Neuro-Ophthalmology · Patient Guide
One pupil bigger
than the other?
Light decides what it means.
Unequal pupils — anisocoria — are usually a harmless quirk of anatomy. Occasionally they are the first visible sign of a carotid dissection or a brain aneurysm. The difference between the two is found with nothing more sophisticated than a light switch. Here is how clinicians read it.
simple anisocoria
difference
that size a pupil
Section 01 — Two Muscles, Two Nerves
A pupil is a balance
between two forces.
Anisocoria is an imbalance.
The pupil is not a structure in itself — it is the hole in the middle of the iris. Its size is set moment to moment by two opposing muscles inside the iris, each wired to a different branch of the autonomic nervous system. When one pupil is larger than the other, one of those two systems is usually doing less on one side.
The sphincter pupillae, a ring of muscle around the pupil edge, makes the pupil smaller. It is driven by the parasympathetic system, whose fibres travel with the third cranial (oculomotor) nerve and relay in the ciliary ganglion behind the eye. The dilator pupillae, radial fibres running outward like spokes, makes the pupil larger. It is driven by the sympathetic system, through a long three-neuron chain that descends from the hypothalamus into the upper chest and climbs back up the neck along the carotid artery.
That anatomy is the whole diagnostic key. A parasympathetic failure leaves a pupil that cannot constrict properly — it is the bigger pupil that is abnormal, and the gap widens in bright light. A sympathetic failure leaves a pupil that cannot dilate properly — the smaller pupil is abnormal, and the gap widens in the dark.
Anisocoria means the pupils differ in size. Around one in five healthy people show a small difference, usually under 1 mm, that stays roughly the same in light and dark — physiological anisocoria, which needs no treatment. To find which pupil is abnormal, compare the difference in bright and dim light. Bigger in the dark means the small pupil cannot dilate: think Horner syndrome. Bigger in the light means the large pupil cannot constrict: think third nerve palsy, Adie's tonic pupil, or a drug or eye drop. A new unequal pupil with a droopy lid, double vision, headache or neck pain is an emergency.
Section 02 — Interactive
Turn the lights down.
Watch which pupil lags.
Pick a cause, then move the room-light slider from dark to bright. The patient's right eye (on your left) carries the abnormality. Notice how the size difference grows in the dark for Horner syndrome and in bright light for every parasympathetic cause — and stays roughly fixed for physiological anisocoria.
Section 03 — Reading the Pattern
The four questions
that sort almost every case.
An eye examination for anisocoria is short and structured. Is the difference bigger in light or dark? Do both pupils react to light? Is the upper lid drooping, and on which side? Do the eyes move together normally? The answers usually narrow the cause before any test drop is used.
| Cause | Abnormal pupil | Difference larger in | Lid & eye movements | Urgency |
|---|---|---|---|---|
| Physiological | Neither | Neither — roughly constant | Normal | None if typical |
| Horner syndrome | Smaller | Dark | Mild ptosis same side; movements normal | Urgent if new or painful |
| Third nerve palsy | Larger | Light | Marked ptosis; eye down and out; double vision | Emergency |
| Adie's tonic pupil | Larger (smaller over years) | Light | Normal; near reaction slow and sustained | Routine |
| Pharmacological dilation | Larger, often very large | Light | Normal | Low once confirmed |
| Iris damage (trauma, surgery, inflammation, angle closure) | Usually larger or irregular | Variable | Normal; eye may be red or painful | Depends on cause |
The last row matters more often than people expect. Blunt injury can tear the sphincter muscle and leave a permanently larger pupil. Inflammation inside the eye (uveitis) can stick the iris to the lens and distort the pupil. An acute angle-closure attack typically leaves a mid-dilated, poorly reactive pupil in a red, painful eye with blurred vision — an eye emergency in its own right rather than a nerve problem. And after cataract or other intraocular surgery, a pupil that remains a little different in size or shape is common, from iris handling, dilating drops wearing off unevenly, or intraoperative miotics; your surgeon will usually have documented it.
Section 04 — Horner Syndrome
A small pupil
with a long pathway
behind it.
Horner syndrome is the classic triad of a smaller pupil (miosis), a mild upper-lid droop (ptosis, typically only 1–2 mm because the lid's sympathetic Müller muscle is small), and sometimes reduced facial sweating (anhidrosis). The signs are subtle. Their importance comes from where the pathway travels.
The first-order neuron runs from the hypothalamus through the brainstem to the upper spinal cord. The second-order neuron exits into the chest, arching over the apex of the lung. The third-order neuron climbs the neck wrapped around the internal carotid artery and enters the skull to reach the eye. A lesion anywhere along that route — a brainstem stroke, a tumour at the lung apex, a neck injury, or a tear in the carotid artery wall — can produce the same small pupil.
Confirming it with eye drops
When the signs are equivocal, pharmacological tests help. The traditional test uses topical cocaine, which blocks noradrenaline reuptake: a normal pupil dilates, a Horner pupil dilates poorly, and Kardon and colleagues showed in 1990 that a post-drop difference of about 0.8 mm or more makes Horner syndrome very likely. Because cocaine is a controlled substance and hard to stock, many clinics now use apraclonidine 0.5% or 1%. A sympathetically denervated dilator becomes supersensitive, so apraclonidine dilates the Horner pupil and lifts the lid while barely affecting the normal side — a reversal of the anisocoria. Koc and colleagues reported good sensitivity and specificity for this test, with the caveat that it may be falsely negative in the first days after onset, before supersensitivity develops.
Section 05 — The Large Pupil
Aneurysm, tonic pupil,
or a patch you touched?
