An afferent pupillary defect typically indicates a lesion in which location?

• A. Retina or optic nerve/optic tract on the affected side
• B. Optic chiasm of the opposite eye
• C. Occipital cortex of the healthy hemisphere
• D. Retina of the unaffected eye

Answer: (A)

Afferent pupillary defect means reduced input along the afferent limb of the pupillary light reflex, which runs from the retina through the optic nerve and optic tract up to the midbrain’s pretectal area. When the afferent signal from one eye is diminished due to retinal disease or pathology of the optic nerve/optic tract on that same side, that eye’s light – even when bright – fails to produce a normal constriction in either pupil. That’s why, on the swinging flashlight test, you see less constriction when light is shone in the affected eye, and relatively better constriction when light is shone in the other eye. This localizes the lesion to the retina or optic nerve/optic tract on the side of the defect.

Lesions at the optic chiasm, in the occipital cortex, or in the retina of the other eye don’t produce this unilateral afferent deficit pattern because the reflex pathway is largely subcortical and depends on the afferent input from the eye being tested.