What Is Eye Miosis?

Medically Reviewed by Whitney Seltman, OD on March 09, 2023
5 min read

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.

If you're in a bright environment, the pupil will shrink in size to allow less light to enter the eye. In dark surroundings, the pupil expands to let more light in. Your pupil's size can also change depending on whether you're looking at nearby or far-away objects. If the pupil doesn't vary in size in response to changes in lighting and gaze, there might be something wrong.

In an adult, the pupil's diameter usually varies between 2 and 4 millimeters in bright light and between 4 and 8 millimeters in dim light.

The maximum pupil size also varies significantly among age groups. For example, the pupil is the widest around age 15, after which it begins narrowing in an inconsistent fashion after age 25. 

Pupil size can change dramatically in response to not only light but also your emotions, periods of intense concentration, recently eaten foods, prescription and recreational drugs, and underlying diseases or disorders. Past physical injuries to the eye or the head can also have a long-lasting impact on the average pupil size.

Response to light. Variations in lighting conditions are the most common reason for a change in pupil size. The pupils in both eyes respond independently to bright or dim light, so it's possible for one pupil to expand or contract while the other remains stable. Pupils also make small adjustments in size to help you focus better on a close or distant object.

Emotions. Processing or experiencing emotions leads to changing pupil sizes too. The pupils slightly expand or dilate every time you feel excited or nervous. Seeing strong emotional cues like a person laughing or crying could also cause differences in pupil size. In general, pupils tend to grow wider if you vividly feel a positive or negative emotion. The pupils can also widen if you're concentrating hard on a particular task like solving a math problem or retrieving a memory.

Medications and drugs. Both prescription and over-the-counter drugs can cause your pupils to expand or contract. Your ophthalmologist or eye doctor might give you eye drops that widen or dilate your pupils. This allows medical professionals to take a better look at your inner eye. Illegal or recreational drugs can also produce noticeable changes in pupil size. The most commonly abused drugs that affect pupil size include cocaine, LSD, MDMA, heroin, methamphetamine, and ketamine.

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

Cluster headaches. These headaches come on in groups -- or clusters -- before stopping. You can have up to eight headaches within a day, each lasting about 30 minutes. Like Horner’s syndrome, miosis and drooping may affect one side of your face.

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.