Pupil Problems

Anisocoria- unequal pupil sizes

Anisocoria – unequal pupils

Your pupils and pupil reactions are an important part of the eye examination.

Pupils should be round, equally sized and equally reactive to light, but often they are not- it is important to be able to differentiate between the pupils that are of concern with those that are “normal for you”.


Firstly pupils should be round- if the pupil is distorted, there may be abnormal adhesions between the iris and the lens behind, which can occur if there is (or more likely has been) anterior uveitis (iritis). There should be a clear history of a condition such as this. Trauma- a blow to the eye can cause damage to the iris causing tearing of the sphincter muscle which causes the pupil to constrict. It will still function, but its effect may be uneven, and therefore the pupil misshapen. Most often, unless a problem is known about, an unusually shaped pupil will be normal for you – Holmes Adie pupil can often show sectoral defects in the sphincter muscle, causing an irregular pupil shape, for example, but close examination is required to rule out anything serious.

Unequal pupil sizes (anisocoria) are relatively common- approximately 1 in 10 people do have a slight inequality. This is called physiological anisocoria, and is normal so long as the difference remains equal under a dull and bright light. If the difference in size increases under a dull or no light, then one pupil is not dilating properly. Conversely if the difference increases in a bright light, one pupil is not constricting properly. Most times these differences can be explained in practice, and do not require onward referral for investigation, but some, particularly if associated with eye movement (motility) weaknesses or a droopy eyelid, do need to be referred.

Pupil reactions are viewed separately to pupil size.

Because of the way the pupils are “wired up”, shining a light in one eye should make both pupils constrict, so there is a direct reaction in one and an indirect reaction in the eye which isn’t being illuminated. By checking these reactions it is possible to ensure that the strength of signal from each eye is equal. An extreme example of this would be where one eye is blind- both pupils constrict normally when the light is shone into the good eye, both will dilate when illuminating the blind eye. If there was a weakness but not failure, when the light is shone into the poorer eye, there will be a constriction, but moving the light to the other eye will show a greater constriction. Moving back to the original will show a relative dilation of both. This Relative Afferent Pupillary Defect, indicates a weakness in the retina or optic nerve in the affected (dilating) side.


I once had a young girl in who had no idea why her parents had booked an eye test – she was having no problems, as far as she was aware, and her father had not come in with her. It later turned out that she had been booked in because her left eye seemed to wander on occasions, but I was not aware of this at the time.

During initial History and Symptoms, I noticed that one eye “drifted” slightly. When questioned what she could see with the right eye, she read the bottom line on the chart (6/5), the other eye she said “nothing”.
“Oh come on Princess” (I thought)! – So many children these days try to fabricate an issue because they fancy a pair of spectacles, or a best friend just got a pair!
“Nothing” she insisted, and then she said she could see a little to the left, but I had already noticed that she was “cheating” – by pulling the cover away from her Right eye, she was able to see a little to the left, but from the right eye!! (She didn’t do that on purpose though).
Pupil reactions pretty much confirmed that the Left eye was unable to see – both constricted when the light was shone in the right eye, but both fully dilated when the light was shone in the left! 

After various other tests, including an examination of the back of her eyes, this child was referred urgently to her GP for onward hospital assessment, and I mentioned to the to the GP after his lunch, that “If this was my child I would be in Dublin already.”
She attended Temple Street Children’s Hospital next morning, was referred to Beaumont an hour or so later, and had brain surgery the same day, diagnosed with a raised intra-cranial pressure (also called “water on the brain”).

Their reckoning was that “yes – her left eye is blind, but she is not going to die, and we saved her right eye”!!


Differences in dilation and constriction will also be evident when doing this part of the test, so although size and reactions are different tests, they can be checked out at the same time.

It is also possible to check the pupils for constriction when looking at a near target- when the eyes attempt to focus at something close, they will also turn in, and the pupils should constrict. It is usually not necessary to check the near reflexes, unless there are problems with the light reflexes, as there are no diseases which will cause a near failure without a light reflex problem. Conversely, in light/near dissociation the near reflexes continue to work, but the light reflex is affected in some way.

It is amazing that most people who have uneven pupils have never noticed it- I saw a lady recently who was unaware, but when it was pointed out, she couldn’t believe when looking in the mirror that she hadn’t seen it before. If uneven pupils are noticed, it should be mentioned to you- I have seen people coming in for urgent tests, fearing the worst, because they have just noticed that their pupils are uneven, yet it was written on the previous test card but not pointed out to the individual.

What is even more amazing, to me at least, is that the number one page on this website in terms of views is this one – parents Googling why their “child’s pupils are uneven”, or “one pupil is not constricting properly”, etc etc – if you think your child has a problem, don’t Google it, go to an Optometrist, get them checked – it could save their life!?

Most causes of uneven pupils are innocuous, but anyone who has an issue such as this should have themselves checked out by their preferred eye care provider.

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