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Anisocoria (Unequal Pupils)

A difference in pupil size between the two eyes, which may be normal or indicate an underlying condition.

3 min read
Close-up photo showing anisocoria - one pupil noticeably larger than the other

Anisocoria means the two pupils are different sizes. While sometimes normal (physiologic anisocoria), it can indicate important conditions affecting pupil control.

Key Takeaways

  • Difference in pupil size between eyes
  • Can be normal (physiologic anisocoria)
  • Or indicate disease-must differentiate
  • Key question: Which pupil is abnormal?
  • Compare in light and dark to help determine

For comprehensive information about understanding and evaluating pupil abnormalities, including when to be concerned and what to expect during evaluation, see our guide: Understanding Pupil Abnormalities - When Unequal Pupils Need Attention.

Is It Normal or Abnormal?

Physiologic Anisocoria

  • Up to 1mm difference is common
  • Equal difference in light and dark
  • No other symptoms
  • Pupils both react normally
  • Present in ~20% of population

Pathologic Anisocoria

  • Greater than 1mm typically
  • Difference changes in light vs dark
  • May have other symptoms
  • Abnormal pupil reactions

Finding the Abnormal Pupil

Difference Greater in Light

Difference Greater in Dark

  • Smaller pupil is abnormal
  • Can't dilate properly
  • Causes: Horner syndrome, pharmacologic constriction, old iritis

Important Causes

Larger Pupil Abnormal

  • Third nerve palsy-with ptosis, diplopia
  • Adie's tonic pupil-benign
  • Pharmacologic mydriasis
  • Trauma to iris

Smaller Pupil Abnormal

Red Flags

What to Bring to the Visit

Old photos can be surprisingly useful. Driver's license photos, phone pictures, and family photos may show whether the pupil difference is new or longstanding. Bring a list of eye drops, motion-sickness patches, inhalers, migraine medicines, and any recent exposure to plants or chemicals, because medication or contact exposure can change pupil size.

Frequently Asked Questions

Can normal people have unequal pupils?

Yes. Physiologic anisocoria is common and usually small, often about 1 mm or less, with normal reactions and no other symptoms.

Why does the room lighting matter?

If the difference is bigger in bright light, the larger pupil is usually the abnormal one. If the difference is bigger in the dark, the smaller pupil is usually the abnormal one.

When is anisocoria an emergency?

New anisocoria with double vision, a droopy eyelid, severe headache, neck pain, trauma, weakness, or confusion needs urgent evaluation.

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