Which visual field defect is characteristic of a lesion at the optic chiasm?

Explanation

## Optic Chiasm Lesions and Bitemporal Hemianopia ### Anatomy of the Optic Chiasm The optic chiasm is where the optic nerves from both eyes meet. Critically, **nasal retinal fibers (which see the temporal visual fields) cross to the contralateral side**, while temporal retinal fibers (which see the nasal visual fields) remain ipsilateral. **Key Point:** A lesion compressing the chiasm from above or below preferentially damages the crossing nasal fibers, resulting in **loss of both temporal visual fields** — bitemporal hemianopia. ### Why Bitemporal? ```mermaid flowchart LR A["Left eye<br/>Nasal retina<br/>sees temporal field"] -->|crosses| B["Optic Chiasm<br/>Compression damages<br/>crossing fibers"] C["Right eye<br/>Nasal retina<br/>sees temporal field"] -->|crosses| B B --> D["Loss of temporal<br/>fields BOTH eyes<br/>= Bitemporal hemianopia"] style B fill:#ffcccc style D fill:#ccffcc ``` ### Common Causes - **Pituitary adenoma** (most common) — compresses from below - **Craniopharyngioma** — compresses from above - **Aneurysm** (anterior communicating artery) - **Meningioma** **High-Yield:** Bitemporal hemianopia is a **red flag for pituitary pathology** until proven otherwise. Always check prolactin, growth hormone, ACTH levels and obtain MRI pituitary. **Clinical Pearl:** Early chiasmal compression may present as **superior temporal quadrantanopia** (loss of upper temporal quadrants first, as superior nasal fibers are compressed first). This progresses to complete bitemporal hemianopia as compression worsens. **Mnemonic:** **CHIASM** — **C**rossing fibers at the **H**iasm **I**nvolve **A**ll **S**patial **M**idlines → bitemporal loss.