Why Are My Pupils Small

The optic nerve and oculomotor nerves both control pupil size. These nerves receive some of their messages from the autonomic nervous system. This part of the nervous system controls all of your vital functions, many of which are automatic.

What Is Eye Miosis?

The black circle in the center of your eye is your pupil. It changes size thousands of times a day. When you’re in dim light, it gets bigger to let more light in. When you’re in bright light, it shrinks to protect your eye and keep light out.

When your pupil shrinks (constricts), it’s called miosis. If your pupils stay small even in dim light, it can be a sign that things in your eye aren’t working the way they should. This is called abnormal miosis, and it can happen in one or both of your eyes.


Age: It’s normal for a newborn’s pupils to stay small for about 2 weeks so their eyes have extra protection from bright light. Your pupils tend to get smaller as you get older, too. The muscles that work your pupils can get weak and have a tough time opening them. This can make it harder for you to see at night.

Inflammation: Swelling inside your eye can make it hard for your pupils to get bigger. Sometimes this happens if you’ve injured your eye. It may also be because of a condition called uveitis, which is swelling in your iris — the part that gives your eye its color — and the tissues around it.

Side effect of a medication: Certain anxiety, muscle spasm, and seizure medications like diazepam (Valium) or antihistamines like diphenhydramine (Benadryl) can make your pupils shrink. So can narcotics, either prescribed or illicit.

Genes: Being born without the muscle that controls your pupils or with pupil muscles that aren’t formed correctly is called congenital miosis or microcoria. You get it when one or both of your parents pass down a problem gene to you. It can happen in one eye or both eyes. If you have it, you may also be nearsighted and have trouble seeing things far away. Or you may have glaucoma, which means there’s too much pressure inside your eyeball.

Horner’s syndrome: This rare condition affects the way your brain “talks” to one side of your face, including one of your eyes. It can make one of your pupils smaller than the other. You can inherit it from your parents, or it can happen after a neck injury or neck surgery. You can also get it if your chest, neck, or brain doesn’t form correctly. Sometimes kids get it if they have a rare type of cancer called neuroblastoma or a tumor in another part of their body.

Horner’s syndrome may cause no other symptoms, or you could have issues like:

  • Droopy upper eyelid (ptosis)
  • Raised lower eyelid
  • Lighter eye color in the eye with miosis (heterochromia)
  • Less sweat on the side of your face with the miotic eye

Some other causes of miosis include:

  • Neurosyphilis (a bacterial infection in your brain that comes from untreated syphilis, a sexually transmitted disease)
  • Severe lack of vitamin D


To find out if you have abnormal miosis, your doctor will take a close look at your eyes in a dark room. They’ll ask you to look at a faraway object. Then they’ll check:

  • The size and shape of your pupils
  • The size of your eyelid opening
  • Whether your pupils are equal in size
  • The position of your pupils
  • How your pupils react to bright light

Normal pupils are 2 to 4 millimeters in bright light and 4 to 8 millimeters in the dark. Your doctor can measure your pupils in both eyes to see how well they shrink and grow.

Sometimes, your doctor may use drops that are supposed to make your pupils big to see how yours react. Or they may order images of your chest, brain, or neck to rule out signs of Horner’s syndrome.


Your doctor’s recommendations will depend on what’s causing your abnormal miosis. If a medication is to blame, they may be able to find a different option that solves the issue.

If your pupils are small because of inflammation in your eye, they can give you long-lasting dilating drops (atropine or homatropine) that make your pupils wider. These are a lot like the drops your eye doctor uses to dilate your eyes during an exam, but they can last up to 2 weeks.

If Horner’s syndrome is causing it, they may need to do several tests to figure out how best to treat it.

Show Sources

American Academy of Ophthalmology: “Normal Vision Development in Babies and Children,” “Congenital Miosis,” “What Is Uveitis?”

American Association for Pediatric Ophthalmology and Strabismus: “Anisocoria and Horner’s Syndrome,” “Dilating Eye Drops.”

British Journal of Clinical Pharmacology: “Relationship between sedation and pupillary function: comparison of diazepam and diphenhydramine.”

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Genetic and Rare Diseases Information Center: “Microcoria, congenital.”

Indian Journal of Ophthamology: “Current approach in diagnosis and management of anterior uveitis.”

Medscape: “Examining the Eyes of an Older Person,” “Horner Syndrome.”

National Institutes of Health: “Your Aging Eyes.”

Schepens Eye Research Institute Massachusetts Eye and Ear: “About the eye.”

