Unequal Pupils (Anisocoria): When Is It an Emergency?

Having unequal pupils, called anisocoria, means one pupil (the dark opening in the center of the eye) is larger than the other. A slight difference can be part of normal variation, so what matters most is whether the difference is new and what comes with it. A new difference is an emergency when it comes with a droopy eyelid, double vision, a severe headache, weakness or a recent head injury: call 911. A new difference on its own still needs a same-day eye exam, or an emergency room when we're closed. Some causes are harmless and temporary, such as eye drops, or medicine from a motion sickness patch getting into the eye.

Key takeaways

Slightly unequal pupils (anisocoria) can be normal, and whether the difference is new and what else you feel matter more than the measurement.
Even with no other symptoms, a new difference in pupil size needs a same-day eye exam, or an emergency room when we're closed.
Dilating drops in one eye, some allergy drops, and medicine from a motion sickness patch that gets into the eye can temporarily leave one pupil larger.
Call 911 when a new pupil difference comes with a droopy eyelid, double vision, a severe headache, weakness or a recent head injury.

Is it normal for one pupil to be bigger than the other?

A slight difference can be. Pupil size shifts a little all the time, and doctors become concerned about the nerves when the change is marked. Under standard testing conditions, pupils usually measure 2 to 5 millimeters across.

About 4% of people have pupils that differ by more than 1 millimeter, and at that size doctors check for a nerve problem.

Whether the difference is new, and whether anything else feels wrong, matter more than the measurement. If you can't remember how long it has been there, a close-up photo from a year or two ago can help your eye doctor tell.

When are unequal pupils an emergency?

When the difference is new and comes with other symptoms. Call 911 for unequal pupils with any of these:

A droopy upper eyelid or double vision. A growing aneurysm (a bulging blood vessel in the brain) can press on nerves and cause a larger pupil, double vision, and pain above and behind the eye.
A severe headache. NINDS says a sudden, severe headache, especially with any other symptoms, calls for 911.
Face drooping, arm weakness, trouble speaking, loss of balance or a sudden change in vision, the stroke warning signs.
A recent blow to the head. After a head injury, one pupil larger than the other is a danger sign that calls for 911 or an emergency department right away. In rare cases, a blood clot that crowds the brain against the skull can develop.

Call us immediately at 310-828-2010 if the larger pupil is in a red eye with intense pain, blurred vision or nausea. Those can be symptoms of angle-closure glaucoma, which needs an eye care provider or an emergency room right away. During office hours (Monday 10 to 5, Tuesday through Friday 8 to 5) we see it the same day; when we're closed, go to an emergency room.

A new difference with none of these symptoms still deserves a look the same day. Call us, and if we're closed and you can't explain the change, go to an emergency room rather than waiting for the next office day.

What causes unequal pupils?

What comes with the difference points to the cause and to how fast you need care.

What you notice	What it can mean	What to do
A slight difference that has been there for years, with nothing else	Normal variation in pupil size	Mention it at your next eye exam
One larger pupil after handling a motion sickness patch	Scopolamine from the patch reached the eye	Wash your hands; the effect is temporary. If the eye also hurts or turns red, take the patch off and call us now (emergency room when we're closed)
A smaller pupil and a droopier upper eyelid on the same side	Horner syndrome	If it's new, call 911
A larger pupil with a droopy eyelid or double vision	Third nerve palsy, which can come from an aneurysm	Call 911
One larger pupil after a blow to the head	A danger sign after a head injury	Call 911
A larger pupil in a red, painful eye with blurred vision or halos	Acute angle-closure glaucoma, which the motion sickness patch can also set off	Call us now; emergency room when we're closed
One larger pupil, sometimes with blurry vision, and no droop, double vision or pain	Adie's tonic pupil, a pupil that constricts slowly in bright light	A same-day exam if it's new; emergency room when we're closed

What is Horner syndrome?

Horner syndrome affects the eye and surrounding tissue on one side of the face. It causes a drooping upper eyelid, a smaller pupil that leaves the pupils unequal, and absent sweating on that side of the face.

