Unequal Pupils

It can be present from birth or a symptom of a medical problem

Pupils are the dark circles in the eyes. They are usually equal in size, but some people are born with unequal pupils.

Many different eye and medical problems can cause the pupils to be unequal—a condition called anisocoria. Certain medications placed in the eyes can cause the pupils to be different sizes, but the effects should wear off, typically within a few hours.

This article will discuss the symptom of unequal pupils, possible causes, tests, and treatment.

Close-up of a person's face and eyes

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Symptoms of Unequal Pupils 

You might notice that your pupils are not the same size when you look in the mirror. Or you might notice someone else’s pupils are not the same size. 

Sometimes unequal pupils can be associated with other symptoms.

Symptoms that can be associated with unequal pupils include:

  • Numbness or weakness of one side of the face or body 
  • Blurred vision or loss of vision in one or both eyes 
  • A droopy eyelid (or both eyelids)
  • Head pain or eye pain 
  • Change in consciousness 

Types of Unequal Pupils

The pupils of the eyes normally constrict (get smaller in size) in response to light and when looking at near objects. These responses should occur symmetrically (the same for both eyes), even when someone naturally has a slight difference in the size of their pupils. When one eye is exposed to light, the other eye also constricts. 

Abnormal pupil size can be:

  • Fixed: The pupil that is not normal in size might not constrict or dilate the way it should.
  • Reactive: The abnormally sized pupil has at least some change with exposure to light or change in visual distance.
  • Dilated: One or both pupils are abnormally large.
  • Constricted: One or both pupils are abnormally small.

Causes of Unequal Pupils 

Some people are born with unequal pupils. This can be a harmless physical feature or a sign of an eye or brain problem. Babies with anisocoria should have an initial physical examination to determine whether a serious cause needs to be addressed. 

Many illnesses can cause pupils to be unequal. Causes of unequal pupils can include:

  • Multiple sclerosis (MS): MS is a chronic neurological disorder that causes symptoms affecting vision, movement, sensation, and more.
  • Cranial nerve damage: This can occur due to a stroke (a blockage of blood flow or bleeding in the brain), brain aneurysm (defect in a blood vessel), or a brain tumor. 
  • Inflammation: An infection affecting the eye or the cranial nerves can cause anisocoria. Inflammatory conditions, such as MS and sarcoidosis, also can do this. 
  • Trauma: An injury affecting the eye or the brain may cause the pupils to be unequal. 
  • Increased intracranial pressure: This can result from a brain tumor, meningitis (inflammation of the fluid around the brain), or a stroke. 
  • Vision loss: Significant vision defects can affect pupil size and reactivity. 
  • Migraine: While it is not common, migraines can cause anisocoria. 
  • Seizure: Sometimes seizures (a disruption of electrical activity in the brain) can cause changes in the pupils, which may be equal or unequal. 
  • Surgery: Eye surgery may result in alterations in pupil size, which can be permanent. 
  • Coma or brain death: Severe brain damage may cause a change in the size and reactivity of the pupils. 

How Does Anisocoria Happen? 

Pupil size and reactivity are controlled by coordination between vision (controlled by cranial nerve two), muscles that move the opening of the pupil, and the nerve that controls these muscles (cranial nerve three). Any injury, disease, or chemical that interferes with one or more of these functions can lead to anisocoria. 

Horner's Syndrome

Horner’s syndrome is a droopy eyelid, pupil constriction, decreased sweating, and possibly eye redness. It occurs due to conditions such as tumors, upper spine disease, stroke, disease in the neck or chest, and MS, which can affect sympathetic stimulation of the nerves that control the pupils.

What Medications Can Cause Unequal Pupils? 

Many medications affect pupil size and can cause anisocoria. Some of these are placed in the eye for the purpose of dilating (enlarging) the pupils as part of an eye examination or a surgical eye procedure. Many medications have the potential to cause changes in pupil size or anisocoria as a side effect. 

