical.com/en/ophthalmology/)

Educational content; not a clinical guideline and no substitute for assessment by a health professional. For sudden visual loss, severe eye pain, a chemical burn or an eye injury, go to an emergency department at once.

Sources: [Anisocoria](https://www.ncbi.nlm.nih.gov/books/NBK470384/) (StatPearls Publishing (NCBI Bookshelf), 2023); [Horner Syndrome](https://www.ncbi.nlm.nih.gov/books/NBK500000/) (StatPearls Publishing (NCBI Bookshelf), 2023).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/ophthalmology/anisocoria-which-pupil-is-abnormal/