Pupillary Pathways
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Direct answer
Light striking either retina triggers a reflex arc that runs through photoreceptors and retinal ganglion cells, the optic nerve, chiasma and tract, and then — before the lateral geniculate body — leaves through the brachium of the superior colliculus to the pretectal nucleus of the midbrain. Each pretectal nucleus projects to both Edinger–Westphal nuclei (crossing in the posterior commissure), so both pupils constrict to a unilateral light; parasympathetic output then travels with the oculomotor nerve to the ciliary ganglion and by the short ciliary nerves to the sphincter pupillae. The dilator pupillae is sympathetic: hypothalamus to ciliospinal centre (T1–T2) to superior cervical ganglion, then along the internal carotid and long ciliary nerves.
What you must remember
- Afferent limb: retina → optic nerve → chiasma (nasal fibres decussate) → optic tract → pretectal nucleus — note the fibres leave the tract before the lateral geniculate body.
- Central connections: each pretectal nucleus innervates both Edinger–Westphal nuclei; this bilateral projection explains the consensual response.
- Efferent limb: Edinger–Westphal nucleus → oculomotor nerve → ciliary ganglion in the orbit → short ciliary nerves → sphincter pupillae (constriction) and ciliary muscle (accommodation).
- Relative afferent pupillary defect: swinging torch test — both pupils dilate when the light swings to the diseased eye (optic neuritis, severe retinal disease); the Marcus Gunn pupil.
- Efferent defect: third nerve lesion — the ipsilateral pupil is dilated and fixed to light and near effort; with ptosis and impaired adduction the triad is complete.
- Light-near dissociation: pupils fixed to light but constricting on convergence — Argyll Robertson pupils (small, irregular, classically neurosyphilis) and dorsal midbrain lesions (Parinaud syndrome).
- Horner syndrome chain: hypothalamospinal tract → lateral medulla → ciliospinal centre of Budge (T1–T2) → preganglionic fibres ascending to the superior cervical ganglion → postganglionic fibres on the internal carotid, then through the cavernous sinus and long ciliary nerves to the dilator pupillae and Müller muscle.
- Pharmacological testing: cocaine drops fail to dilate a Horner pupil; apraclonidine 0.5–1% reverses the anisocoria; 0.1% pilocarpine constricts the denervation-supersensitive Adie pupil.
Working through anisocoria at the bedside
A nurse reports unequal pupils; three questions resolve the anatomy. First, which is abnormal — the small or the large? In dim light the difference grows if the small pupil is at fault (sympathetic failure); in bright light it grows if the large pupil cannot constrict (parasympathetic or third nerve failure). Second, is the large pupil light-fixed? If yes, examine eye movements and the lid: a complete third nerve palsy with ptosis and adduction failure plus a dilated pupil screams compression — posterior communicating aneurysm — and is a neurosurgical emergency; a blown pupil with normal movements in a young woman with normal reflexes suggests Adie's tonic pupil, which constricts sluggishly to near and to dilute pilocarpine. Third, test the small-pupil side for the rest of Horner syndrome — mild ptosis from Müller muscle, apparent enophthalmos, and facial anhidrosis distinguishing central, preganglionic and postganglionic lesions: anhidrosis over the whole face places the lesion before the superior cervical ganglion (carotid dissection, apical lung tumour), while face sparing with normal sweating localises after the ganglion, along the carotid. The swinging torch completes the screen — bilateral dilation on swinging to one eye marks an afferent lesion even when both pupils look equal at rest.
Where students slip
The recurrent written error is routing the light reflex through the lateral geniculate body or the visual cortex; the reflex fibres leave the optic tract proximal to the geniculate and reach the pretectum through the brachium of the superior colliculus — that is why cortical blindness spares the pupillary light reflex. The second slip is the decussation count: from each retina, the temporal fibres stay ipsilateral and the nasal fibres cross in the chiasma — mirror the visual pathway exactly. And the exam favourite "which lesion gives light-near dissociation?" needs both answers, Argyll Robertson and dorsal midbrain, with the sites — pretectal damage sparing the more ventral near-response fibres.
Frequently asked questions
Trace the pathway of the pupillary light reflex.
Retina → optic nerve → optic chiasma → optic tract → pretectal nucleus (via the brachium of the superior colliculus) → both Edinger–Westphal nuclei → oculomotor nerve → ciliary ganglion → short ciliary nerves → sphincter pupillae. The bilateral pretectal projection produces both direct and consensual constriction.
Why does optic neuritis spare the consensual response in the other eye?
An afferent defect reduces input from the affected eye, so light in that eye constricts both pupils weakly (swinging torch dilation), but light in the healthy eye constricts both pupils normally. The efferent limbs on both sides are intact.
What is an Argyll Robertson pupil?
A small, irregular pupil that accommodates for near but does not react to light — light-near dissociation — classically of neurosyphilis. The lesion is in the pretectal region interrupting light-reflex fibres while sparing the near-response pathway.
Which lesions cause Horner syndrome along its pathway?
Central: lateral medullary stroke or syringobulbia. Preganglionic: apical lung cancer (Pancoast), brachial plexus avulsion. Postganglionic: carotid artery dissection, cavernous sinus lesion. Features are miosis, mild ptosis, anhidrosis and apparent enophthalmos.
Why is a painful dilated fixed pupil an emergency?
It suggests compression of the third nerve, most critically by a posterior communicating artery aneurysm. The pupillomotor fibres run on the surface of the nerve, so they fail before somatic fibres — surgical clipping or coiling must follow urgently.