The pupil is the black center part of the eye. Pupils get larger (dilate) in dim light and smaller (constrict) in bright light. Usually both pupils are about the same size and respond to light equally. Unequal pupil size is called anisocoria.

If pupil sizes are very unequal, a person may notice the discrepancy. More often, unequal pupils are noticed only during a doctor's examination. Unequal pupils themselves usually cause no symptoms, but, occasionally, a person may have trouble focusing on near objects. Also, the underlying disorder sometimes causes other symptoms such as eye pain and redness, loss of vision, drooping eyelid, double vision, or headache. These more noticeable symptoms are often the reason people seek medical care rather than the unequal pupils.

Causes

The most common cause of unequal pupils is

• Physiologic anisocoria

Physiologic anisocoria is pupils that are naturally different in size. No disorder is present. About 20% of people have this lifelong condition, which is considered a normal variation. In such people, both pupils react normally to light and darkness and there are no symptoms.

Less commonly, people have unequal pupils because of

• Eye disorders
• Nervous system disorders

Either the larger or the smaller pupil may be the abnormal one depending on the cause. Often, the larger pupil is unable to constrict normally. However, sometimes, as in Horner syndrome, the smaller pupil is unable to widen (see Autonomic Nervous System Disorders: Horner's Syndrome). If the larger pupil is abnormal, the difference between pupil sizes is greater in bright light. If the smaller pupil is abnormal, the difference is greater in the dark.

Eye disorders that cause unequal pupils include birth defects and eye injury. Also, certain drugs that get into the eye may affect the pupil. Such drugs may be drops intended to treat eye disorders (for example, homatropineSome Trade Names
TUSSIGON
used for certain inflammatory disorders or injuries or pilocarpineSome Trade Names
SALAGEN
used for glaucoma) or they may be drugs or other substances that accidentally get into the eye (for example, scopolamineSome Trade Names
TRANSDERM SCOP
used as a patch for motion sickness, plants such as jimsonweed, or certain insecticides). Inflammation of the iris (iritis) and certain types of glaucoma cause unequal pupils, but this finding is usually overshadowed by severe eye pain.

Nervous system disorders that cause unequal pupils are those that affect the 3rd cranial nerve, or certain parts of the sympathetic or parasympathetic nervous system (the autonomic nervous system—see Autonomic Nervous System Disorders: Overview of the Autonomic Nervous System). These pathways carry nerve impulses to the pupil and to the muscles that control the eye and eyelid. Thus, people with nervous system disorders that affect the pupil often also have a drooping eyelid, double vision, and/or visibly misaligned eyes. Brain disorders that can affect these pathways include strokes, brain hemorrhage (spontaneous or due to head injury), and, less commonly, certain tumors or infections. Disorders outside the brain that affect the sympathetic nervous system include tumors and injuries that involve the neck or upper part of the chest. Horner syndrome refers to the combination of a constricted pupil, drooping eyelid, and loss of sweating around the affected eye caused by interruption of the sympathetic nerves to an eye due to any cause.

Evaluation

Doctors' first goal is to determine whether the pupils have always been unequal or whether there is another cause such as a drug or disorder. Then, the goal is to decide whether the larger or the smaller pupil represents the problem. The following information can help people decide when a doctor's evaluation is needed and help them know what to expect during the evaluation.

Warning signs: In people with unequal pupils, certain symptoms and characteristics are cause for concern. They include

• Drooping eyelid (ptosis)
• Double vision
• Loss of vision
• Headache or neck pain
• Eye pain
• Recent head or eye injury

When to see a doctor: People with warning signs should see a doctor right away. People without warning signs but who have any other symptoms should call the doctor. The doctor can decide how quickly they need to be seen based on their symptoms. People who simply happen to have noticed unequal pupils and feel well can usually wait a week or two to see a doctor.

What the doctor does: Doctors first ask questions about the person's symptoms and medical history, including questions about smoking. Doctors then do a physical examination. What they find during the history and physical examination often suggests a cause of the unequal pupils (see Symptoms of Eye Disorders: Some Causes and Features of Unequal Pupils) and the tests that may need to be done.

