A 45-year-old woman presents with a 3-month history of progressive visual disturbance described as "loss of the outer edges of my vision" and new-onset frontal headache. MRI of the sella turcica with gadolinium shows a 18 mm hypointense lesion arising from the anterior pituitary with superior extension. The structure marked **B** in the diagram shows compression of the crossing nasal retinal fibers. Which of the following visual field defects is most characteristic of this mass effect?

Explanation

## Why Bitemporal hemianopsia affecting the peripheral temporal fields is right Compression of the **optic chiasm** (marked **B**) by an expanding pituitary macroadenoma classically produces bitemporal hemianopsia. The crossing nasal retinal fibers (which represent the temporal visual fields) are compressed first as the tumor grows superiorly from the sella turcica. This results in loss of the peripheral temporal fields bilaterally—the "sailor's view" disturbance—before progressing to complete bitemporal hemianopsia. This is the pathognomonic mass effect sign of sellar/suprasellar pathology compressing the chiasm, as documented in Harrison 21e Ch 380 and Williams Textbook of Endocrinology 14e. ## Why each distractor is wrong - **Homonymous hemianopsia with preserved pupillary light reflex**: This pattern results from lesions posterior to the optic chiasm (optic tract, lateral geniculate nucleus, or optic radiations), not from direct compression of the chiasm itself. Pituitary macroadenomas compress the chiasm anteriorly. - **Concentric visual field constriction with central scotoma**: This pattern is seen with chronic papilledema, advanced glaucoma, or retinal pathology—not from chiasmal compression. Chiasmal compression produces a field defect that respects the vertical meridian (bitemporal pattern), not concentric loss. - **Monocular vision loss with afferent pupillary defect**: This indicates optic nerve pathology (anterior to the chiasm), such as optic neuritis or compression of a single optic nerve. Chiasmal compression affects both eyes symmetrically in the temporal fields. **High-Yield:** Bitemporal hemianopsia = chiasmal compression until proven otherwise; always check prolactin, cortisol, and formal visual fields (Humphrey perimetry) in any patient with sellar mass and vision loss. [cite: Harrison 21e Ch 380; Williams Textbook of Endocrinology 14e]