One pupil bigger than the other: when it matters
Pupils that have always been slightly different sizes are common and usually mean nothing. Pupils that have become different are a different conversation. Here is how I tell the two apart in clinic, and how you can start before you get there.
- A small, steady difference in pupil size that has always been there, with nothing else wrong, is usually just how that child is made.
- The word that changes everything is new. Unequal pupils that have appeared recently are seen urgently rather than at the next free slot.
- An unequal pupil together with a lid on the same side that has started to droop is the combination that eye services put in their highest tier.
- Old photographs are the single most useful thing you can bring, because they answer the only question I really need answered.
- After a knock to the head or the eye, an unequal pupil, a pupil that does not react or blood in front of the coloured part of the eye is a same day emergency.
Almost every week a parent shows me a photograph on a phone, zoomed in on one small face, and asks the same thing. Is that pupil bigger than the other one, or am I imagining it? Usually they are not imagining it. Usually they noticed it in a photograph rather than in the room, because a camera flash catches both eyes at the same instant and your own eyes never quite do.
Unequal pupils have a name, anisocoria, which means nothing more than the two black circles are different sizes. It is a description, not a diagnosis. And the question I ask next is never how big the difference is. It is how long it has been there.
One of my child's pupils is bigger than the other, is that dangerous?
Usually not. A small, steady difference in pupil size, in a child whose eyes are white and comfortable, whose lids sit level and who sees normally with each eye, is common and is usually just how that child is made. It does not blur vision, and it tends to stay exactly as it is. Some people carry it their whole lives without anyone mentioning it.
What decides everything is whether it is new. That single word separates a routine eye clinic booking from an urgent one, and it is not my opinion, it is how children's eye services sort these referrals. Long standing and isolated goes into the ordinary queue. Recent onset goes to an acute eye clinic. Same sign, same child, two completely different appointments, decided by a date.
So please do not spend the evening comparing millimetres. Spend it comparing photographs.
It has always been like that, does it still need checking?
Yes, once. Not tonight, not this week, but at an ordinary appointment, because "it has always been like that" deserves to be confirmed rather than assumed. What I am checking is short and undramatic. Do both pupils get smaller when I shine a light, and does each one react when the light goes into the other eye? Is the sight equal in the two eyes? Do the lids sit at the same height? Do the eyes move fully in every direction? Are the two irises the same colour?
That last one surprises parents. Two eyes of genuinely different colours in a child is worth pointing out to the doctor rather than admiring, because it belongs to the same short list of things we look at alongside a pupil difference. It is often nothing. It is still worth saying.
When that examination is normal and the difference has been in every photograph since your child was two, you have a baseline and you can stop wondering. That is a real outcome and I count it as a good visit. If you are not sure where such an appointment happens, the different people who check children's eyes in Malaysia do not all do the same job, and this one needs a doctor's look rather than a sight test.
One pupil is bigger and that eyelid looks droopy, what does that mean?
It means the appointment moves to this week, and it means saying both signs on the phone rather than only the one that bothers you more. A recent droopy lid, a difference in pupil size, and a child younger than five: eye services put that combination in their highest tier and send it straight to a hospital eye clinic. Those two signs together do not point at the eye. They point at a nerve pathway that supplies both the pupil and part of the eyelid, and that pathway does not run only through the head. It loops down through the neck and the very top of the chest before it comes back up.
Which is why nobody examines just the eye. The examination looks at the child.
I want to be careful here, because this is the paragraph where a frightened parent starts searching. The reason these referrals move quickly is not that a serious cause is likely. It is that in a small number of children something is found that is very much better found early, and the child usually looks entirely well at the time. Being seen this week is the whole intervention. It is not a reason to sit up all night. A droopy lid on its own, without a pupil difference, is a slower and much more familiar story, and a lid that has drooped since birth is a different article altogether.
Could something in the house have done this?
Quite possibly, and it is one of the first questions I ask. A number of ordinary household things can widen one pupil for a day or two if they reach an eye, usually through a hand.
