What Your Pupils Can Tell You About Your Health

The pupil is the dark-colored opening at the center of your eye that lets light in. Doctors can look at your pupils for clues about your health.

The size of your pupils and how they react to light can help her diagnose certain health problems. For instance, if you’ve had a blow to the head and one or both of your pupils are dilated -- larger than normal -- that can be a sign of a serious brain injury.

Your doctor also can use what’s called a “swinging light test” to find out if your pupils react to light the same way. Sometimes she’ll put medicated eye drops into your eyes to dilate them and make it easier to see into your eyeball.

If you notice any sudden change in the size of your pupils and there’s no known reason, see your doctor right away.


When one pupil is larger than the other, that’s known as anisocoria. About 1 in 5 people may have this. Perhaps the best-known person with this condition was the singer David Bowie, whose left eye was permanently dilated after an injury.

If you don’t have other symptoms, you might compare the size of your pupils with older photos of yourself to try to figure out when it happened.

This is rare, but it can be a sign of a bigger problem if anisocoria just shows up or the size of your two pupils is suddenly different without a reason.

Other Conditions

Sometimes other conditions affect the pupil itself, instead of just being a clue to another problem. These include:

  • Coloboma, which happens when part of your eye doesn’t form the right way before you’re born. A coloboma in the iris usually leads to the pupil being longer than it should be, sometimes giving it a keyhole-like shape.
  • Third cranial nerve palsy, a dangerous condition that can make one pupil dilate. It’s often caused by pressure on one of the nerves that control eye movements. If you also have a headache and double vision, it can be a sign of an aneurysm -- a weak area in the wall of a blood vessel. If it’s small, you may not even know it’s there, but it can be dangerous if it grows, ruptures, and leaks blood into the space around your brain.
  • A tumor on your pituitary gland, which controls several other glands that make hormones, can make your pupil bigger.
  • Horner's syndrome makes a pupil shrink. Sometimes you’re born with this condition, but it’s usually caused by something that affects the nerves around your eyes.

Adie syndrome, sometimes called Holmes-Adie syndrome, makes one pupil larger than normal and slow to react to light. The cause is often unknown, but it sometimes happens after an injury or lack of blood flow.

WebMD Medical Reference Reviewed by Alan Kozarsky, MD on March 29, 2017


American Academy of Ophthalmology.

American Association for Pediatric Ophthalmology and Strabismus: “Anisocoria and Horner's Syndrome.”

American Stroke Association: “What You Should Know About Cerebral Aneurysms.”

Walker, H., Hall, W., and Hurst, J., editors. Clinical Methods: The History, Physical, and Laboratory Examinations, third edition, Butterworths, 1990.

The Mayo Clinic: “Concussions.”

University of California Irvine, Gavin Herbert Eye Institute: “Neuro-Ophthalmology.”

National Organization for Rare Disorders. 

National Institutes of Health, National Eye Institute.

National Library of Medicine, Genetics Home Reference: “Coloboma.”

Wills Eye Hospital: “Pituitary Tumor.”

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