One Pupil Bigger Than Other?

When you look in the mirror, are your pupils different sizes? The scientific name for having one pupil bigger than the other is called anisocoria. The word anisocoria means that that the pupils are not the same size. There are different types of anisocoria and it is important to understand the difference between them. Because having different size pupils can be a sign of something dangerous in the brain or body, it is always a good idea to see an ophthalmologist if you notice anything abnormal. 

Physiologic Anisocoria

Physiologic anisocoria, also called simple anisocoria, means that the pupil is a different size because that is just the way your body is. It is not a sign of something else harmful going on in the body. Even though physiologic anisocoria is usually present for years, people may notice it suddenly and become concerned. 

Many people have physiologic anisocoria. It is estimated that 1 in 5 people, or 20% of the population has simple anisocoria with a difference in pupil size of less than or equal to 1 mm. Both pupils react to light normally. Interestingly, physiologic anisocoria can be intermittent, which means it is not present all the time. In some cases, it can even completely resolve on its own. 

Diagnosis Of Anisocoria

An ophthalmologist can diagnose anisocoria during an in person eye exam. The eye doctor will test your pupils in light and in the dark, and also take a close look at your pupils using a machine called a slit lamp. How your pupil reacts in both situations will help determine the cause of the anisocoria. In the light, if the bigger pupil does not constrict, then it is likely that that pupil is the problem pupil. In the dark, if the pupil stays small, when it is suppose to dilate, then it is likely that this pupil is the issue. 

The doctor may also ask to see an old photo of you or ask to see your photo ID. This is because they want to see if they can notice a difference in your pupil size from years earlier. If you have always had a difference in pupil size then it is more likely that this is not an emergent situation. However, if the difference in your pupil size is new, or if it is accompanied by other symptoms, it may be something they need to send you to the emergency room for. Some concerning signs and symptoms with anisocoria are also having double vision, eyelid ptosis (droopy eyelid) or having abnormal eye movements. 

Causes Of Anisocoria

Aside from physiologic anisocoria, there are disease in the eye, body and brain that can cause a difference in pupil size. Some of these include Horner’s syndrome, Adie’s Tonic Pupil, stroke, intracranial hemorrhage, and a brain tumor. Sometimes patients may also have anisocoria from eyedrops or medications. This is called pharmacologic anisocoria. 

One Pupil Bigger Than Other: Take Home Points

Related: Best Eye Drops For Chlorine Irritation

If you notice that one pupil is bigger than the other, you should seek medical attention right away. While physiologic anisocoria is not harmful and happens to 20% of people, there are dangerous causes of anisocoria that need to be ruled out by a medical professional. It may be helpful for you to show your doctor old picture of yourself to see if the anisocoria was present at the time you took the photo. Be sure to also tell your doctor about any other symptoms you might be having and if you are taking any medications. 


Gross JR, McClelland CM, Lee MS. An approach to anisocoria. Curr Opin Ophthalmol. 2016 Nov;27(6):486-492. doi: 10.1097/ICU.0000000000000316. PMID: 27585208.

Falardeau J. Anisocoria. Int Ophthalmol Clin. 2019 Summer;59(3):125-139. doi: 10.1097/IIO.0000000000000276. PMID: 31233420.

Steck RP, Kong M, McCray KL, Quan V, Davey PG. Physiologic anisocoria under various lighting conditions. Clin Ophthalmol. 2018 Jan 4;12:85-89. doi: 10.2147/OPTH.S147019. PMID: 29379269; PMCID: PMC5757963.

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