## Chiasmal Compression and Visual Field Defects ### Anatomical Basis The optic chiasm is where nasal (medial) retinal fibres from both eyes decussate to form the optic tracts. Lesions at the chiasm affect these crossing fibres preferentially. ### Bitemporal Hemianopia Mechanism 1. Compression from **above** (e.g., pituitary adenoma, craniopharyngioma) affects the inferior nasal fibres first 2. These fibres represent the **superior temporal visual fields** 3. As compression worsens, superior nasal fibres are affected → superior temporal fields lost 4. Result: **bilateral temporal field loss** (bitemporal hemianopia) **Key Point:** Bitemporal hemianopia is pathognomonic for chiasmal pathology. The pattern reflects the crossing of nasal retinal fibres at the chiasm. **Clinical Pearl:** Compression from **below** (rare) causes superior bitemporal hemianopia first; compression from **above** (common — pituitary) causes inferior bitemporal hemianopia first. **High-Yield:** Pituitary adenomas are the most common cause of bitemporal hemianopia in clinical practice. Always obtain MRI pituitary when this defect is found. ### Differential Table | Lesion Location | Visual Field Defect | Mechanism | | --- | --- | --- | | Optic chiasm (compression) | Bitemporal hemianopia | Nasal fibre crossing affected | | Optic tract | Homonymous hemianopia | Post-chiasmal fibres | | Optic nerve | Central scotoma or arcuate | Axonal loss | | Lateral geniculate nucleus | Homonymous hemianopia | Post-chiasmal | [cite:Neuro-Ophthalmology by Burde, Maybodi & Savino] 
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