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Anisocoria (unequal pupils)

What is anisocoria?

Anisocoria is a difference in the pupil size between the eyes. The iris regulates the amount of light entering the eye by enlarging or shrinking the pupil size. In normal eyes this will happen equally in each eye. Below is an example of a small right pupil and a larger one in the child’s left eye.

Image of a small right pupil in the left eye and a larger one in a child’s right eye

What is the iris?

The iris is the coloured part of the eye. It is a muscular tissue which causes the pupil to vary in size depending on the lighting conditions and so controls the amount of light entering the eye.

What is the pupil?

The pupil is the black circle at the centre of the iris that allows light to enter the eye so we are able to see. It appears black (sometimes red in photographs) but is actually clear.

What causes anisocoria (unequal pupils)?

Children may be born with unequal pupils (anisocoria) or may develop the condition. Approximately 20% of the population have unequal pupils, but it carries no more significance than having slightly different shoe sizes. The cause of this anisocoria is unknown. The difference in pupil size remains fairly constant whatever the lighting conditions in these cases, but both pupils react to light. You may hear your eye specialist refer to this difference as “physiological” or “essential” anisocoria.

There are more significant types of anisocoria where the difference in size between the pupils varies in different lighting conditions.  There are several possible underlying reasons. Here are some examples of possible causes of significant anisocoria.

Miosis (smaller pupil):

  • Horner’s syndrome
  • Inflammation

Mydriasis (larger pupil):

  • Injury
  • Medications
  • Glaucoma
  • Nerve damage
  • Previous eye surgery


Physiological anisocoria has no associated symptoms and rarely requires further investigation.

Significant anisocoria will require further investigation into the possible cause, which may require treatment.

Your consultant will be happy to discuss further treatment if necessary.

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Please note: this is a generic information sheet relating to care at Sheffield Children’s NHS FT. These details may not reflect treatment at other hospitals. This information is not intended as a substitute for professional medical care. Always follow your healthcare professionals’ instructions. If this resource relates to medicines, please read it alongside the medicine manufacturer’s patient information leaflet. If this information has been translated into another language from English, efforts have been made to maintain accuracy, but there may still be some translation errors. If you are unsure about any of the guidance in this resource or have specific questions about how it relates to your child, always ask your healthcare professional for further advice.

Resource number: EYE53

Resource Type: Article

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If you have any questions, feel free to contact the eye department on 0114 2717468.


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S10 2TH

United Kingdom

Switchboard: 0114 271 7000

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