Pupil of the Eye

The pupil is the black opening in the middle of the colored part of your eye (iris). The pupil gets bigger or smaller in response to changes in light. Muscles in the iris control the size of the pupil. Pupil issues can suggest certain diseases.


Vision happens because the pupil in your eye (located in the iris) lets light through to the lens.
The pupil of your eye, the black hole in your iris, works with the lens to send light to the retina, where it becomes electrical signals sent on to the brain through the optic nerve.

What is the pupil of the eye?

The pupil of your eye is the black hole in the middle of your iris. Your iris is the colored part of your eye. Muscles in your iris control how big the pupil is. Usually, this happens as a result of how much light is available. This makes the pupil an important part of your vision system.

When you’re in bright light, your pupil constricts and gets smaller. When the light is dim, your pupil dilates and gets bigger. The name of this reaction is the pupillary light response.

The pupil has protection. The cornea is a clear dome over the eye that covers the pupil and provides protection. Another layer of tissue called the conjunctiva also protects the pupil and the whole eye.


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What does the pupil do?

Your pupil lets light into your eye as the muscles of your iris change its shape. The lens in your eye focuses light that passes through your pupil. Light then goes to the back of your eye and hits your retina.

Your retina turns light into electrical signals. Your brain receives these signals and turns them into images. This is how you’re able to see.

Two muscles in your iris control how you pupil opens and closes. The iris dilator muscle opens your pupil from the inside out. It sort of looks like a sun with rays extending out from the middle. The iris sphincter is a circular muscle that controls closing your pupil.

Nerves that affect the pupil connect to the afferent pathway (the path from your eye to your brain). They also connect to the efferent pathway (the path from your brain to your eye).

The pupil also gives aqueous humor a pathway to the front of your eye. Aqueous humor is a fluid that nourishes your eyes.


What does the pupil look like?

The pupil looks like a black hole. It looks black because light usually stays inside your eye. One of the few times that you might see light coming back out of your eye is in a photograph. That’s why you might see red-looking eyes in pictures taken with a flash. This is light reflecting off the blood vessels in your eyes.


Conditions and Disorders

What are the common conditions and disorders that affect the pupil?

A variation in pupil size can be an issue itself or a symptom of other conditions. Common conditions of the pupil of the eye include:

Conditions of pupil size and behavior

  • Marcus Gunn pupil: This condition means that your pupils don’t respond normally when an eye care specialist shines a light into your eyes.
  • Anisocoria (uneven pupil size): This is the medical term for one pupil being bigger than the other. It can be normal, temporarily, but it can also be caused by serious conditions.
  • Polycoria: This is the medical term for having more than one working pupil in one eye.
  • Argyll Robertson pupil: This term refers to small pupils that get smaller (constrict) correctly when focusing on nearby objects, but not when they’re exposed to bright light.
  • Miosis (small pupil): The medical name for very small pupils (constricted) is miosis. Small pupils can happen when you’ve had a stroke or when you’ve used opiates, among other reasons. Mydriasis is the name of the opposite condition, having dilated pupils.
  • Adie syndrome: This condition means that your pupil stays dilated and doesn’t react normally to changes in light. It can also make your pupil stay constricted rather than dilated.

Other conditions that affect the pupils

  • Horner syndrome: This rare condition affects the eyes and the face, usually on one side. It’s also called oculosympathetic palsy. Palsy is a word used for nerve paralysis. There are three types of Horner syndrome.
  • Third cranial nerve palsy: The third cranial nerve is the oculomotor nerve. It controls the muscles that move your eye, constrict your pupils, focus your eyes and position your upper eyelid. Palsy of this nerve affects many functions.
  • Traumatic brain injury: These types of brain injuries involve your brain hitting against the skull. One of the most common types is a concussion. One of the first things a provider will check is how your pupils look.
  • Cataracts: Cataracts make the normally clear lenses in your eyes foggy or cloudy. Many people will have cataract surgery. This type of surgery may affect your pupil size. Also, the size of your pupils may affect complications after the surgery.
  • Headaches: Cluster headaches often affect one side of your head, and therefore, one of your eyes/pupils. You may have one eye that waters.

What tests may be done to test the health of the pupil of the eye?

An eye care specialist may do tests to tell if your eyes are healthy, including:

  • A comprehensive eye exam: As part of this exam, a provider will shine a light on your pupil to see if it gets bigger. Both pupils should react to the light.
  • Using different types of medicated eye drops to test pupil reaction: Medications include things like pilocarpine and phenylephrine.
  • Tests for other diseases: If your pupils aren’t working the way they should, you might have a health condition that’s affecting them. Some of them may be very serious, like a stroke or a brain injury. A provider may order blood tests or imaging tests.


What are common treatments for conditions involving your pupils?

Treating eye conditions depends on what’s causing them. For instance, if your current medication affects your pupils, a provider may suggest that you stop taking that drug or substitute another medicine.

Some conditions may require medication. Often, these may be in the form of eye drops or topical preparations (like ointments that you apply to your skin).

Other forms of medications may come in oral form (pills, capsules or liquids). You’ll get some drugs as an injection (a shot) and others through a vein (intravenously or IV).

Providers may treat some eye conditions with eye exercises or surgery.


What can I do to take care of my eyes, including my pupils?

There are things you can do to keep your eyes healthy. These include:

  1. Don’t smoke: Try to quit smoking if you use tobacco.
  2. Eat a diet that’s healthy for you: Check with a provider, but you may want to include vegetables and fruit, especially green leafy vegetables like spinach and kale. You may want to eat fish like tuna, salmon and halibut. You can add in things like walnuts and flax seeds. Remember to drink enough water.
  3. Exercise: Try to move 30 minutes per day at least five days per week. Some people with obesity may have higher risks for eye problems related to other health conditions, like diabetes.
  4. Wear protective eyewear: Protective gear includes sunglasses and safety glasses while you work or participate in certain sports.
  5. Use care if you wear contact lenses: Make sure your hands are clean when you put your lenses in or take them out. Follow manufacturer or provider instructions on how long to wear your lenses and how to clean them.
  6. Rest your eyes: If you use a computer or other screens, or do a lot of reading, use the 20-20-20 rule. This means you look at something 20 feet away for 20 seconds about every 20 minutes to help prevent eye strain.
  7. Get regular eye exams: Check with your provider about how often you need vision screenings or exams. If you have some conditions, like glaucoma or diabetes, you may need more frequent visits.

A note from Cleveland Clinic

Your pupils are important parts of your vision system. Problems with your pupils can point a healthcare provider toward a diagnosis of other illnesses. See an eye care specialist as often as they suggest. It’s also good to do what you can to keep yourself and your eyes healthy.

Medically Reviewed

Last reviewed on 10/04/2022.

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