Anisocoria - Unequal Pupil Sizes

Anisocoria is a term that refers to an abnormal condition in which the pupils of the two eyes in the same individual are of different sizes. Normally, the size of the pupil is identical in both eyes. Pupillary diameter varies as both irises normally constrict or dilate simultaneously according to the light in the environment.

Anisocoria is a common condition, and it is estimated that one in five individuals have anisocoria to a varying extent. Most of these are what is known as physiologic anisocoria, which is mild (less than one-millimeter difference in size) and considered to be normal.

However, anisocoria can also be indicative of an underlying medical condition in some cases. In fact, severe cases of anisocoria, such as those associated with Horner syndrome, have the potential to be life threatening.


There are many possible causes for anisocoria, including:

  • Aneurysm
  • Head injury
  • Brain tumor
  • Glaucoma
  • Hemorrhage or stroke
  • Infection of the brain membranes or meningitis
  • Migraine
  • Seizures
  • Eye drops
  • An adverse effect to certain medications that may enter the eyes such as inhalers

Factors to Consider

There are certain characteristics about anisocoria that can help physicians to understand whether the sign is indicative of an underlying condition. These include:

  • The time of presentation of anisocoria
  • Any related incident at the time when anisocoria first presented
  • The extent of difference in the pupil size in a brightly-lighted environment
  • The extent of pupil size difference in a dimly-lighted environment

A pediatric ophthalmologist will usually perform a number of tests to examine the extent and implications of anisocoria, including an evaluation of:

  • Vision
  • Eyelid position
  • Eye movement
  • Pupil size and reaction to lighting changes
  • Overall health of the front and back portion of the eyes

Abnormal Pupil

During the investigative process, it is important to establish which pupil is abnormal. The miotic pupil is the one which is smaller in size. It is likely to be abnormal if the difference in size becomes more noticeable in the dark. The mydriatic pupil or the one which is larger in size is more likely to be abnormal if the difference in size is exaggerated by bright lighting.

An abnormally large pupil, known as a dilated or mydriatic pupil, occurs when the iris is not able to constrict as usual in response to bright light. This may be caused by:

  • Injury to the iris tissue
  • Adie’s tonic pupil syndrome
  • Abnormality of the third cranial nerve

An abnormally small pupil, known as a miotic pupil, may occur when the eye becomes inflamed. This may be due to trauma, Horner’s syndrome or another cause.

Underlying Health Conditions

A difference in pupil size of more than one 1mm that develops later in life and does not return to the normal size spontaneously may be indicative of an underlying health condition. This may include disease of the eyes, brain, blood vessels or nervous system.

Underlying health conditions such as Adie’s tonic pupil syndrome, Horner’s syndrome or an abnormality in the third cranial nerve may be diagnosed following the presentation of anisocoria.

These conditions require specific care to manage the symptoms and improve patient outcomes. In most cases, an ophthalmologist will be involved in the diagnostic process, to make treatment decisions according to the specific case.


Further Reading

Last Updated: Feb 26, 2019

Yolanda Smith

Written by

Yolanda Smith

Yolanda graduated with a Bachelor of Pharmacy at the University of South Australia and has experience working in both Australia and Italy. She is passionate about how medicine, diet and lifestyle affect our health and enjoys helping people understand this. In her spare time she loves to explore the world and learn about new cultures and languages.


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