When the larger pupil is the abnormal one, three very different causes have to be separated — and the first of them is one of the true emergencies in eye care.
Third nerve palsy
The parasympathetic pupil fibres run on the outer surface of the third cranial nerve, which is why compression from outside — classically an aneurysm of the posterior communicating artery — tends to affect the pupil early. A pupil-involving third nerve palsy usually comes with a droopy lid and an eye that rests down and out, causing double vision, often with headache. It needs emergency vascular imaging. By contrast, third nerve palsies from small-vessel ischaemia in people with diabetes or hypertension more often spare the pupil, although no rule is absolute and clinicians image cautiously.
Adie's tonic pupil
Damage to the ciliary ganglion, often presumed post-viral, leaves a larger pupil that reacts poorly to light but constricts slowly and holds that constriction when the person focuses up close (light-near dissociation). Thompson's 1977 observations described the segmental, worm-like iris movements seen at the slit lamp and the supersensitivity of the pupil to dilute pilocarpine (around 0.1%) that weakly affects a normal pupil. It is most common in women aged 20 to 50, generally benign, and over years the pupil often becomes smaller.
The pharmacological pupil
A very large, round pupil that barely reacts, in an otherwise well person with a normal lid and normal eye movements, is frequently pharmacological. Common culprits include scopolamine motion-sickness patches (touch the patch, then rub the eye), nebulised ipratropium leaking around a mask, some garden plants such as Datura and Brugmansia, and accidental exposure to dilating drops. The bedside clue is 1% pilocarpine: a pupil blocked at the receptor by an anticholinergic drug fails to constrict, whereas a third-nerve or tonic pupil generally does constrict. The effect wears off over hours to days.
Section 06 — The Numbers
How common is
harmless anisocoria?
The most cited data on normal pupils come from Lam, Thompson and Corbett, who photographed pupils of healthy volunteers repeatedly in dim light. Around a fifth showed a difference of 0.4 mm or more at any single sitting, and the side and size of the difference could vary from day to day. A small asymmetry, on its own, is ordinary.
≥0.4 mm difference at one visit
typical physiological anisocoria
sympathetic pupil pathway
What converts an ordinary asymmetry into a clinical concern is not the size alone but the company it keeps: a new onset, a lid droop, double vision, pain, a pupil that does not react, or a difference that changes markedly between light and dark. Reviewing old photographs — a driving licence, a passport, a phone gallery — is one of the simplest and most useful steps, because an asymmetry visible years ago is very unlikely to represent a new dangerous lesion.
Section 07 — When to Act
Red flags that turn
a curiosity into
an emergency.
A 36-year-old notices one pupil looks larger after a long-haul flight. The lid is normal, there is no double vision and no headache. On examination the difference is 2.5 mm in bright light and almost nothing in the dark, and the large pupil does not constrict to 1% pilocarpine. A scopolamine patch had been worn for the flight and removed by hand. The pattern — isolated, large, light-predominant, pilocarpine-resistant, normal lid and motility — is characteristic of pharmacological blockade and resolves over a few days.
Section 08 — FAQ
Frequently asked questions
about unequal pupils.
Often, yes. Population studies have found that roughly one in five healthy people have a measurable pupil size difference of 0.4 mm or more at any given moment, with no disease behind it. This physiological (simple) anisocoria is usually under about 1 mm, keeps a similar size difference in bright and dim light, both pupils react briskly, and there is no droopy eyelid, double vision or pain. Old photographs showing the same difference years ago are reassuring.
Seek same-day emergency care if new unequal pupils come with a droopy eyelid, double vision, an eye that will not move normally, a severe or sudden headache, neck or face pain, recent head or neck injury, confusion, weakness, or a red painful eye with blurred vision. A large pupil with a droopy lid and double vision can signal a third nerve palsy from a brain aneurysm, and a small pupil with a droopy lid and neck pain can signal a carotid artery dissection. Both need urgent imaging.
They compare the size difference in bright light and in dim light. If the difference is larger in the dark, the smaller pupil is failing to dilate, which points to a sympathetic problem such as Horner syndrome. If the difference is larger in bright light, the bigger pupil is failing to constrict, which points to a parasympathetic problem such as a third nerve palsy, Adie's tonic pupil, or dilating eye drops or medication.
Horner syndrome is damage anywhere along the three-neuron sympathetic pathway that runs from the brain, down into the chest, and back up the neck alongside the carotid artery to the eye. It causes a smaller pupil that dilates slowly in the dark, a mild droop of the upper eyelid, and sometimes reduced sweating on that side of the face. Because the pathway passes the lung apex and the carotid artery, a new Horner syndrome needs investigation for causes such as carotid dissection or a tumour at the top of the lung.
Adie's tonic pupil is a usually benign condition, most common in young and middle-aged women, in which the parasympathetic nerve supply to one pupil is damaged, often after a presumed viral insult to the ciliary ganglion. The affected pupil is larger, reacts poorly to light, constricts slowly and tonically when focusing up close, and often becomes supersensitive to very dilute pilocarpine. It may be accompanied by reduced tendon reflexes, which is called Holmes-Adie syndrome.
Yes, and it is one of the most common causes of a sudden, isolated large pupil. Anticholinergic exposure such as a scopolamine motion-sickness patch touched and then rubbed into the eye, nebulised ipratropium leaking around a mask, some plant material, or dilating drops can produce a widely dilated pupil that barely reacts. A key clue is that it does not constrict with 1% pilocarpine, whereas a pupil dilated by a third nerve palsy usually does. The eyelid and eye movements are normal.
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