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.

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 writtten on the previous test card but not pointed out to the individual.

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

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