A 52-year-old woman presents with progressive loss of vision in the outer (temporal) fields of both eyes over 6 months. MRI pituitary shows a 2.2 cm sellar mass compressing the structure marked **C** from below. Which of the following best explains the visual field defect observed in this patient?

Explanation

## Why option 1 is right The optic chiasm (structure **C**) lies immediately above the pituitary gland within the sella turcica. Pituitary macroadenomas (>1 cm) grow superiorly and compress the chiasm from below. The crossing nasal retinal fibers (which correspond to the temporal visual fields) are damaged first, producing the classic sign of bitemporal hemianopia—loss of the outer visual fields bilaterally. This is a hallmark of suprasellar compression and is the most common visual complication of pituitary tumors. Harrison 21e Ch 380 emphasizes this anatomical relationship as the key to understanding pituitary tumor mass effects. ## Why each distractor is wrong - **Option 2**: Compression of the optic nerve at the optic foramen would cause monocular (one-sided) visual loss, not bilateral temporal field loss. The optic nerves are lateral to the chiasm and are not compressed by a sellar mass growing upward. - **Option 3**: The lateral geniculate nucleus is part of the visual pathway in the midbrain/thalamus, far removed from the pituitary. Compression here would cause homonymous hemianopia (same side of visual field in both eyes), not bitemporal hemianopia. - **Option 4**: The optic tract lies posterior and lateral to the chiasm. Compression of the tract would cause contralateral homonymous hemianopia (loss of the same side of the visual field in both eyes), not bitemporal loss. **High-Yield:** Pituitary macroadenomas compress the optic chiasm from **below** → bitemporal hemianopia (nasal fibers crossing); this is the most common visual sign of pituitary tumor mass effect. [cite: Harrison 21e Ch 380]