Doctors ask when the person noticed the unequal pupils, whether vision is blurred in the light or dark, and whether the person has any other symptoms. Other important symptoms involving the eyes include a droopy eyelid, double vision, pain with bright light, loss of vision, and eye pain. Other important symptoms that do not involve the eyes include headache, dizziness or loss of balance, cough, chest pain, or shortness of breath. Doctors ask whether the person has recently had a head or eye injury, what eye drops the person has used, and whether the person has ever had an eye disorder or eye surgery.

The physical examination focuses on the head and eyes. Doctors examine the person's pupils in light and dark rooms. Doctors examine whether the eyes move normally when the person follows a doctor's finger moving up, down, left, right, and toward the eyes. Doctors examine the entire eye, usually using a slit lamp (an instrument that enables a doctor to examine the eye under high magnification). Other eye symptoms are evaluated as necessary. Doctors may use eye drops to test how the pupils respond to drugs that cause the pupils to constrict or widen.

Sometimes doctors examine an old photograph of the person (for example, on the person's driver's license) to see whether pupils were previously unequal.

Typically people with eye symptoms such as pain, redness, blurry vision, or light sensitivity have an eye disorder. People who have a droopy eyelid, double vision, headache, or balance difficulties have Horner syndrome or a 3rd cranial nerve paralysis and possibly a brain disorder. People whose only symptom is recent blurry vision, particularly when focusing on near objects, may have a pupil that has been widened by a drug. People with no other symptoms or abnormalities often have chronic conditions such as physiologic anisocoria, birth defects of the iris, or Adie (tonic) pupil (see Symptoms of Eye Disorders: Some Causes and Features of Unequal Pupils).

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Some Causes and Features of Unequal Pupils Cause Common Features* Adie (tonic) pupil (a pupil that does not constrict normally in response to light) One or both pupils are is too wide, do not fully constrict in response to light, and widen slowly after being constricted by light No other symptoms Birth defects of the iris Features that are present lifelong Usually other birth defects Chemicals and drugs (including scopolamineSome Trade Names TRANSDERM SCOP patches, drugs in animal flea collars or sprays, certain aerosol drugs for asthma or COPD such as ipratropiumSome Trade Names ATROVENT or tiotropiumSome Trade Names SPIRIVA , and organophosphate insecticides) if they contact the eye and some eye drops In people who use or have been exposed to these substances Sometimes difficulty focusing, particularly on nearby objects Horner syndrome (disruption of certain nerve fibers that connect the eye and the brain) On one side of the face, a drooping eyelid, a small pupil that is slow to widen in response to darkness, and decreased sweating If the cause is a disorder (such as migraines or a lung tumor) or an injury, other symptoms An eye injury or eye surgery In people who have had an eye injury or eye surgery Sometimes pain with exposure to bright light and/or eye redness Physiologic anisocoria (pupils that are normally different sizes) Present for a long time No symptoms or abnormalities found during the examination A difference of less than 1/16 inch (about 1 millimeter) in pupil size and pupils that constrict normally in response to light Third cranial nerve paralysis Double vision and a drooping eyelid Sometimes in people who have had a head injury or who have a bulge (aneurysm) in an artery supplying the brain, bleeding in the brain, a brain tumor, or diabetes *Features include symptoms and the results of the doctor's examination. Features mentioned are typical but not always present. COPD = chronic obstructive pulmonary disease.

Testing: Testing is usually unnecessary unless people have other symptoms. People with Horner syndrome or 3rd cranial nerve paralysis usually require magnetic resonance imaging (MRI) or computed tomography (CT).

Treatment

Treatment of unequal pupils is unnecessary. However, the underlying disorder may need to be treated.

Key Points

• Unequal pupils are very common and are often only a normal variation.
• Doctors examine the pupils in light and dark rooms to help determine the cause.
• People with a drooping eyelid or double vision may have a serious disorder.

Last full review/revision August 2012 by Kathryn Colby, MD, PhD