A few of the medications that can cause unequal pupils are:

In addition to medications, some recreational drugs may also cause changes in pupil size. For example, using alcohol, marijuana, or cocaine can cause dilated pupils, and opiates can cause pupillary constriction or dilation. These effects are usually symmetrical, but they may be unequal. 

Temporary Effects

The effects of medications or drugs on pupil size are temporary and wear off within minutes, hours, or days, depending on the drug, dose, and a person’s ability to break down and remove the drug from the body.

How to Treat Unequal Pupils 

The treatment of unequal pupils varies. In some circumstances, the pupil size can indicate a severe medical emergency. That’s why prompt medical attention is necessary. 

Treatment is not necessarily focused on making the pupils look equal. Instead, the priority of treatment is to manage the underlying condition. 

Some of the treatment options include:

  • Disease-modifying medication to treat MS 
  • Medications to reduce intracranial pressure, which can include high-dose steroids 
  • Acute stroke treatment, such as blood thinners 
  • Antibiotics to treat bacterial meningitis 
  • A procedure to lower intracranial pressure, such as the removal of cerebrospinal fluid (CSF)
  • Surgical removal of a tumor 
  • Placement of a ventricular shunt to prevent the buildup of intracranial pressure 

Are There Tests to Diagnose the Cause of Unequal Pupils? 

Diagnosis of the cause of unequal pupils involves a comprehensive physical examination, including an eye examination and a neurological examination. Diagnostic tests are often needed as well. 

Your healthcare provider will determine whether you have any visual problems. And they will also define whether you have fixed or reactive pupils and whether either of your pupils is dilated or constricted. 

Other symptoms and signs can help to identify underlying medical conditions. For example, a severe headache may indicate increased intracranial pressure, while fever and stiff neck are signs of meningitis. 

Diagnostic tests may include:

  • Brain imaging, such as computerized tomography (CT), magnetic resonance imaging (MRI), or angiogram, can identify problems with the brain or with the blood vessels of the brain. 
  • Visual evoked potential can identify signs of eye disease or MS.

Marcus Gunn Pupil

Marcus Gunn pupil, also called a relative afferent pupillary defect, occurs when both pupils constrict in response to light in one eye but do not constrict in response to light in the other. This is a sign of vision problems in the eye that don’t elicit pupil constriction.

When to See a Healthcare Provider

You should see a healthcare provider immediately if you suddenly develop unequal pupils. This can be a sign of a medical emergency. 

Discuss a plan of action with your healthcare provider if you have a chronic condition that can cause unequal pupils, such as MS or a ventricular shunt. The plan will include which symptoms you need to call about and who to call.

Summary 

Unequal pupils (anisocoria) has many different causes. It can be a natural physical trait, a temporary effect of medication, alcohol, or illicit drugs, or a sign of neurological or eye disease.

If you develop new anisocoria, it’s important to get medical attention. The diagnostic process involves a detailed eye exam and neurological examination. Treatment is necessary if your unequal pupil size has a medical cause. 

A Word From Verywell 

If you have any asymmetry of your eyes or pupils, a healthcare provider will work quickly to reach a diagnosis because sometimes anisocoria is caused by serious problems, such as a tumor or a stroke. Being observant about your health can help you get appropriate and effective treatment at the right time. 

Frequently Asked Questions

  • Can you correct unequal pupils?

    Yes, unequal pupils can often be corrected, but the purpose of treatment goes beyond correction—the goal is to manage the underlying condition. 

  • Do pupils dilate and constrict together?

    Yes, normally, pupils constrict and dilate together simultaneously and symmetrically. However, an asymmetry of these movements often signals a problem with one or both sides.

  • Can drugs affect pupil size?

    Yes, certain medications and recreational drugs can affect pupil size. Most of the time, this affects both pupils equally.

    Some medicated eye drops can cause the pupils to be unequal if the drops are placed only in one eye. Eye drops placed in both eyes can sometimes have unequal effects, causing uneven pupils.

3 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.
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