The pupillary light reflex makes both pupils constrict when light enters one eye. The signal travels through the optic nerve to the pretectal nuclei in the midbrain, then through the Edinger-Westphal nuclei and parasympathetic fibres in the oculomotor nerve to the sphincter pupillae via the ciliary ganglion. Because of crossing connections, the lit eye constricts (direct response) and so does the other eye (consensual response).
| Nerve root | CN II (afferent), CN III (efferent) |
|---|---|
| Area | Pupil, with light shone into each eye in turn |
| How to test | In dim light, the examiner shines a bright light into one eye and watches both pupils constrict, then swings the light between the eyes to compare responses. |
| Muscles | |
| Nerve Supply | Optic nerve (CN II); Oculomotor nerve (CN III, parasympathetic fibres); Short ciliary nerves |
Controls how much light reaches the retina, protecting it from glare and improving focus.
The swinging flashlight test detects a relative afferent pupillary defect, a sign of optic nerve disease such as optic neuritis. A dilated pupil that does not react to light in a drowsy patient suggests oculomotor nerve compression from brain herniation or a posterior communicating artery aneurysm and is an emergency. Pupils that react to near focus but not to light (light-near dissociation) occur in Argyll Robertson pupils and Adie tonic pupil.
Damage to one optic nerve or severe retinal disease makes both pupils dilate when the light swings to the affected eye.
Compression of CN III, as from an aneurysm or uncal herniation, causes a dilated pupil that does not constrict to light, often with ptosis and a down-and-out eye.
Damage to the ciliary ganglion produces a dilated pupil with a poor light reaction and slow constriction to near focus.
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