If one pupil is noticeably dilated, especially if your cheek is tingling, this is a medical emergency.

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“It can be scary to look into the mirror and see one of your pupils dilated,” says Kaushal M. Kulkarni, MD, board certified ophthalmologist and neuro-ophthalmologist in private practice in New York.

“Or to be at work or with friends or family and have one of them point it out to you. Or, even worse, to have other symptoms, such as headache, cheek tingling or numbness, double vision or eyelid drooping.

“There are many reasons you could have a dilated pupil on one side, and although some of them are benign — as we will talk about below, the fact is that a dilated pupil on one side is an emergency, especially if you have any other symptoms, and you should seek medical attention immediately.

“It could represent a third nerve palsy, which could be due to a brain tumor, stroke or aneurysm.

“Or it could be that your opposite pupil is actually smaller, making the other eye look ‘dilated.’ This could be due to a condition called Horner’s syndrome and is also an emergency since it could represent a carotid artery dissection.

“Ultimately, in order to find the correct diagnosis, you may need to see a neuro-ophthalmologist, who is an eye doctor or neurologist who specializes in the pupil.

“That being said, people with migraines often get a dilated pupil on one side associated with their episodes of migraine and headache.

“This is usually benign and gets better within a few hours. This could theoretically cause some cheek tingling at the same time if it is a complicated migraine.

“Other benign causes of a dilated pupil include trauma, Adie’s tonic pupil (usually due to idiopathic nerve degeneration), or being exposed to chemicals or drops that cause pupil dilation (drops for allergy or redness, motion sickness patches or chemicals found in plant or soil).”

The symptom duo of a dilated pupil with a tingling cheek, and no other symptoms, is uncommon, and in fact, may also have two unrelated causes.

For example, the tingling cheek could be related to an undiagnosed TMJ disorder or a compression of the trigeminal nerve by blood vessels.

So if you’re experiencing both a dilated pupil and a tingling cheek that seem to have started up at about the same time, consider the possibility of two unrelated causes.

There’s also a possibility that what you perceive as a new-onset dilation of one pupil is actually a harmless phenomenon called physiologic anisocoria – in which one pupil is just a tiny bit larger than the other, barely enough to be noticed by the patient, but not enough to be noticed by someone facing them.

The condition can affect both pupils on a transient and random basis and is normal.

Dr. Kulkarni has a special interest in optic nerve regeneration.