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What causes unequal pupils? 18 possible conditions

What Is Anisocoria?

Anisocoria is a condition where the pupil of one eye differs in size from the pupil of the other. Your pupil is the black circle in the center of your eye. The pupils of your eyes are usually the same size.

Anisocoria has several causes and it is only found in a small number of people. You can be born with this condition, or it can develop over time. In some cases, there is no cause for the difference in size. In addition, some people with anisocoria experience a difference in pupil size only temporarily and then the size returns to normal without treatment.

What Causes a Difference in Pupil Size?

A difference in pupil size might be a result of a medical condition or disease, or the side effect of a drug. The most common causes for abnormal pupil size include:

  • brain tumor
  • abscess in the brain
  • meningitis
  • migraine
  • seizure
  • bleeding into the skull
  • aneurysm

Diagnosing the Cause of Uneven Pupil Size

You will normally notice an uneven pupil size yourself when you look into a mirror. Your doctor may also notice the condition during an eye examination.

You should seek medical care if the size of your pupils changes suddenly. Always contact a doctor right away if your pupils change in size following a massive headache or head injury.

During your appointment, your doctor will examine your eyes and take your vital signs. You should tell your doctor about any symptoms you have been experiencing along with the anisocoria. Be sure to mention if you recently had a headache, a change in your vision, a fever, or a stiff neck. Also notify your doctor if you have had any of the following symptoms:

  • sensitivity to light
  • eye pain
  • vision loss

Several tests can be done to find a diagnosis, depending on what your doctor suspects the cause to be. These tests include:

  • complete blood count (CBC)
  • blood differential
  • lumbar puncture, or spinal tap
  • CT (computed topography) scan
  • MRI (magnetic resonance imaging) scan
  • X-ray

What Other Symptoms May Be Present?

Uneven pupils do not usually cause symptoms on their own. However, the underlying medical condition or emergency may cause other symptoms, in addition to an uneven pupil size.

Other symptoms that may be present include:

  • blurred vision
  • double vision
  • fever
  • headache
  • loss of vision
  • nausea
  • stiff neck
  • blurred vision

Let your doctor know if you are also experiencing any of the symptoms mentioned above. This will help him or her diagnose the underlying cause of your anisocoria.

How Are Uneven Pupils Treated?

Treatment options for anisocoria can vary depending on what caused your pupils to become uneven in size.

If an infection is to blame, you may be given antibiotic or antiviral eye drops.

Abnormal growths—such as a brain tumor or a brain abscess—may be removed through surgery. If surgery is not an option, your doctor may use medical treatments to shrink the growth.

Some cases of uneven pupil size are temporary and do not need to be treated.

If you had a head injury before your pupils changed size, you should seek emergency attention immediately. Your treatment options will be discussed with doctors at the hospital.

Preventing Uneven Pupil Size

There is no way to completely prevent changes in the size of your pupils. However, you can reduce your risk of developing uneven pupils by reporting changes in sight to your doctor immediately.

You can also reduce your risk of developing this condition by wearing protective gear while playing contact sports or using heavy machinery.

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See a list of possible causes in order from the most common to the least.



This condition is considered a medical emergency. Urgent care may be required.

A concussion is a mild traumatic brain injury. Usually it occurs after an impact to your head or after a whiplash-type injury. A concussion can cause many severe symptoms that affect brain function.

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Intracerebral Hemorrhage

This condition is considered a medical emergency. Urgent care may be required.

Intracerebral hemorrhage (ICH) is when blood suddenly bursts into brain tissue, causing damage to the brain. Symptoms usually appear suddenly during ICH.

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Subdural Hematoma

This condition is considered a medical emergency. Urgent care may be required.

Subdural hematoma is an accumulation of blood on the brain's surface beneath the skull that's usually caused by a head injury and can be fatal. Headache, seizure, and slurred speech are signs.

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Intracranial Hemorrhages

Intracranial hemorrhage (ICH) is bleeding inside the skull. It is a life-threatening emergency. If you think you or someone you know is experiencing ICH, go to the emergency room right away or call 911. There are fou...

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Head Injury

This condition is considered a medical emergency. Urgent care may be required.

A head injury could be an injury to the brain, skull, or scalp. It can vary in severity depending on the cause. In some cases face swelling can be a sign of a head injury.

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Epidural Hematoma

This condition is considered a medical emergency. Urgent care may be required.

An epidural hematoma occurs when blood fills the area between the skull and the protective covering of the brain. It usually results from a traumatic injury to the head, and puts you at risk for brain damage or death.

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Optic Neuritis

The optic nerve carries visual information from your eye to your brain. Optic neuritis (ON) is when your optic nerve becomes inflamed, causing vision loss.

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Shaken Baby Syndrome

This condition is considered a medical emergency. Urgent care may be required.

Shaken baby syndrome is caused by forcefully and violently shaking a baby. Other names for this condition include abusive head trauma, shaken impact syndrome, whiplash shake syndrome, and inflicted head injury. Shake...

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Skull Fractures

This condition is considered a medical emergency. Urgent care may be required.

A skull fracture is any break in the cranial bone, or the skull. It can result in bleeding, bruising, pain, and swelling. Less severe symptoms include headache, nausea, confusion, and blurred vision.

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Brain Tumor

A brain tumor is a collection or mass of abnormal cells in your brain. A brain tumor can be cancerous (malignant) or non-cancerous (benign).

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Eye Emergencies

This condition is considered a medical emergency. Urgent care may be required.

An eye emergency is when chemicals or a foreign object gets in the eye, or an injury affects the eye area. Emergencies require immediate medical attention to help prevent permanent vision damage.

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Seizures are changes in the brain's electrical activity that cause violent shaking and loss of bodily control. Bruises can result from injuries sustained during a seizure.

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Brain Aneurysm

An aneurysm in the brain is a weak area in an artery in the brain that bulges out and fills with blood. It can be unpredictable and life-threatening, and can cause extremely serious conditions.

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Acoustic Neuroma

Acoustic neuromas are noncancerous tumors that grow on the nerve that connects the brain and ear. They can cause dizziness, vertigo, headache, vision problems, and pain.

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Meningitis is an inflammation of the membrane that covers the brain and spinal cord. It may cause headache and fever in teens and adults, irritability in babies, and trouble breathing in young children.

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Glaucoma is a term for several eye conditions that can damage your optic nerve. It has many types, and over time it can lead to vision loss.

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Necrotizing Vasculitis

This condition is considered a medical emergency. Urgent care may be required.

Necrotizing vasculitis is the inflammation of blood vessel walls. It can interrupt blood flow, causing skin, muscle, and blood vessel damage, and death of tissues and organs. Its symptoms can affect the entire body.

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Low Blood Sugar (Hypoglycemia)

This condition is considered a medical emergency. Urgent care may be required.

Also known as hypoglycemia, low blood sugar can be a dangerous condition. Hypoglycemia is rare in people who are not suffering from diabetes, the chronic disease that affects the body's ability to regulate blood sugar...

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This feature is for informational purposes only and should not be used to diagnose.
Please consult a healthcare professional if you have health concerns.