A 52-year-old woman presents with progressive bitemporal hemianopia and headaches. A lateral skull X-ray shows enlargement and ballooning of the structure marked **A** (sella turcica). Which of the following best explains the visual field defect in this patient?

Explanation

## Why "Compression of the optic chiasm by upward extension of a pituitary macroadenoma" is right The sella turcica (marked **A**) is a saddle-shaped depression in the sphenoid bone that houses the pituitary gland. When a pituitary macroadenoma enlarges, it expands superiorly and compresses the optic chiasm, which lies directly above the sella. This superior compression of the chiasm causes bitemporal hemianopia—the classic visual field defect in pituitary tumors—because the crossing nasal fibers of the chiasm are compressed first. The ballooned appearance of the sella on lateral skull X-ray is a key radiological sign of pituitary macroadenoma. (Gray's Anatomy 42e Ch 28; Harrison 21e Ch 380) ## Why each distractor is wrong - **Compression of the optic nerves by enlargement of the frontal sinuses**: The frontal sinus (marked **B**) is anterior and superior to the optic nerves and chiasm. Frontal sinus enlargement does not cause bitemporal hemianopia; it would cause different visual symptoms if any. The clinical presentation and X-ray findings point to a sellar pathology, not a frontal sinus process. - **Compression of the lateral geniculate nucleus by cavernous sinus thrombosis**: Cavernous sinus involvement by pituitary tumors causes lateral (not superior) extension and results in cranial nerve palsies (CN III, IV, VI, V1, V2), not bitemporal hemianopia. The lateral geniculate nucleus is in the thalamus and is not directly compressed by sellar or cavernous sinus pathology in this context. - **Ischemia of the optic nerve head secondary to increased intracranial pressure**: While increased intracranial pressure can occur with large sellar masses, it causes papilledema and generalized visual obscuration, not the specific pattern of bitemporal hemianopia. Bitemporal hemianopia requires direct mechanical compression of the chiasm, not ischemic injury. **High-Yield:** Bitemporal hemianopia = pituitary tumor with superior chiasmal compression; ballooned sella on lateral skull X-ray = pituitary macroadenoma. [cite: Gray's Anatomy 42e Ch 28; Harrison 21e Ch 380]