A 52-year-old man with hypertension presents with sudden-onset visual field loss. Perimetry shows a homonymous hemianopia respecting the vertical midline. All of the following are characteristic features of homonymous hemianopia EXCEPT:

Explanation

## Understanding Homonymous Hemianopia **Key Point:** Homonymous hemianopia results from a lesion in the optic tract, lateral geniculate nucleus, optic radiations, or visual cortex — anywhere POSTERIOR to the optic chiasm. The critical distinguishing feature is that the defect respects the vertical meridian and affects corresponding visual fields in both eyes identically. ## Characteristics of Homonymous Hemianopia | Feature | Status | Explanation | | --- | --- | --- | | Respects vertical meridian | ✓ Always | Anatomical consequence of post-chiasmal anatomy | | Affects both eyes equally | ✓ Always | Both eyes see the same half-field loss | | Pupillary light reflex abnormal | ✗ Not always | Pupillary pathways are separate from visual pathways | | Post-chiasmal lesion | ✓ Always | Defines homonymous hemianopia | ## Why Pupillary Reflex Is NOT Always Abnormal **High-Yield:** The pupillary light reflex depends on the afferent pupillary pathway (retina → optic nerve → pretectal nucleus → Edinger-Westphal nucleus). A lesion in the optic radiations or visual cortex does NOT interrupt this pathway — it only affects conscious vision. Therefore, pupils remain reactive despite severe visual field loss. **Clinical Pearl:** A patient with a large occipital lobe infarct causing complete homonymous hemianopia may have perfectly normal, reactive pupils. This dissociation between visual loss and pupillary function is pathognomonic for post-chiasmal lesions. ## Localization of Homonymous Hemianopia ```mermaid flowchart TD A[Homonymous Hemianopia]:::outcome --> B{Location?}:::decision B -->|Optic tract| C[Incongruous defect]:::outcome B -->|LGN| D[Sectoral or sectoranopia]:::outcome B -->|Optic radiations| E[Incongruous, upper/lower quad]:::outcome B -->|Visual cortex| F[Congruous, macular sparing possible]:::outcome G[Pupillary reflex pathway<br/>bypasses all these]:::action ``` **Mnemonic:** **TRACT** = The Radiations And Cortex are Tested (for visual fields, not pupils). [cite:Walsh & Hoyt's Clinical Neuro-Ophthalmology Ch 5]