Relative afferent pupillary defect (RAPD)

The Marcus Gunn pupil results from a lesion anterior to the optic chiasm affecting the afferent limb of the pupillary light reflex. When light is shone into the affected eye, reduced signal transmission leads to diminished direct and consensual pupillary constriction compared to the unaffected eye.

Clinical Test - Swinging Flashlight Test:

• Shine a light alternately between each eye every 2-3 seconds.
• In a normal response, pupils constrict equally regardless of which eye is stimulated.
• In RAPD, when the light swings to the affected eye, both pupils paradoxically dilate or constrict less because of decreased afferent input.

Differentiation from Argyll-Robertson Pupil:

• Argyll-Robertson pupils are small, irregular, and bilaterally constrict poorly to light but constrict normally during accommodation (light-near dissociation).
• Marcus Gunn pupil shows an asymmetric pupillary light reflex due to unilateral optic nerve or retinal pathology without accommodation abnormalities.

Clinical Significance:

RAPD indicates unilateral or asymmetric optic nerve dysfunction (e.g., optic neuritis, ischaemic optic neuropathy) or severe retinal disease. It does not occur in lesions posterior to the optic chiasm.

Causes

• retina: detachment
• optic nerve: optic neuritis e.g. multiple sclerosis

Pathway of pupillary light reflex

• afferent: retina → optic nerve → lateral geniculate body → midbrain
• efferent: Edinger-Westphal nucleus (midbrain) → oculomotor nerve