The iris is a structure behind the transparent cornea of the eye. It has a circular opening called the pupil that changes size in response to the light level. Your pupils are usually the same size as each other, dilating and constricting together as the light level decreases and increases.
Anisocoria is a condition in which the two pupils of the eye are unequal in size. If you notice your pupils are unequal, you should talk to your doctor. Anisocoria might be a sign of a severe eye or nervous system disorder.
Anisocoria is a condition in which your two pupils appear unequal in size. The pupils dilate (widen) when illumination levels are low to let more light into the eye. When there is intense light, the pupils constrict (become smaller). Usually, your pupils dilate and constrict together and are the same size as each other.
Unequal pupils happen when one pupil is not reacting to light as it should. If the iris muscle is paralyzed, the pupil will be dilated. When the other pupil contracts in response to strong light, the pupil in the affected eye remains large. Similarly, some disorders cause one pupil to be constantly small. In low light, the other pupil will dilate, causing anisocoria.
Sometimes. Some people have anisocoria from birth, which causes no trouble. Anisocoria caused by medicines or recreational drugs will resolve when the drug is stopped. But anisocoria can also be a sign of severe diseases of the brain, nervous system, or other body systems.
About 20% of people have pupils of unequal sizes. The pupils react to changes in illumination as expected but remain of different sizes. This is known as physiological or essential anisocoria and does not affect vision. If your pupils have been unequal for a long time and you have no other vision problems, you don't need to worry.
Some birth defects of the iris can cause the pupil to have an irregular, fixed shape. These defects include aniridia (absent iris), coloboma (partial absence of the iris), and ectopic pupil. These are almost always noticed in childhood.
There are several disorders that cause anisocoria. Some of them are life-threatening. If one of your pupils is suddenly larger than the other, you should go to the emergency room immediately.
Adie's Pupil. Also called the tonic pupil, this is caused by damage to the nerves that supply the muscles of the pupil or the ciliary ganglion, a nerve complex in the eye socket. The affected side's pupil is usually dilated and reacts slowly to light. Adie's pupil is more common in women.
Horner's Syndrome. This is a disorder of the nerves of half the face and eye. It can be a genetic condition, in which case it is usually noticed at birth. It can also happen later in life. The pupil on the affected side is small, that side of the face does not sweat, and the upper eyelid is droopy (ptosis). The eye may be sunken into its socket.
Horner's syndrome may be a dangerous sign. It can be caused by several disorders, many of which are life-threatening:
Migraine. A migraine is usually a moderate or severe throbbing headache affecting one side of the head. It is often associated with nausea, vomiting, intolerance to light or sound, and visual problems. One of the eye disorders of migraine is mydriasis, dilation of the pupil.
This dilation of the pupil remains even in bright light when the other pupil constricts. The anisocoria can be significant. Migraine often causes anisocoria that is episodic. Both pupils are equal at most times but become unequal during a migraine episode.
Mechanical Anisocoria. This is caused by some injury or disorder of the iris or its supporting structures. Surgery, eye injuries, inflammation of the iris, tumors in the eye, and angle-closure glaucoma can cause one pupil to be distorted.
Strokes. A stroke is a life-threatening condition in which the blood supply to a part of the brain is cut off. It can cause death. People who recover may have paralysis of some parts of their body. The sooner treatment of stroke is begun, the better the outcome.
Anisocoria can be one of the first symptoms of a stroke. It may be seen when a stroke is about to happen.
Third Nerve Palsy (TNP). The third cranial nerve, also called the oculomotor nerve, controls some of the eye's muscles. A disorder of this nerve reduces the mobility of the eye as well as the reaction of the pupils to light. The pupil on the affected side stays wide open and does not react to light.
Many disorders cause third nerve palsy. One of the most dangerous is an aneurysm of the posterior communicating artery. An aneurysm is a swelling of an artery with thin walls. It can burst, leading to bleeding into the brain.
Medicines and Drugs. Prescription drugs you are taking may cause your pupils to dilate (mydriasis) or constrict (miosis). Among the medicines causing dilated pupils are decongesting nose drops, scopolamine patches, some deodorants, and some herbal medicines. Small pupils are caused by opiates, pilocarpine, prostaglandins, clonidine, and others.
Recreational drugs like cocaine can cause dilated pupils.
Drugs, prescription or recreational, will usually affect both eyes. Sometimes, however, one eye may be more affected. This is more likely with local use, such as nose and eye drops, or nebulized drugs that reach one eye.
Anisocoria by itself may cause you no discomfort. But if there are underlying disorders causing your unequal pupils, you may have symptoms related to those:
If you have any of these symptoms along with unequal pupils, you should talk to your doctor as soon as possible or go to the emergency room.
Your primary care physician may refer you to an ophthalmologist. These are doctors with special training in disorders of the eye. They'll examine your pupils in a dark room and then in bright light to see their reaction to light. You may also need a slit lamp examination and a fundus examination to see the insides of your eyes. If your doctor suspects a serious disorder inside your brain, they may ask for a computed tomography scan (CT scan) or magnetic resonance imaging scan (MRI scan).
If your doctor diagnoses essential anisocoria, no treatment is needed. Unequal pupils without any other symptoms are not a problem. But if any underlying disorder is detected, you will need treatment for it. Some disorders causing anisocoria are dangerous and need hospitalization and emergency treatment.
SOURCES: American Academy of Ophthalmology: "Anisocoria," "What Is Anisocoria?" Cleveland Clinic: "Anisocoria." International Journal of General Medicine: "Migraine with benign episodic unilateral mydriasis." National Health Service: "Stroke." National Organization for Rare Disorders: "Horner’s Syndrome." Payne WN., Blair K., and Barrett MJ. Statpearls, "Anisocoria," Statpearls publishing, 2022
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