Some people are born with pupils of different sizes, known as anisocoria. However, anisocoria can also result from a trauma, such as a blow to the head, medication use, or an underlying health condition.

Pupils, or the black parts at the center of the eyes, change size to regulate the amount of light entering the eye. Pupils are large in the dark to let more light in and small in bright light.

Usually, the pupils in each eye dilate or constrict at the same time. When they do not, the pupils may appear to be different sizes.

In this article, learn about the possible causes for pupils of different sizes, as well as when to see a doctor.

If a person’s pupils are suddenly different sizes, it is best to seek medical attention. While not always harmful, a sudden change can indicate serious and dangerous medical conditions.

It is especially important to seek medical attention if the change occurs after an injury or with other symptoms.

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Physiological anisocoria is the most common type of anisocoria.
Image credit: ThomasBonini, 2019

There are three types of anisocoria:

  • physiological
  • mechanical
  • pathological

Physiological anisocoria: This is when the pupils are naturally different sizes. It is the most common type of anisocoria, and the difference between the pupil sizes is no more than 1 millimeter. Physiological anisocoria may be temporary or permanent, depending on individual cases.

About 15–30% of the population experiences physiological anisocoria. The difference between the pupil sizes is more or less constant, even when the light changes, and is not usually of concern.

Mechanical anisocoria: This type of anisocoria is the result of physical damage to the eye, such as an injury or a condition that causes inflammation to the eye.

Pathological anisocoria: This type is when the difference in pupil size comes from one of the following:

  • a disease affecting the iris, or colored area
  • a disease affecting the pupil
  • a disease affecting the information pathways to the pupil

Anisocoria may not have an underlying cause. Physiological anisocoria is when there is a natural, small difference in the size of a person’s pupils. This is not harmful and does not require treatment.

However, a sudden and pronounced change in one pupil size can indicate a medical condition.

Medical conditions that may lead to anisocoria include:

Third nerve palsy

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Third nerve palsy or Horner’s syndrome may cause anisocoria.

Third nerve palsy (TNP) can cause the pupils to be different sizes.

Causes of TNP include a brain hemorrhage, trauma, or aneurysm. Anyone experiencing symptoms after a head injury must see a doctor.

Aside from the pupils being different sizes, other symptoms of TNP include:

  • slight drooping of the eyelid, known as ptosis
  • abnormalities in the muscles around the eye
  • potential loss of ability to focus on objects

Other causes of TNP in children include migraine and severe infections, such as meningitis. However, some cases of TNP in children are congenital, meaning they occur from birth.

TNP can be due to pressure on the third nerve from an artery aneurysm. An aneurysm causes pain and is a life-threatening condition that needs immediate attention.

Horner’s syndrome

People with Horner’s syndrome have one abnormally small pupil. They will also have all or some of the following symptoms:

  • a drooping upper eyelid on that side
  • slightly higher lower eyelid
  • an eye with a sunken appearance
  • little or no sweating on that side of the face

Tonic pupil or Adie syndrome

Tonic pupil is where one pupil will appear abnormally large in light, taking a long time to constrict. It is not a life-threatening condition.

About 90% of cases occur in women between 20 and 40 years of age.

Although usually not harmful, having a tonic pupil may lead to someone becoming miotic, or chronically having a small pupil.


Certain medications can also cause pupils to be different sizes.

These include:

Mechanical causes

Physical problems with the eyes that can cause the pupils to be different sizes include:

  • eye trauma, such as being hit or poked in the eye
  • iritis and uveitis
  • acute angle glaucoma
  • intraocular tumors

In cases of pathological anisocoria, medical professionals need to determine which pupil is abnormal by looking at how they react in different lights.

In the light, the pupil that is bigger is usually the abnormal pupil. In this case, doctors may suspect third nerve palsy and would do a CT or MRI scan for diagnosis.

In the dark, the smaller pupil is usually the abnormal pupil. In this case, doctors may diagnose Horner’s Syndrome.

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With pathological anisocoria, a doctor may recommend a CT scan to rule out dangerous underlying causes.

People with physiological anisocoria do not usually need treatment, as it is typically harmless.

Mechanical anisocoria may require surgery to correct the damage a trauma has caused.

If the damage is due to an underlying condition, such as uveitis or glaucoma, a doctor will develop an individualized treatment plan.

If a person’s medication is causing anisocoria, a doctor may recommend a different drug. The symptoms of tonic pupil often subside with glasses and a prescription for pilocarpine.

With pathological anisocoria, such as TNP or Horner’s Syndrome, a doctor must first rule out dangerous underlying causes, such as stroke, tumor, hemorrhage, and aneurysm. They will usually use physical tests and scans, such as CT scans or MRIs, as soon as possible.

The medical term for pupils of different sizes is anisocoria, and doctors categorize this into three further types, depending on the underlying cause.

Many people have very slight differences in the size of their pupils, which is usually not harmful. However, significant size differences or symptoms that come on suddenly can be a sign of dangerous health problems, including a brain aneurysm.

If the pupils change suddenly or with other symptoms, such as drooping or other abnormalities, contact a doctor immediately.