Walker, HK. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. Boston: Butterworths, 1990.

Pupil Size and Your Health

Troy L. Bedinghaus, OD, board-certified optometric physician, owns Lakewood Family Eye Care in Florida. He is an active member of the American Optometric Association.

Updated on October 26, 2022

Bryan Wolynski, OD, is a board-certified community optometrist who has been in the eye care field for over 30 years. He works in private practice in New York City.

Table of Contents
Table of Contents

The size of your pupil can tell your healthcare provider quite a bit about your health. It’s an important key to unlocking possible medical conditions you might not otherwise know about.

There are many parts of the eye, and the pupil is among the most important. It controls the amount of light that enters your eye. It also continually changes size.

Your pupil naturally widens and narrows based on the brightness of the light around you. It also changes size depending on whether you are looking at near or far objects.

This article discusses how the size of your pupils can give healthcare providers clues about your health and help them diagnose medical conditions.

pupils constantly change their size

What Is the Pupil?

The pupil is the round, black circle in the center of the iris. Your iris is the colored part of your eye. The pupil is actually a hole through which light passes to the retina, the light-sensitive layer in the back part of the eye.

Your pupils are similar to a camera aperture. They widen or narrow to let more or less light in. Pupils can expand to become larger (dilate) or contract to become smaller (constrict).

Your iris contains muscles that respond to outside stimuli to control the amount of light that reaches your retina. In bright light, the pupil constricts to reduce the amount of light entering the eye. In dark or dim light, the pupil dilates to allow more light into the eye so you can see better.

Your pupils also constrict slightly to look at close objects and dilate slightly to look far away.

Normal pupil size ranges between 1/16 to 5/16 of an inch (2.0 to 8.0 millimeters), depending on the lighting. The younger you are, the larger your pupils tend to be in normal light.

Testing Pupil Size

When your healthcare provider examines your pupils, they will first look for anisocoria —a condition in which the pupils aren’t the same size. Twenty percent of the general population has slight anisocoria that does not signal anything abnormal. But in some cases, unequal pupil sizes can be a symptom of a disease.

Your healthcare provider will also look at the size and shape of your pupils in bright and dim light. Healthcare providers will note the quality and speed that your pupils respond to bright and dim light as well. They may also test your pupils’ response to objects that are nearby, such as small print. Any differences between your pupils are also noted.

The optic nerve and oculomotor nerves both control pupil size. These nerves receive some of their messages from the autonomic nervous system. This part of the nervous system controls all of your vital functions, many of which are automatic.

A disruption of the autonomic nervous system might cause changes in how your pupils react to stimuli. That’s why the size of your pupils can indicate health problems completely unrelated to your eyes.

Associated Conditions

Pupil size abnormalities can sometimes mean you have a disease. This is just a small sample of medical conditions you could have. That’s because there are also other conditions that can cause irregular pupil function.

  • Brain aneurysm: An aneurysm that pushes on nerve pathways in the brain can cause a dilated pupil, as well as other symptoms.
  • Lung cancer: Lung cancer that affects the top part of the lung can spread and impact the nerves that control the pupil.
  • Brain tumor: A tumor or mass close to the origin of the pupil’s nerves can cause problems in the pupil’s function.
  • Multiple sclerosis: Multiple sclerosis can cause optic nerve damage, which leads to an abnormal response of the pupils known as afferent pupillary defect (APD).
  • Head trauma: Head injury or concussion can cause unequal pupils.
  • Cluster headaches: Cluster headaches can cause a constricted pupil on one side.
  • Stroke: A stroke can sometimes cause changes in the size of a pupil.
  • Syphilis: Syphilis can cause an Argyll-Robertson pupil. These are small, unequal, misshapen pupils that constrict with near focusing but do not react normally to light.

In addition, recreational drugs and alcohol can cause the pupils to dilate or constrict abnormally. This is the reason why healthcare providers check your pupils when there is a concern about intoxication or overdose.

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Some prescription and over-the-counter medications, including antihistamines and a few medications used to treat glaucoma, can sometimes dilate your pupils as well.

How Pupil Size Affects LASIK Surgery

It is possible that the size of your pupils can prevent you from having LASIK eye surgery to correct your vision. People with very large pupils are generally not good candidates for LASIK and other refractive procedures.

Eye specialists may use a device called an infrared pupillometer to measure the size of the pupils. The device consists of a large camera that shines infrared light (an invisible type of light) on the eye and senses the reflected light.

