A 52-year-old man with hypertension presents with sudden-onset visual field loss. Perimetry shows a homonymous hemianopia with preserved pupillary light reflex and no afferent pupillary defect. Which feature best distinguishes a post-geniculate (optic tract/radiations/cortex) lesion from a pre-geniculate (retina/optic nerve/chiasm) lesion?

Explanation

## Distinguishing Pre-Geniculate from Post-Geniculate Lesions ### Anatomical Basis The pupillary light reflex pathway diverges from the visual pathway at the optic nerve. Pre-geniculate lesions (retina, optic nerve, chiasm) affect both pathways, while post-geniculate lesions (optic tract, lateral geniculate nucleus, optic radiations, visual cortex) spare the pupillary reflex pathway. ### Key Discriminating Feature **Key Point:** Preserved pupillary light reflex with visual field defect is pathognomonic for post-geniculate lesions. The pupillary afferent fibres branch off before the lateral geniculate nucleus and travel via the pretectal nucleus, bypassing the visual cortex. ### Comparison Table | Feature | Pre-Geniculate | Post-Geniculate | |---------|---|---| | **Pupillary light reflex** | Abnormal (RAPD if unilateral) | **Normal** | | **Visual acuity** | Often reduced | Usually normal | | **Visual field pattern** | Non-homonymous (arcuate, altitudinal, central scotoma) | Homonymous | | **Congruity** | N/A (unilateral) | Variable (more congruent in cortex) | | **Colour vision** | Often affected early | Relatively spared | ### Clinical Pearl **Clinical Pearl:** A patient with homonymous hemianopia who can still perceive light in the blind field (preserved pupillary response) has a post-geniculate lesion — most commonly acute stroke in the middle cerebral artery territory affecting the optic radiations or occipital cortex. **High-Yield:** The mnemonic "**RAPD = Retina, Optic nerve, or chiasm (pre-geniculate)**" helps recall that relative afferent pupillary defects only occur with pre-geniculate pathology. ### Why Preserved Pupillary Reflex is the Best Discriminator While congruity and visual acuity can vary, the presence of a normal pupillary light reflex in the setting of visual field loss is virtually diagnostic of post-geniculate pathology and is the single most reliable discriminating feature on clinical examination.