It can appear at any age. The nerves involved can be damaged by an injury, including surgery or a difficult birth, by a benign or cancerous tumor, or by a problem with the artery that supplies blood to the head and neck. When it starts before age 2, the iris (the colored part of the eye) on the affected side can be lighter than the other.

Because some of those causes are urgent, a new droop with a smaller pupil is a 911 call, as our page on droopy eyelids explains.

What is a third nerve palsy?

The third cranial nerve, also called the oculomotor nerve, lifts the upper eyelid, moves the eye in most directions and narrows the pupil.

When it stops working, the lid droops and double vision follows, and in a complete palsy the eye turns down and out. If the pupil is involved, it becomes fixed and dilated. That raises concern that something, such as an aneurysm, is pressing on the nerve, and it calls for urgent brain imaging.

When the pupil is spared, the cause is usually poor blood flow to the nerve, particularly in people with diabetes or high blood pressure. Most of these palsies improve within a month, with complete recovery by 3 months. You can't tell the two apart at home, so new double vision with a droopy lid means 911 either way.

What is Adie's tonic pupil?

Adie's pupil is a pupil in one eye that is larger than normal and constricts slowly in bright light. When the deep tendon reflexes, usually at the ankle, are also missing, the condition is called Holmes-Adie syndrome.

Symptoms tend to start in adulthood. Most of the time the cause is unknown, and some cases follow trauma, surgery, poor blood flow or an infection.

How we check unequal pupils at Cohenmehr Optometry

Unless it's a 911 call, phone 310-828-2010 first and tell us when you noticed the difference, whether you've used any eye drops or a patch, and what else you feel. A person answers and triages the call, and urgent cases are worked into the same day; when we're closed, go to an emergency room for a new difference you can't explain.

At Cohenmehr Optometry, the exam starts with a light shone into each eye to check how the pupils react. The urgent work-up adds the slit lamp, pressure checks and imaging as your symptoms demand. If an eyelid has been drooping for a while too, we measure its position and check the muscle that lifts it, which is part of ruling out a nerve cause; see our page on droopy eyelid evaluation. When the exam points to a nerve or blood vessel problem, we coordinate an urgent referral, and the referral and your records move the same day.

For a pupil that stays wide after an eye injury, we fit custom soft lenses with a printed iris that block the extra light, matched in color and pupil size to the other eye.

Questions patients ask

Can allergy eye drops make your pupils bigger?

Some can. Over-the-counter allergy drops that combine naphazoline, a redness reliever, with pheniramine, an antihistamine, carry a label warning that pupils may become enlarged temporarily, causing light sensitivity. Used in only one eye, they can leave the pupils unequal until the effect wears off. The same label says to ask a doctor before using them if you have narrow-angle glaucoma and to stop and ask a doctor if you feel eye pain or your vision changes.

Can lazy eye drops make one pupil bigger?

Yes, and it's expected. Instead of a patch, some children with amblyopia (lazy eye) get a once-a-day atropine drop in the stronger eye, which blurs near vision so the brain uses the weaker eye. Atropine 1% also dilates the pupil, so that eye's pupil looks bigger, and its label warns that light sensitivity and blurred vision may last a couple of weeks.

Can an eye exam leave my pupils different sizes?

Yes, if dilating drops went in only one eye: that pupil stays larger until the drops wear off. The National Eye Institute says vision may be blurry and light may bother you for a few hours after a dilated exam, so bring sunglasses and ask someone to drive you home. At a comprehensive eye exam we examine the retina by dilation or wide-field imaging, whichever your eyes need.

Why is one of my eyes more sensitive to light?

A larger pupil is one reason. Dilating a pupil lets more light into the eye, the way opening a door lets light into a dark room, so a pupil widened by drops, a medicine or an injury can make that eye sensitive to light. Light that suddenly bothers you, especially with eye pain or redness, is a different matter: our eye symptoms guide lists it as a same-day problem, so call us, or go to an emergency room when we're closed.

Sources

Written by

Dr. Kevin Cohenmehr, OD

Medical eye care: glaucoma, diabetic eye exams, macular degeneration and eye infections, plus vision therapy for children. A graduate of UCLA and SUNY College of Optometry.

About Dr. Kevin →

Unequal Pupils (Anisocoria): What It Means and When It's an Emergency