A 68-year-old woman with diabetes presents with progressive bitemporal hemianopia and headaches. MRI shows a suprasellar mass compressing the optic chiasm. Which visual field defect pattern best distinguishes compression of the chiasm from compression of the optic tract?

## Chiasmal vs Optic Tract Compression: Visual Field Patterns ### Anatomical Basis of Field Defects The optic chiasm is composed of crossing nasal fibres from both eyes. Compression from above (pituitary adenoma, craniopharyngioma) preferentially affects the inferior crossing nasal fibres first, causing **superior bitemporal hemianopia**. In contrast, optic tract lesions cause **homonymous hemianopia** because the tract contains all fibres from one side of the visual field. ### Comparison Table | Feature | Chiasmal Compression | Optic Tract Compression | |---------|---|---| | **Field pattern** | **Bitemporal** (both eyes) | **Homonymous** (same side both eyes) | | **Superior vs inferior** | **Superior > inferior** (inferior fibres compressed first) | Equal involvement | | **Pupillary reflex** | Normal initially, RAPD late | Normal (post-geniculate) | | **Progression** | Gradual, starts superior | Acute or gradual | | **Optic atrophy pattern** | Temporal atrophy (nasal fibres spared) | Homonymous atrophy | | **Common cause** | Pituitary adenoma, craniopharyngioma | Stroke, tumour, demyelination | ### Key Discriminating Feature **Key Point:** **Bitemporal hemianopia with superior field loss greater than inferior** is pathognomonic for chiasmal compression from above. This occurs because the inferior nasal crossing fibres (representing the superior temporal fields) are compressed first as the mass grows superiorly from the sella turcica. **High-Yield:** The mnemonic **"CHIASM = Compression from above → Inferior fibres first → Superior fields lost"** helps recall the pattern of visual field loss in chiasmal compression. ### Clinical Pearl **Clinical Pearl:** In a patient with bitemporal hemianopia and headaches, always obtain pituitary MRI. If the superior fields are preferentially lost, chiasmal compression from below (rare) or from above (common — pituitary adenoma) is likely. The **superior > inferior pattern** is the most reliable clinical discriminator from optic tract lesions, which cause congruent homonymous hemianopia. ### Why This Discriminates Chiasm from Optic Tract Chiasmal lesions produce **bitemporal** defects (affecting both temporal fields because nasal fibres cross). Optic tract lesions produce **homonymous** defects (affecting the same side of both eyes). The superior predominance in chiasmal compression reflects the anatomical vulnerability of inferior crossing fibres to compression from above — the most common direction of mass growth in the sellar region.