Having naturally large pupils or pupils that dilate heavily in dim light may increase the chances of glare and halos following LASIK. This would interfere with the clear vision you are hoping for from the surgery. For this reason, measuring pupil size is an important step in deciding if LASIK is right for you.


The size of your pupils can give your healthcare provider clues about your health. Pupil size constantly changes according to the amount of light entering it. It also changes depending on whether the objects you are looking at are near or far away.

When your healthcare provider notices your pupils aren’t acting normally, it tells them that you may have a disease or medical condition. It can also tell healthcare provider if you are under the influence of recreational drugs or alcohol.

If your healthcare provider notices that you have naturally large pupils, you probably are not a candidate for LASIK surgery. That’s because people with naturally large pupils may experience glares and halos following a LASIK procedure.

A Word From Verywell

Though you may not think about them much, your pupils are a very active part of your body. Not only do they help you see better in various situations, but they can also be a sign of your health.

For this reason, it’s important to get regular eye exams. If you notice that your pupils are an unusual size or reacting differently than normal, be sure to see your healthcare provider and have it checked out.

Frequently Asked Questions

What does pupil size indicate?

Pupils naturally constrict and dilate to control the amount of light that gets in. This helps you focus your eyes on things both near and far. Abnormal changes in pupil size can indicate a person has been drinking or is on drugs. Certain health conditions, like a brain aneurysm or head trauma, can also cause abnormal pupil size changes.

Can a person have naturally large pupils?

Yes. It is rare, but some people can have naturally larger pupils. The standard pupil size is 2 mm to 8 mm. People with naturally large pupils are prone to light sensitivity. The larger pupil allows more light in the eye, which can be uncomfortable but not harmful.

What do uneven pupils mean?

Pupils that are not the same size are known as anisocoria. This can occur naturally or be a sign that something is wrong. Roughly one in five people have naturally occurring anisocoria.
Conditions that can cause pupils to be uneven include a brain aneurysm, cluster headache, or stroke.

15 Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

  1. Aguirre GK. A model of the entrance pupil of the human eye. Sci Rep. 2019;9(1):9360. doi:10.1038/s41598-019-45827-3
  2. Michigan Medicine. Eye anatomy and function.
  3. Mathôt S, Fabius J, Van Heusden E, Van der Stigchel S. Safe and sensible preprocessing and baseline correction of pupil-size data. Behav Res Methods. 2018;50(1):94–106. doi:10.3758/s13428-017-1007-2
  4. MedlinePlus. Anisocoria.
  5. LaRoche MJ. Anisocoria and an array of neurologic symptoms in an adult with Ewing sarcoma. J Adv Pract Oncol. 2017;8(1):18–23. doi:10.6004/jadpro.2017.8.1.2
  6. U.S. National Library of Medicine. What kinds of eye examinations are there? April 20, 2016.
  7. McDougal DH, Gamlin PD. Autonomic control of the eye. Compr Physiol. 2015;5(1):439–473. doi:10.1002/cphy.c140014
  8. Johns Hopkins Medicine. Pupillary disorders including anisocoria.
  9. American Academy of Ophthalmology. 4 neuro conditions not to be missed. March 23, 2016.
  10. Brigham And Women’s Hospital. What is a brain aneurysm?
  11. Liu SL, Nie YH, He T, Yan XX, Xing YQ. Iris metastasis as the first sign of small cell lung cancer: a case report. Oncol Lett. 2017;13(3):1547–1552. doi:10.3892/ol.2017.5648
  12. Chen JW, Gombart ZJ, Rogers S, Gardiner SK, Cecil S, Bullock RM. Pupillary reactivity as an early indicator of increased intracranial pressure: the introduction of the Neurological Pupil index. Surg Neurol Int. 2011;2:82. doi:10.4103/2152-7806.82248
  13. Peragallo J, Biousse V, Newman NJ. Ocular manifestations of drug and alcohol abuse. Curr Opin Ophthalmol. 2013;24(6):566–573. doi:10.1097/ICU.0b013e3283654db2
  14. Schallhorn SC, Kaupp SE, Tanzer DJ, Tidwell J, Laurent J, Bourque LB. Pupil size and quality of vision after LASIK. Ophthalmology. 2003;110(8):1606-14.
  15. American Academy of Ophthalmology. Why are my pupils so naturally large?

Additional Reading

  • Doran M, Karmel M, Stuart A. 4 neuro conditions not to be missed. American Academy of Ophthalmology. 2012.

By Troy Bedinghaus, OD
Troy L. Bedinghaus, OD, board-certified optometric physician, owns Lakewood Family Eye Care in Florida. He is an active member of the American Optometric Association.

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