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Anisocoria: What causes unequal pupil sizes?

  1. What is anisocoria?
  2. Anisocoria types and causes
  3. What to do if you have anisocoria

Look at your eyes carefully in a mirror. Does one pupil look noticeably larger or smaller than the other? If so, you may have anisocoria (“an-eye-so-CORE-ee-ah”).

What is anisocoria?

Anisocoria simply means you have unequal pupils. One pupil may be bigger than normal, or one pupil may be smaller than normal, resulting in unequal pupil sizes. The two pupils may or may not respond normally to light.

In most cases, anisocoria is benign and no cause for concern. But if your pupils suddenly become unequal in size, this less-common type of anisocoria can be a symptom of a serious medical condition.

Anisocoria types and causes

There are four main types of anisocoria:

  1. Simple anisocoria

  2. Pathologic anisocoria

  3. Mechanical anisocoria

  4. Pharmacologic anisocoria

Simple anisocoria

Simple anisocoria — also called essential anisocoria or physiologic anisocoria — is the most common type of anisocoria. It’s a benign (harmless) condition that affects approximately 20% of the population

In simple anisocoria, the difference in pupil size is usually 1 millimeter (mm) or less, and both pupils react normally to light. The presence of simple anisocoria does not appear to be influenced by sex, age or eye color

The exact cause of simple anisocoria is unknown. It may be intermittent or constant, and sometimes it goes away on its own.


Top: Normal pupils. Middle: Anisocoria with one pupil bigger than normal. Bottom: Anisocoria with one pupil smaller than normal.

SEE RELATED: Small pupils: What do they mean?

Pathologic anisocoria

Pathologic anisocoria is unequal pupils due to an underlying condition or disease. Examples include:


Iritis is a form of uveitis (an inflammatory disease of the eye). Acute iritis is characterized by eye redness and pain, photophobia, inflammatory cells in the anterior chamber of the eye, and constriction of the pupil of the affected eye (causing anisocoria).

Iritis has many causes, including eye infection, underlying inflammatory diseases and trauma. Your eye doctor can treat the symptoms of iritis while the underlying cause of the condition is determined and controlled. 

In some cases, anisocoria from iritis can remain after the iritis has been successfully treated.

Horner's syndrome

Though signs and symptoms may vary from person to person, most people with Horner’s syndrome have these three signs: 

  • Ptosis (drooping eyelid)

  • Miosis (constriction of one pupil, causing anisocoria)

  • Facial anhidrosis (loss of sweating around the affected eye)

Horner's syndrome also can be differentiated from simple anisocoria by how quickly the pupil dilates in dim lighting. Normal pupils (including normal pupils that are slightly unequal in size) dilate within five seconds of room lights being dimmed. A pupil affected by Horner's syndrome generally takes 10 to 20 seconds to dilate in dim lighting or a darkened room.

Horner's syndrome typically is caused by an underlying medical problem, such as a stroke, tumor or spinal cord injury. But in some cases, no cause can be found.

Adie’s tonic pupil

Adie’s tonic pupil is a dilated pupil caused by damage to nerve fibers that control muscles in the eye that constrict the pupil. The affected pupil also reacts poorly to light. Adie’s tonic pupil occurs primarily in women between the ages of 20 to 40 years, and in 80% of cases, only one eye is affected. In most cases, the cause of Adie's tonic pupil is unknown.

Third nerve palsy

The third cranial nerve — also called the oculomotor nerve — controls several muscles that control movements of the eyes and eyelids and also influences a muscle that controls pupil size. Paralysis (palsy) of the oculomotor nerve causes the affected eye to have a dilated pupil, resulting in anisocoria. 

In addition to anisocoria, third nerve palsy also often causes ptosis (drooping eyelid), a “down and out” misalignment of the affected eye and loss of accommodation (ability to focus on near objects).

Causes of third nerve palsy include pressure on the nerve from an aneurysm, tumor or brain hemorrhage. Causes of oculomotor nerve palsy in children can include migraine and severe infections, such as meningitis. 

If you or a family member develop symptoms of third nerve palsy, seek medical attention immediately.

SEE RELATED: Mydriasis: Definition and causes

Mechanical anisocoria

Mechanical anisocoria is unequal pupil sizes as a result of damage to the iris or its supporting structures. Causes of this type of anisocoria include trauma to the eye, complications of eye surgery (including cataract surgery), angle-closure glaucoma and inflammatory conditions such as iritis or uveitis. 

Congenital anomalies in the structure of the iris also can be considered a cause of mechanical anisocoria. Examples include:

  • Aniridia (a complete or partial absence of the iris of one eye)

  • Coloboma (a gap in the iris present at birth, giving the pupil a distinct “keyhole” or “cat-eye” appearance)

  • Ectopic pupil (an inherited condition that causes displacement of the pupil and dislocation of the lens)

Tumors inside the eye also can cause mechanical anisocoria. 

Pharmacologic anisocoria

This is unequal pupil size that occurs as a side effect of a medication.

Drugs that have been identified as potential causes of pharmacologic anisocoria are selective serotonin re-uptake inhibitors (SSRIs) that are used to treat depression.

Transdermal scopolamine patches used to treat motion sickness and nausea from chemotherapy also have been shown to have the potential to cause anisocoria. 

Certain glaucoma eye drop medications also can cause anisocoria, especially if they are used to treat glaucoma in just one eye. Examples identified include pilocarpine, which can cause a smaller pupil in the treated eye, and brimonidine (Alphagan P) and apraclonidine (Iopidine), which can cause a larger pupil in the treated eye.   

What to do if you have anisocoria

If you or someone else notices that you have unequal pupil sizes, see your eye doctor immediately — especially if you have any of the following:

  • Drooping eyelid (ptosis)

  • Double vision

  • Loss of vision

  • Headache or neck pain

  • Eye pain

  • Recent head or eye injury

If the anisocoria is minor and your pupils react normally to tests your eye doctor performs, there may be nothing to worry about. But you should have your unequal pupils evaluated by an eye care professional before you assume all is well.

If you have anisocoria and one pupil is bigger than the other, ask your eye doctor about photochromic lenses. These eyeglass lenses will darken automatically in sunlight to reduce any light sensitivity (photophobia) you may be experiencing.

Photochromic lenses also will protect your eyes from harmful UV rays and high-energy blue light — especially the eye with the larger pupil if it doesn't react normally to light.

READ NEXT: What is a blown pupil?

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Gary Heiting, OD

Gary Heiting, OD, is a former senior editor of AllAboutVision.com. Dr. Heiting has more than 30 years of experience as an eye care provider, health educator and consultant to the eyewear ... Read more