See your provider or go to the ER right away if one of your pupils is bigger than the other. Anisocoria is sometimes the first sign people notice of a life-threatening underlying condition like a stroke or aneurysm.


Anisocoria makes pupils irregularly sized.
Anisocoria makes pupils irregularly sized.

What is anisocoria?

Anisocoria is the medical term for when one of your pupils is bigger than the other.

The pupil is the black center of your eyes that expands and contracts to help you see in different amounts of light. It naturally changes size without you noticing or controlling it. It shrinks (contracts) in bright light and expands (dilates) in dim light.

Visit your provider or go to the emergency room if you notice one of your pupils is suddenly larger than the other. Some people develop anisocoria with no long-term complications, but it can also be a sign of a much more dangerous issue in your body.


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Anisocoria vs. Horner’s syndrome

Anisocoria is the medical term for asymmetric (different sized) pupils. It’s similar to the way you might see swelling in your body referred to as edema. Anisocoria can be caused by lots of conditions. Some are temporary, but some can be life-threatening.

Horner’s syndrome is a rare genetic condition that affects the tissue around your eyes. It can cause a drooping eyelid (ptosis), irregular pupils and a lack of sweating on half your face.

Both anisocoria and Horner’s syndrome can be caused by serious, life-threatening conditions like a stroke, brain aneurysm or some cancers. That’s why you should talk to your provider as soon as you notice any changes in your eyes or vision.

How common is anisocoria?

Anisocoria can affect anyone. Experts estimate that around 15% of people experience anisocoria at some point in their lives.


Some babies are born with anisocoria. Talk to your provider about what you should expect if your child’s pupils are different sizes at birth.

How does anisocoria affect my body?

The most obvious way anisocoria affects your body is one of your pupils is bigger than the other. Usually, one of your pupils will be different enough from the other that you can notice it in a mirror.

Other than physically looking out of proportion, the difference in size might affect your vision. If one pupil can’t adjust to light like it usually does, you might have trouble seeing clearly, or you might be light sensitive in your affected eye.

Symptoms and Causes

What are the symptoms of anisocoria?

Anisocoria can be accompanied by serious symptoms. Go to the emergency room if one pupil is bigger than the other and you experience any of the following:

  • Eye pain.
  • Loss of vision.
  • Blurry vision.
  • Double vision (diplopia).
  • Light sensitivity.

You might have symptoms outside your eyes too, including:

Depending on what’s causing the anisocoria, you might have no symptoms. If that’s the case, you still need the change in your eyes examined by a provider. You might also still need imaging tests to rule out one of the causes of anisocoria that can be life threatening.


What causes anisocoria?

Anisocoria can be caused by a lot of conditions in your body, injuries, traumas and even some medicines.

Some of the most common causes include:

Anisocoria can also be caused by serious, life-threatening conditions, including:

Your healthcare provider will classify your anisocoria as either pathological (caused by a disease) or physiological (caused by something malfunctioning inside your body).

This distinction isn’t as important as getting your symptoms evaluated right away. No matter what’s causing your pupils to be irregularly sized, you should see your provider as soon as possible.

Diagnosis and Tests

How is anisocoria diagnosed?

Your healthcare provider will diagnose anisocoria by looking at your eyes and performing a physical exam to check for other symptoms. You might need imaging tests, including:

You might also need blood tests or a lumbar puncture if your provider thinks you have an infection.

Management and Treatment

How is anisocoria treated?

Treating what’s causing anisocoria is more important than treating the irregularity in your pupils.

If you have no other symptoms — and your provider rules out any serious underlying conditions — you probably won’t need any treatment.

If anisocoria is the first sign of a more serious condition, the treatment you’ll need depends on what’s causing the anisocoria. Talk to your provider about what you’ll need to do next.

How do I take care of myself with anisocoria?

Tell your healthcare provider about any changes in your eyes or vision. If you wear glasses or contact lenses, have your eyes examined regularly so your provider can adjust your prescription as often as necessary.


How can I reduce my risk?

Because anisocoria can be caused by such a wide range of conditions, there’s no one surefire way to prevent it. Have your eyes examined regularly, and see your provider annually for a checkup.

Maintaining good overall health is the best way to make sure your body can function at its best.

Outlook / Prognosis

What can I expect if I have this condition?

It depends on what’s causing your anisocoria. Some issues like migraines or reactions to medications will resolve themselves on their own. If you experience something more serious like an aneurysm or stroke, your life might be changed permanently.

For most people, anisocoria is a minor part of a larger health issue, and as you treat your underlying condition, your pupils will return to their usual size.

Talk to your provider about what to expect, especially if you’ll need longer-term treatment to recover.

Living With

When should I see my healthcare provider?

Visit your healthcare provider or go to the emergency room right away if you notice any changes in your eyes or vision, including if your pupils are irregularly sized.

Go to the emergency room right away if your pupils are different sizes and you experience any of the following symptoms at the same time:

  • Eye pain.
  • Blurry, double or loss of vision.
  • Light sensitivity.
  • Neck pain.
  • Nausea and vomiting.

What questions should I ask my doctor?

  • What’s causing the anisocoria?
  • Will I need any tests?
  • What treatment will I need?
  • How long will recovery take?

A note from Cleveland Clinic

Anisocoria can be a sign that something inside your body is seriously wrong. Visit your provider or the emergency room as soon as you notice any irregularity in your pupils. Even if it turns out that you’re having a short-term reaction, you should always take any changes in your eyes seriously and have them checked out by a provider right away.

Medically Reviewed

Last reviewed by a Cleveland Clinic medical professional on 02/16/2022.

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