- Eye drops belonging to someone else in the house, including a grandparent's glaucoma or dilating drops.
- A motion sickness patch worn behind an adult's ear, touched and then transferred by little fingers.
- An inhaler or a nebuliser mask used near a small face, where some of the mist reaches the eye.
- Certain garden and houseplants, handled and then rubbed into an eye.
None of this is anyone's fault, and I have never once thought less of a parent who worked it out in the waiting room. Tell the doctor about every drop, patch, inhaler and plant in the house before anything else is done. It costs one sentence and it can change the entire shape of the visit. This is also the reason the same small habits that keep little eyes safe at home include keeping other people's medicines out of reach, not just the sharp and hot things.
She banged her head and now the pupils look different, what do I do?
Go today, to an emergency department, and do not wait for an eye appointment. This is the one version of unequal pupils where the timescale is hours.
After a knock to the head or to the eye, the signs that need to be seen the same day are a pupil that has become unequal, a pupil that does not get smaller in bright light, a pupil that has changed shape, or blood visible in front of the coloured part of the eye. Alongside those, take just as seriously any vomiting, a bad headache, drowsiness or confusion, an eye that cannot move properly or cannot be kept open, severe eye pain, and any change in sight at all. If your child has become newly sensitive to light after the bump, that counts too.
You do not need to decide which of those is the important one. You need to describe them, in order, to someone who can look tonight.
What should I bring to the appointment?
Photographs, and more of them than you think. Take one in ordinary room light and one in a dim room, because pupils change size as the light changes and how the difference behaves between the two is genuinely part of the answer. It is almost impossible to describe in words and it takes ten seconds to capture.
Then go backwards. Scroll through your camera roll for the same face at four years old, at two, as a baby, and look at the pupils. The birthday party photo, the passport photo, the one from Raya where everyone is squinting. Old photographs answer the only question I really need answered, and they answer it better than any of our memories do. Parents have shown me a picture from three years ago that settled the whole thing before I had finished washing my hands.
Bring the list of what is in the house too, and mention anything else you have noticed even if it seems unrelated: an eye that turns, a head that tilts, one eye being screwed up in bright sun, a complaint about the whiteboard. And if you are worried the wait will be long, asking for a referral in the public system works better when you name the two signs together and say when they started, in that order.
Most of the time this ends quietly. A normal examination, a difference that has been there since babyhood, a parent who can put the phone down. I would rather look at fifty untroubled pupils than miss the one that changed last week.
- The difference in pupil size is new, or you cannot find it in any older photograph.
- The eyelid on the same side has started to droop, at any age.
- It has appeared after a knock to the head or a knock to the eye, especially with vomiting, a bad headache, drowsiness or any change in sight.
- The pupil has changed shape, does not get smaller in bright light, or there is blood visible in front of the coloured part of the eye.
- There is eye pain, a very red eye, double vision or blurred vision alongside it.
Common questions
One of my son's pupils is bigger than the other, is that dangerous?
It has always been like that, does it still need checking?
One pupil is bigger and that eyelid looks a bit droopy, what does that mean?
She banged her head yesterday and now her pupils look different, what do I do?
Could my asthma inhaler or my eye drops have done this?
Does he need a brain scan?
Should I measure the difference myself at home?
Can unequal pupils affect her sight?
- Moorfields Eye Hospital · Anisocoria (unequal pupils), children's eye conditions management · www.moorfields.nhs.uk
- Moorfields Eye Hospital · Ptosis (droopy eyelid), children's eye conditions management · www.moorfields.nhs.uk
- Moorfields Eye Hospital · Eye trauma, children's eye conditions management · www.moorfields.nhs.uk
- NHS · Eye injuries · www.nhs.uk
- Fierz FC, Gerth-Kahlert C · Long-term follow-up in children with anisocoria, Journal of Ophthalmology 2017 · pmc.ncbi.nlm.nih.gov