Bitemporal vision loss is most likely due to a lesion at the optic chiasm caused by which condition?

• A. Arterial embolism
• B. Optic nerve lesion
• C. Pituitary adenoma
• D. Occipital lobe stroke

Answer: (C)

Bitemporal vision loss occurs when the crossing fibers at the optic chiasm are damaged, because the nasal retinal fibers from both eyes cross there and subserve the temporal (outer) visual fields. A lesion at the chiasm classically produces loss of the outer halves of the visual fields in both eyes, i.e., bitemporal hemianopia. The most common cause is a pituitary adenoma that expands upward to compress the chiasm.

Other causes don’t fit the pattern: an arterial embolism or other vascular event affecting the optic nerve or retina typically causes loss in one eye (monocular) or acute focal deficits rather than the characteristic bitemporal pattern. An optic nerve lesion also causes monocular vision loss in one eye. An occipital lobe stroke yields a homonymous hemianopia—loss of the same visual field side in both eyes—rather than bitemporal loss.