Most people have one pupil in each eye — but that's not the case for everyone. Polycoria is an extremely rare eye disease in which a patient has multiple pupils in one eye. In cases of true polycoria, each pupil reacts to light and functions independently of the other one.

This guide to polycoria explains the difference between true and false polycoria, how polycoria may affect vision and how the condition can be treated.

The pupil is an opening in the center of the iris that changes in size to allow more or less light into the eye. The pupil is surrounded by muscles that make it smaller (constrict) in bright light and make it larger (dilate) in dim light.

The pupil is a part of the eye that's extremely important for vision. By allowing just the right amount of light to reach the retina, the pupil helps you see clearly and lessens glare both during the day and at night.

Polycoria is one of several eye conditions that affects the pupil. Other conditions affect how the pupil responds to light, including: 

• Adie's tonic pupil
• Argyll Robertson pupil
• Marcus Gunn pupil

SEE RELATED: Anisocoria: Unequal pupil sizes

There are two main types of polycoria: true polycoria and false polycoria. Even an eye doctor can't necessarily tell the difference at first glance.

In true polycoria , the eye has two or more fully functioning pupils within one iris. Each pupil will react to light independently by constricting or dilating on its own.

In false polycoria ( pseudopolycoria ), an eye seems to have multiple pupils but actually only one pupil reacts to light. The other holes in the iris look like, but don't function as, pupils.

True polycoria is so rare that only a few cases have ever been reported. The condition can affect a patient's ability to see clearly with the affected eye. It's important to see an eye doctor right away if you have any issues with your vision.

Depending on the case, polycoria may or may not cause vision issues. Symptoms of polycoria may include:

• Appearance of more than one “pupil” in one or both eyes

• Blurry vision

• Double vision (diplopia)

• Glare sensitivity

In one case of polycoria reported in Arquivos Brasileiros de Oftalmologia (a Brazilian ophthalmology journal), the patient told doctors he had had poor vision in his left eye since childhood. During an eye exam, doctors found two pupils in his left eye. They put dilating eye drops into the eye and found that both pupils dilated. The patient was diagnosed with true polycoria.

Doctors don't know exactly what causes a person to have multiple pupils. However, polycoria may be caused by problems with eye development before birth.

Polycoria has also has been linked to these two other uncommon eye conditions:

• Axenfeld-Rieger syndrome – Axenfeld-Rieger syndrome (ARS) is a genetic condition that affects the eyes. Patients who have this condition may also have false polycoria — extra holes in the iris that look like multiple pupils — or a single pupil that is off center.
• Iridocorneal endothelial syndrome – Iridocorneal endothelial (ICE) syndrome is an eye disorder that can lead to distortion of the iris and pupil, including polycoria. Patients with this condition also often develop secondary glaucoma. ICE syndrome is more common in women and patients 20 to 50 years old.

A patient may not know they have polycoria until they go to an eye doctor for a comprehensive eye exam. During the visit, the eye doctor will likely perform a slit-lamp exam, which uses a special light and microscope, to examine the front and back of the eyes. This exam also allows the eye doctor to see any abnormalities in the pupils.

It may not be necessary to treat polycoria if it does not affect the patient's vision.

However, an eye doctor may recommend surgery for a patient with true polycoria if it is causing poor vision. The surgeon may treat polycoria by doing a pupilloplasty, an operation in which the surgeon cuts the iris "bridge" that separates the multiple pupils, joining them into a single pupil.

In the case of the patient with true polycoria in Brazil, his doctors performed a pupilloplasty. The patient's vision improved in the affected (then surgically treated) eye, and he was happy with the results of the surgery.

It's important to see your eye doctor for regular comprehensive eye exams and to schedule a visit any time you experience vision problems. In one case, polycoria was diagnosed in an adult who went to the eye doctor regarding vision issues.

Annual eye exams allow your eye doctor to check your eye health and your vision. If you do have an eye condition, going to your eye doctor regularly is the best way to get timely diagnosis and treatment. If you're due for an exam, contact your eye doctor to schedule a visit today.

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