Relative afferent pupillary defect (RAPD)

ophthalmology8 free questions

Australian guidance: RANZCO clinical resources; eTG Neurology (optic neuritis)

1. Definition

A relative afferent pupillary defect (RAPD), or Marcus Gunn pupil, is an asymmetry in the afferent (optic nerve) limb of the pupillary light reflex between the two eyes. It is detected on the swinging light test and localises the problem to the optic nerve or to extensive retinal disease, not to the iris muscles or the efferent pathway. It is one of the highest-yield objective signs in ophthalmology because it cannot be faked or masked by a cooperative or uncooperative patient.

2. Anatomy of the pupillary light reflex

Light strikes the retina and the signal travels along the optic nerve (cranial nerve II, the afferent limb), through the optic chiasm and tract, to the pretectal nucleus in the dorsal midbrain. Each pretectal nucleus projects to both Edinger-Westphal nuclei. From there parasympathetic fibres run with the oculomotor nerve (cranial nerve III, the efferent limb) to the ciliary ganglion and then the sphincter pupillae. Because the pretectal nucleus drives both Edinger-Westphal nuclei, light in one eye constricts both pupils: the direct response in the illuminated eye and the consensual response in the other.

3. Mechanism of an RAPD

An RAPD reflects a difference in the strength of the afferent signal from each eye. When one optic nerve is diseased, the light entering that eye generates a weaker pupillomotor drive to the midbrain than the healthy eye does. On the swinging light test, as the torch moves from the healthy eye to the affected eye, total drive to both Edinger-Westphal nuclei falls, so both pupils dilate rather than hold their constriction. The affected pupil appears to paradoxically dilate to direct light because the weak direct response is less than the consensual constriction it was receiving a moment earlier from the good eye.

4. How to test (swinging light test)

Dim the room and use a bright torch. Have the patient fixate on a distant target to remove the near reflex. Swing the light rhythmically from one eye to the other, pausing 2 to 3 seconds on each. Watch for a pupil that dilates as the light reaches it: this is the eye with the relative afferent defect. The defect is graded by severity, formally with neutral density filters over the good eye. Importantly, an RAPD can still be assessed when one pupil is fixed (for example a third nerve palsy or a pharmacologically dilated pupil) by watching only the reactive pupil, since the reflex is consensual.

5. Causes

Causes are lesions of the optic nerve or extensive retina: optic neuritis, arteritic and non-arteritic anterior ischaemic optic neuropathy, central retinal artery occlusion, severe (ischaemic) central retinal vein occlusion, extensive retinal detachment, advanced unilateral or asymmetric glaucoma, and optic nerve compression or trauma. A dense cataract or refractive error does not cause an RAPD, because it blurs the image without damaging the optic nerve, so the afferent signal reaching the midbrain remains symmetric.

6. Clinical significance

An RAPD is the objective sign that separates optic nerve disease from media and refractive causes of visual loss. In a young adult with painful monocular loss and reduced colour vision it points to optic neuritis, which is frequently the presenting feature of multiple sclerosis and prompts MRI of the brain and orbits. In an older patient with sudden painless loss it points to a vascular optic nerve or retinal event such as central retinal artery occlusion, a time-critical emergency. Its absence in a blurry eye is just as informative, pointing away from the optic nerve.

VIDEO SHORT (rapd-swinging-light) — not parsed by build_topic.py; for the lucan-shorts pipeline

TITLE: The pupil that gets bigger when you shine light in it DURATION: ~40s

HOOK (0-4s): "This is the one eye sign that looks like a glitch. You shine a light in, and the pupil gets wider."

CORE (4-30s): "It is called a relative afferent pupillary defect. Both pupils are wired to react to light together, so shine a light in a healthy eye and both constrict. Now swing the torch to an eye with a damaged optic nerve. That eye can barely sense the light, so the brain thinks the light just got dimmer, and both pupils drift back open. Swing back to the good eye and they snap shut again. The affected pupil widens as the light arrives on it."

PAYOFF (30-40s): "A positive swinging light test means the problem is the optic nerve itself, not the lens and not the surface of the eye. Think optic neuritis or a central retinal artery occlusion. The pupil that dilates to light is telling you which nerve is dying."

VISUAL CUES: - Two-eye animation with a torch swinging left to right. - Healthy eye: both pupils constrict, tint green. - Swing to affected eye: both pupils dilate, red flag pulse. - Overlay label "afferent = input" on the optic nerve; arrow from retina to midbrain. - End card: "RAPD = optic nerve or big retinal problem. Never cataract."

BACKLOG — future SBAs (not parsed by build_topic.py)

Candidate additions when RAPD is next expanded (flagged 2026-07-19, to take the pack toward ~10):

Bilateral / symmetric anterior visual pathway disease → NO RAPD. e.g. bilateral toxic-nutritional optic neuropathy, or symmetric bilateral cataracts, with markedly reduced acuity but no RAPD — because an RAPD is relative and requires asymmetry between the eyes. Reinforces that poor vision without an RAPD does not exclude optic nerve disease when the disease is symmetric. Lead-in: pupillary finding / underlying mechanism.
Traumatic optic neuropathy. Blunt orbital or brow trauma → sudden monocular loss with an RAPD from optic nerve injury (often at the canalicular segment), with a normal or near-normal fundus early. Adds a trauma cause of RAPD alongside the vascular, inflammatory, and compressive ones already covered. Lead-in: single most likely diagnosis, or next investigation (CT orbits).

Practise this in Lucan

Written for Australian practice and grounded in the guidance named above. Study material only — not medical advice. Always check against current guidelines.