Eye Pupils

Medically Reviewed by Mahammad Juber, MD on September 12, 2022
3 min read

Pupils are the dark-colored openings at the center of your eyes that let 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 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 they’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.

Some conditions that can affect your pupils include:

Anisocoria: This is when one pupil is larger than the other. 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.

Though it is rarely the case, it can be a sign of a bigger problem if anisocoria just shows up or the size of your two pupils is suddenly different for no apparent reason.

Coloboma: This 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: This 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 the aneurysm is small, you might not even know it’s there, but it can be dangerous if it grows, ruptures, and leaks blood into the space around your brain.

Pituitary gland tumor: This gland controls several other glands that make hormones. A tumor in this gland can make your pupil bigger.

Horner’s syndrome: This condition makes a pupil shrink. You may have been born with it, but it’s usually caused by something that affects the nerves around your eyes.

Adie syndrome: Sometimes called Holmes-Adie syndrome, it 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.

Brain injury: A head injury can sometimes cause your pupils to become bigger than normal or two different sizes. If you have a head injury and your pupils change size, you should go to an emergency room.

Cluster headache: These headaches cause pain on one side of your head. Your pupil on the side of the headache may get smaller when you have this kind of headache.

Iritis: This condition causes irritation and swelling around your pupil. If you don't get it treated it can leave you with scar tissue that can make your pupil’s shape irregular.

Some medications and illegal drugs can cause your pupils to change size in one or both eyes. Medications that can affect the way your pupils look include:

  • Amphetamines
  • Anticholinergics
  • Antihistamines 
  • Antipsychotics
  • Botulinum toxin (Botox)
  • Decongestants
  • Medications for Parkinson’s disease
  • Medications to prevent seizures 
  • Muscle relaxants
  • Tricyclic antidepressants

Illegal drugs including “bath salts,” cocaine, and LSD can also affect your pupil size.

Some emotional responses can cause your pupils to get larger. Studies have found that when people hear someone laugh or cry, for example, their pupils get larger. Your pupils may also get bigger if you see something that causes an emotional reaction.