Noticing it is one pupil is the whole point
Anisocoria is, by definition, a difference between the two sides, so the very fact that you are comparing one pupil against the other is what makes it meaningful. The useful questions are whether the difference is new, which of the two pupils is the abnormal one, and whether anything else has changed on that side. The rest of this page works through each of those in turn.
How the size of each pupil is controlled
The pupil is the round opening at the centre of the coloured iris, and it controls how much light enters the eye. Its size is set by two muscles working against each other. A ring of muscle constricts the pupil, making it smaller, under the control of the parasympathetic nerves. A second set of fibres dilates the pupil, making it larger, under the control of the sympathetic nerves. In normal circumstances both eyes receive the same instructions at the same moment, so the pupils stay equal and move together, constricting in bright light and dilating in the dark.
Anisocoria simply means the two pupils are a different size. Because size depends on this balance between constriction and dilation, a difference can arise either from a pupil that will not constrict properly, or from one that will not dilate properly. Which of the two it is turns out to be the key to understanding the cause.
A pupil difference is easier to see in people with lighter-coloured irises, whether blue, green, or hazel, where the dark pupil stands out clearly against the iris. In very dark brown eyes the edge of the pupil is harder to make out, so a genuine difference is sometimes missed, or only noticed later in a close-up photograph.
Physiological anisocoria, and why pupils are still treated with respect
Around one in five people has a small, natural difference in pupil size with no underlying problem at all. This is called physiological anisocoria. It has a recognisable pattern: the difference is usually small, often less than a millimetre, and stays about the same in bright and in dim light. Both pupils still react briskly, and there is nothing else wrong, no drooping eyelid, no double vision, no pain, and no change in vision. It may even vary slightly from day to day or seem to swap sides. If you have had a small, steady difference for as long as you can remember, this is much the most likely explanation.
It is worth being honest, though, about why pupils are treated so carefully in an eye clinic. A difference in pupil size is one of the few signs a person can notice in themselves that can, in a minority of cases, point to a serious problem, including some that are urgent or even life-threatening. So while physiological anisocoria is common and harmless, a pupil difference that is new, that is changing, or that comes with any other symptom should never simply be assumed to be the harmless kind. It deserves to be checked.
Working out which pupil is the odd one
When a difference does have a cause, the most useful question is which pupil is behaving abnormally, and there is a simple way to judge it. Compare the two pupils in bright light, and then again in a dim room.
If the difference grows in the dark, it is the smaller pupil that is failing to dilate as it should. If the difference grows in bright light, it is the larger pupil that is failing to constrict. This single observation separates the causes into two groups, and it is exactly the assessment made in clinic.
Causes that need looking into
When the smaller pupil is the abnormal one, and the difference is greater in dim light, the sympathetic nerve supply that dilates the pupil is not working on that side. The best known cause is Horner's syndrome, in which that nerve supply is interrupted somewhere along its path, producing a slightly smaller pupil and a mildly drooping upper lid on the same side. Horner's syndrome is sometimes harmless, but the sympathetic pathway travels a long way, from the brain down into the chest and back up through the neck, so a new Horner's syndrome should always be investigated promptly to find out where and why it has arisen. A cluster headache can also produce a smaller pupil and a drooping lid on one side during an attack, alongside severe one-sided head pain and a red, watering eye.
When the larger pupil is the abnormal one, and the difference is greater in bright light, that pupil is failing to constrict. A palsy of the third cranial nerve, which drives most of the eye's movements and the muscle that constricts the pupil, can leave the pupil enlarged and sluggish. It usually comes with a drooping eyelid and an eye that sits turned down and outwards, causing double vision. A third nerve palsy that involves the pupil, particularly with pain, is a medical emergency, because one cause is an aneurysm, a swelling of a blood vessel pressing on the nerve, which can be life-threatening and must be excluded urgently.
Not every enlarged pupil is sinister. An Adie's tonic pupil, most common in younger adults, is an enlarged pupil that reacts slowly and poorly to light and is usually benign. A pupil can also be dilated simply by contact with a substance, such as certain plants, some medicated patches, nebulised medicines, or eye drops, which is harmless once recognised and settles as it wears off. Migraine can occasionally cause a temporary dilation of one pupil during or around an attack, which then returns to normal.
There are also causes within the eye itself. A blow or injury to the eye can damage the small muscle that constricts the pupil, leaving it larger and often slightly irregular in shape. Inflammation inside the eye, called uveitis, and some previous eye operations can leave adhesions, known as posterior synechiae, that tether the iris and pull the pupil into an uneven or irregular shape. An eye that is also red, painful, or seeing poorly points towards a cause of this kind, and should be examined.
- A drooping eyelid, double vision, or an eye that will not move normally, especially with pain or a severe headache
- A red, painful eye with blurred vision, haloes around lights, or nausea
- Sudden weakness, numbness, difficulty speaking, a facial droop, or a severe sudden headache
- A recent injury to the eye or head
Look back through old photographs
One genuinely useful thing you can do is look through old photographs of yourself. Pupils are often visible in close-up pictures. If the same small difference can be seen in photographs from months or years ago, it is very likely the long-standing, harmless kind. If your pupils looked equal until recently, the difference is new, and that is the situation that needs assessment rather than reassurance. Old photos help answer the single most important question, which is whether this is new, but they are a clue and not a substitute for being examined if anything else has changed.
New, changing, or accompanied means get it checked
A small pupil difference that has been present for years, stays steady, and comes with no other symptoms is very rarely a problem, and needs no more than a mention at a routine eye examination. Everything else deserves attention. A pupil difference that is genuinely new, that is getting larger, or that arrives with any other symptom, a drooping eyelid, double vision, eye pain, a severe headache, a red eye, or reduced vision, should be assessed without delay, and the combinations listed above should be treated as an emergency.
Because a pupil difference is occasionally the first outward sign of a serious problem, it is one of the few eye findings where it is genuinely better to be seen and reassured than to wait. If you cannot be sure whether the difference is old or new, treat it as new.
Common questions
Is it normal to have one pupil bigger than the other?
It can be. Around one in five people has a small, natural difference called physiological anisocoria, which is harmless. The reassuring pattern is a small difference that has been present for a long time, stays the same in bright and dim light, and comes with no other symptoms. This only applies when the difference is long-standing and isolated. A new or changing difference, or one with any other symptom, should always be checked, because a pupil difference is one of the few eye signs that can occasionally be serious.
How do I know if uneven pupils are serious?
The key questions are whether the difference is new, and whether anything else has changed. Long-standing and completely isolated is usually harmless. New, changing, or accompanied by a drooping eyelid, double vision, pain, headache, or a red eye should be assessed promptly, and some combinations are emergencies. When in doubt, be examined.
Can migraine or headaches cause uneven pupils?
Yes, occasionally. Migraine can cause a temporary dilation of one pupil around an attack that then settles. A cluster headache can cause a smaller pupil and a drooping lid on one side during an attack, with severe one-sided head pain and a red, watering eye. A pupil change that does not settle, or that comes without a recognised headache pattern, should still be checked.
One of my pupils suddenly became much bigger. What should I do?
A sudden, clear change deserves prompt attention, and should be treated as an emergency if it comes with a drooping eyelid, double vision, eye pain, or a severe headache, or if the eye is red and painful with blurred vision or haloes. A substance or eye drop reaching the eye is a common and harmless cause, but a genuinely new, sudden change should still be confirmed rather than assumed to be harmless.
