A patient has a right homonymous superior quadrantanopia. Which anatomical structure is most likely affected?

Explanation

## Homonymous Quadrantanopia and Meyer's Loop ### Anatomical Basis of Optic Radiations The optic radiations carry post-chiasmal fibres from the lateral geniculate nucleus (LGN) to the primary visual cortex. Different portions of the radiations represent different quadrants of the visual field. ### Meyer's Loop — Inferior Radiations **Key Point:** Meyer's loop (inferior optic radiations) carries fibres representing the **superior visual quadrants**. Damage to Meyer's loop causes **superior quadrantanopia** (pie-in-the-sky defect). ### Anatomical Pathway 1. **Superior optic radiations** → represent inferior visual quadrants → lesions cause inferior quadrantanopia 2. **Inferior optic radiations (Meyer's loop)** → represent superior visual quadrants → lesions cause superior quadrantanopia ("pie-in-the-sky") 3. **Optic tract** → causes homonymous hemianopia (entire half of visual field) 4. **Occipital cortex** → causes homonymous hemianopia or quadrantanopia depending on extent ### Why Right Inferior Radiations? Right homonymous superior quadrantanopia means: - Both eyes lose the **right superior quadrant** of vision - This is a **post-chiasmal** lesion (homonymous = same side in both eyes) - The lesion affects fibres carrying superior field information - Meyer's loop (inferior radiations) carries superior field fibres - Therefore: **right inferior optic radiations** (Meyer's loop) **Mnemonic:** **PIE-IN-THE-SKY** = Meyer's loop damage → superior quadrantanopia. The "pie" is in the upper visual field (sky). **Clinical Pearl:** Meyer's loop lesions are common with temporal lobe pathology (tumours, stroke, temporal lobe epilepsy surgery). The defect is often the first clue to temporal lobe involvement. ### Comparison Table | Structure | Visual Field Defect | Typical Cause | | --- | --- | --- | | Optic tract | Homonymous hemianopia | Midbrain/brainstem lesion | | Superior radiations | Inferior homonymous quadrantanopia | Temporal lobe lesion | | Inferior radiations (Meyer's loop) | Superior homonymous quadrantanopia | Temporal lobe lesion | | Occipital cortex (entire) | Homonymous hemianopia | Posterior cerebral artery stroke | | Occipital pole | Central visual field sparing | Anterior circulation stroke | **High-Yield:** Temporal lobe surgery (e.g., for epilepsy) risks Meyer's loop damage → superior quadrantanopia. This is a recognized complication. [cite:Neuro-Ophthalmology by Burde, Maybodi & Savino; Harrison 21e Ch 29]