Which pattern of visual field defect is most expected with a pituitary lesion compressing the optic chiasm?

• A. Bitemporal VF defects
• B. Homonymous VF defects
• C. Quadrantanopia
• D. Monocular defect

Answer: (A)

When a pituitary lesion presses on the optic chiasm, the crossing fibers from the nasal retina—which carry information for the temporal (outer) visual fields of both eyes—are affected. Since these fibers decussate at the chiasm, compression there produces loss of the temporal visual fields in both eyes, i.e., a bitemporal hemianopia.

If the lesion were behind the chiasm, you’d expect a homonymous field defect (the same side of the field lost in both eyes) due to post-chiasmal involvement. A quadrantanopia would point to involvement of the optic radiations or a focal occipital lesion affecting one quadrant, and a monocular defect would indicate damage to the optic nerve before the chiasm.