Pupils: light reflex and abnormal pupils (Marcus Gunn, Horner, Argyll Robertson) !

Sign

A check of pupil size and response to light, which localises lesions of the eye, the nerves and the brain.

Updated 2026-11-01

See it in 3D: Iris · Optic nerve · Oculomotor nerve (III) · Eyelids · Optic chiasm

What it is

The pupils reflect the balance between the parasympathetic system (cranial nerve III, constricts) and the sympathetic system (dilates). The light reflex enters through the optic nerve and leaves through nerve III, and light in one eye constricts both pupils. Testing them separates an input defect (optic nerve) from an output defect (nerve III or iris) and picks up urgent signs such as the dilated pupil of herniation.

The anatomy it tests

Afferent pathway: retina and optic nerve, to the midbrain.
Parasympathetic efferent pathway: Edinger–Westphal nucleus, nerve III and ciliary ganglion, which contract the iris sphincter.
Sympathetic pathway: hypothalamus, cervicothoracic cord, cervical sympathetic chain alongside the carotid, to the iris dilator and the Müller muscle.
The pupillary fibres run on the surface of nerve III, which is why compression affects them first.

How to elicit it

Dim the room light and ask the patient to look at a distant point.
Note pupil size, shape and symmetry, and the eyelids.
Shine a light into one eye from the side and watch the direct response and the consensual response of the other eye.
Swinging flashlight test: move the light from one eye to the other every 2 to 3 seconds and watch whether a pupil dilates as the light reaches it.
Accommodation: ask the patient to look far away and then at a finger about 15 centimetres away; the pupils constrict.
Record the size in millimetres; nursing records normal pupils as PERRLA.

Abnormal pupils

Pupil	Finding	Typical cause
Marcus Gunn (relative afferent pupillary defect)	When the light swings to the affected eye, both pupils dilate	Optic neuritis, severe retinal or optic nerve lesion
Horner	Small pupil, mild ptosis and less sweating on the same side; difference larger in the dark	Sympathetic pathway lesion: lung apex tumour, carotid dissection, brainstem stroke
Argyll Robertson	Small, irregular pupils; no reaction to light but reaction to accommodation	Neurosyphilis
Compressive third nerve	Dilated, unreactive pupil with ptosis and the eye down and out	Posterior communicating aneurysm, uncal herniation
Adie tonic	Dilated pupil, slow reaction to light, better to accommodation	Ciliary ganglion damage; young women
Bilateral pinpoint	Very small pupils	Opioids, pontine haemorrhage, organophosphates

What a positive result means

Afferent pupillary defect: a lesion of the optic nerve or retina of the eye that dilates.
Anisocoria greater in the dark (the small pupil fails to dilate): Horner.
Anisocoria greater in the light (the large pupil fails to constrict): third nerve palsy, tonic pupil or mydriatic drops.
In a patient with trauma or falling consciousness, a pupil that dilates is a sign of herniation and is reported at once.

Usefulness and limits

Mydriatic drops, some inhalers and other drugs change the pupils; always ask about them.
Examining with the room light on hides the anisocoria of Horner syndrome.
A pilocarpine drop test helps separate drug-induced from neurological mydriasis.

Test yourself

Think of the answer, then tap the question to see it.

What is seen in the swinging flashlight test with an afferent pupillary defect?

When the light swings to the affected eye, both pupils dilate instead of constricting, because that optic nerve carries less light.

What is the triad of Horner syndrome?

Miosis, mild ptosis and anhidrosis on the same side, from a lesion of the sympathetic pathway.

What is an Argyll Robertson pupil?

A small, irregular pupil that reacts to accommodation but not to light; it is characteristic of neurosyphilis.

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

← All of the clinical exam

Educational content on the clinical exam; it is learned and practised under supervision. It does not replace assessment by a health professional and is not for self-diagnosis.

Sources: Common Pupillary Abnormalities (MSD Manual Professional Edition, 2024); Anisocoria (MedlinePlus, U.S. National Library of Medicine, 2024); Horner syndrome (MedlinePlus, U.S. National Library of Medicine, 2024); 16.3 The Cranial Nerve Exam (OpenStax, Anatomy and Physiology 2e, 2022).

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