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.
See it in 3D: Iris · Oculomotor nerve (III)
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 | Small | Mild ptosis, slow dilation in the dark | Emergency if painful or new |
| Third 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 | 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.
Elsewhere in DurgiPedia
- Pupils: light reflex and abnormal pupils (Marcus Gunn, Horner, Argyll Robertson)
- The visual pathway, pupillary reflexes and eye movements
Connections
See it in 3D2
The structures of Durgical's 3D anatomy where it is found.
In Pharmacology2
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 (StatPearls Publishing (NCBI Bookshelf), 2023); Horner Syndrome (StatPearls Publishing (NCBI Bookshelf), 2023).
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Representative model, for educational purposes only.