One Pupil Bigger Than the Other? Mirror Check and Red Flags

A mirror and photo check shows whether one pupil is bigger than the other and whether that difference is new. A small difference that has been there for years is common and usually harmless, but a sudden change with headache, weakness, confusion or blurred vision needs emergency help. Here is how to check it properly, what can throw the result off, and when to call for help.

What does it mean when one pupil is bigger than the other?

The medical name for unequal pupil size is anisocoria. The American Academy of Ophthalmology’s EyeWiki describes it as relatively common. The most common cause is the harmless kind, called physiologic, which EyeWiki says affects up to 20% of people. In that kind the difference is 1 mm or less, and the light and near responses of the pupils are normal. MedlinePlus adds that slight differences are found in up to 1 in 5 healthy people.

The other causes sit in the nerves that control the pupils, in the iris itself, after an injury, or from certain medicines and eye drops. Those causes can look similar at home, which is why a difference that is new, changing or accompanied by symptoms needs a clinician to examine it. A photo can show that a difference exists. It cannot show why.

How do you check pupil size at home, step by step?

Use the same room, the same distance and the same light every time, so that any change you see is in your eyes and not in the conditions.

1. Sit or stand in an ordinary, even room light. Keep windows and bright lamps out of your face, and turn off any screen brightness boost.
2. Before you start, take out coloured or cosmetic contact lenses and remove eye makeup. Note any eye drops you have used and the time you last used them, because drops change pupil size.
3. Look straight ahead at something far away for about 10 seconds, then look at your face in a mirror from about arm’s length.
4. Compare the dark centres of the two pupils side by side. Note which one looks bigger and whether the difference is slight or obvious.
5. Take a photo of your face from the same distance, with the flash off. Put the date and time in the file name, or write them in a notes app.
6. Turn off the main light, sit in a dim room for two minutes, and take a second flash-free photo from the same spot.
7. Repeat the whole check at the same time of day for three to five days, and keep the photos in one dated folder.
8. Write down any headache, vision change, drooping lid, double vision, weakness or numbness at the same time.

What the result means in bright and dim light

Clinicians compare the difference in bright light and in the dark. In the harmless pattern it stays about the same in both. A difference that changes between bright and dim light is a reason to ask a clinician to check it, and your photos help that conversation.

A home photo cannot give a precise size. A photo cannot show whether the gap is exactly 1 mm, so do not try to decide that from a phone picture. If the difference is visible at a glance in several photos, that is enough reason to mention it.

What can throw off the reading?

• Flash or a torch: bright light makes pupils shrink, which can exaggerate a small difference or hide a real one.
• Eye drops: drops that widen or narrow the pupil change its size for a time, in one eye or both.
• Coloured or cosmetic contact lenses: the printed ring can hide part of the pupil edge and make it look uneven.
• Eye makeup: dark liner or shadow close to the pupil can make one side look smaller.
• Changing light: a photo taken straight after walking from a bright room into a dark one can show a difference that is not there in steady light.
• Glare and angle: a shiny reflection or a head tilted to one side changes how the pupil edge appears.
• Tinted glasses: dark or photochromic lenses in a photo change how both pupils look.

When is a pupil difference an emergency?

A pupil difference that is new, sudden or unexplained, especially with symptoms, needs urgent assessment. The NHS stroke guidance lists blurred vision or loss of sight in one or both eyes and a severe headache among stroke warning signs, alongside a drooping face, arm weakness and slurred speech. It says to call 999 if you think you are having, or have had, a stroke, including signs in the last 24 hours even if they have stopped, and not to drive yourself to A&E.

EyeWiki notes that pain, headache, drooping of the upper lid, double vision, blurred vision, numbness, weakness or unsteadiness may warrant further evaluation, and that a headache with confusion or focal neurological deficits suggests a mass effect that needs a neurological workup, with bleeding and tumour ruled out. Head trauma is one of the listed causes. MedlinePlus says a difference after an eye or head injury needs medical help immediately, and that a difference which develops later in life and does not return to equal size may be a sign of an eye, brain, blood vessel or nerve disease.

Call emergency services now if a pupil difference comes with any of these: a sudden severe headache, a drooping lid, double vision, one-sided weakness or numbness, slurred speech or confusion, drowsiness that is hard to rouse, a fit, or a head injury. Do not wait for the next appointment in those cases.

What should you do about a steady difference that is not urgent?

See more in Eye Care. A difference that is the same in every photo, has been there for as long as you can remember, and comes with no other symptoms is worth mentioning to a doctor at your next visit. Bring the dated photos, a list of any eye drops or lenses you use, and your notes about light and symptoms. A clinician can test the reaction to light and eye movements, which a home check cannot.

Unequal pupils: questions people ask

Sources

• NHS: Stroke symptoms (FAST, and when to call 999)
• EyeWiki (American Academy of Ophthalmology): Anisocoria
• MedlinePlus (US National Library of Medicine): Anisocoria