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Anisocoria
Not the size decides but the course: unchanged since always is usually normal — newly appeared belongs in assessment.
Also known as: pupil size difference · one pupil larger than the other
Open in the tool — with search and all the links →This compilation is for information and replaces neither medical diagnosis nor treatment. If you have symptoms, see a doctor.
Anisocoria means only one thing: the two pupils are of different size. That is not a diagnosis but an observation — and in 10 to 20 per cent of people it is simply part of the body. This simple anisocoria usually amounts to about 0.4 millimetres of difference, seldom more than 0.8, and it stays the same over the years. Often it is noticed only after decades, in a photograph.
In older age harmless reasons are added that make an existing difference more visible. After cataract surgery the pupil sometimes stays slightly irregular or slower. A past inflammation of the iris can leave fine adhesions holding it at one point. Glaucoma drops narrow the pupil; spray from an inhaler that reaches the eye widens it for hours or days. And pupils become smaller and slower with the years anyway — so a difference shows up more than it used to.
What matters is not the size but the course. A difference someone has always had and that stays the same is almost always a normal variant. A difference that appears suddenly — especially together with a drooping lid, double vision, headache or neck pain — belongs in immediate assessment. Old photographs are astonishingly useful here: they show whether it was always there. If a newly drooping lid or double vision is joined by a wide pupil that barely reacts to light, that speaks more for a cause pressing from outside — the fibres for the pupil run on the outside of the nerve and are affected first.
The simplest test there is. A picture from ten years ago shows whether the difference is new — and that decides more than any measurement.
If the difference grows in darkness it is the smaller pupil — it dilates poorly. If it grows in the light it is the larger one — it constricts poorly. That is the decisive fork, and it costs nothing.
Is a lid drooping, and if so on which side? On the side of the smaller pupil it points elsewhere than on the side of the larger one.
Go through the eye drops and inhalers before searching further. Often that is exactly the answer.
Appeared suddenly, together with a drooping lid, double vision, severe headache or neck pain: get it assessed at once, do not wait for the next appointment.
A difference noticed late has not therefore newly arisen. In older people especially it is often seen only on closer looking or in a photograph — not because something changed but because one is now looking. A lifelong difference is not a missed warning sign.
Simple anisocoria, in 10 to 20 per cent of people. The difference stays about the same in light and in darkness, both pupils react normally to light. No disease value, no treatment needed.
After cataract surgery the pupil can stay slightly irregular or slower; a past iris inflammation sometimes leaves fine adhesions. At the slit lamp this is seen at once.
Glaucoma drops narrow the pupil. Inhaler spray reaching the eye, a scopolamine patch or certain plant saps on the fingers widen it for hours or days. This cause is very often missed.
Horner syndrome. The difference shows up more in darkness, because the smaller pupil dilates less well. Newly appeared and with neck or head pain it belongs in immediate assessment.
A third cranial nerve palsy. Appearing suddenly this is an emergency and belongs in assessment without detour.
Adie’s pupil, mostly in younger women. It barely reacts to light but slowly and with delay on near gaze. Benign — but it often persists.
AWMF 030-137 · S1 · 2021-11 — in force
Augenmotilitätsstörung inklusive Nystagmus
checked 2026-09 · last looked for a successor 2026-09-20