Pupillary Reflexes
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Direct answer
The pupillary light reflex constricts both pupils when light enters either eye, and it succeeds because the afferent signal splits at the pretectal nuclei to reach both Edinger-Westphal nuclei. Fibres leave each retina in the optic nerve, and just after the chiasma a contingent leaves the optic tract before the lateral geniculate body to synapse in the pretectal area; each pretectal nucleus projects to both Edinger-Westphal nuclei, whose parasympathetic outflow runs in the third nerve to the ciliary ganglion and the sphincter pupillae. This double crossing explains the direct and consensual responses, and its specific interruptions explain the relative afferent pupillary defect, the Argyll Robertson pupil and nerve III palsies.
What you must remember
- Pathway in order: retina, optic nerve, chiasma, optic tract, pretectal nucleus (before the lateral geniculate body), bilateral Edinger-Westphal nuclei, third nerve, ciliary ganglion, short ciliary nerves, sphincter pupillae.
- Why both pupils react to one eye: pretectal nuclei project bilaterally via the posterior commissure — the anatomic basis of the consensual reflex.
- Swinging torch test: a relative afferent pupillary defect (Marcus Gunn pupil) shows paradoxical dilation when light swings to the affected eye, indicating optic nerve disease (commonly optic neuritis) with an intact iris apparatus.
- Argyll Robertson pupil: accommodation preserved but light reflex absent, small irregular pupils; the lesion sits in the pretectal region — classically neurosyphilis, also diabetes.
- Horner syndrome: miosis with ptosis, apparent enophthalmos and ipsilateral anhidria from interruption of the oculosympathetic pathway; confirm with the cocaine or apraclonidine test.
- Near reflex dissociation: light-near dissociation also occurs with dorsal midbrain lesions (Parinaud syndrome) and severe diabetic autonomic neuropathy.
- Pharmacologic pupils: atropine dilates (mydriasis with cycloplegia); morphine and opioids constrict (pin-point pupil); adrenaline affects the dilator pupillae.
How to work through an unequal pupil
Approach anisocoria by asking which pupil is abnormal and whether it reacts to light. A 30-year-old with a large right pupil that is fixed to light but constricts on near effort has a tonic (Adie) pupil — denervation supersensitivity of the sphincter, constricting briskly to dilute pilocarpine 0.1%, which does nothing to a normal pupil. A 60-year-old with a large pupil, ptosis and down-and-out eye deviation has a third nerve palsy; the blown pupil localises to compression (posterior communicating artery aneurysm is the feared cause) because the parasympathetic fibres run on the surface of the nerve.
Now the small pupil. If it reacts poorly to light and better to near view, and the knees show lost vibration sense, think Argyll Robertson and test for syphilis. If instead the small pupil comes with a slight lid droop and a dry half of the face, the pathway broken is sympathetic: Horner syndrome, localised further by the sweating pattern — face-plus-arm anhidria points centrally (e.g., lateral medullary stroke), forehead-only anhidria to a lesion below the superior cervical ganglion such as a Pancoast tumour. Finally, shine light alternately in each eye of a patient recovering from unilateral visual blur: dilation on swinging to the bad side despite both pupils constricting when the good eye is lit clinches a Marcus Gunn pupil, telling you the problem is the optic nerve, not the retina or the iris.
Where students slip
The commonest diagram error is routing the light reflex through the lateral geniculate body or the visual cortex; the decussating fibres for this reflex peel off the optic tract to the pretectal nucleus well before the geniculate, which is exactly why cortical blindness leaves the light reflex intact — a favourite viva point. The second confusion is between afferent and efferent limbs: an optic nerve lesion gives a consensual response in the blind eye when the good eye is lit (efferent intact) but a Marcus Gunn pattern on swinging; a third nerve lesion gives a fixed pupil on that side no matter which eye is lit. State the limb, then the site, then the classic disease — that sequence reads like a textbook answer.
Frequently asked questions
Where exactly do the fibres for the light reflex leave the visual pathway?
Before the lateral geniculate body, in the proximal optic tract, to reach the pretectal nuclei of the dorsal midbrain near the superior colliculi.
Why is the pupil fixed in cortical blindness?
The reflex pathway diverges from the visual pathway at the optic tract, so occipital cortex destruction abolishes sight but leaves the light reflex completely normal.
What is a Marcus Gunn pupil?
A relative afferent pupillary defect detected by the swinging torch test: both pupils dilate when light moves to the affected optic nerve, showing conduction delay in the afferent limb.
Which lesion produces light-near dissociation with small irregular pupils?
The Argyll Robertson pupil of neurosyphilis, located in the pretectal region; accommodation and convergence work, the light reflex does not.
Why does Horner syndrome cause miosis rather than mydriasis?
The sympathetic dilator pupillae loses its tone (and the Muller muscle of the lid its lift), so the parasympathetic sphincter acts unopposed, constricting the pupil and drooping the lid.