Bitemporal hemianopia with central vision spared localizes a lesion to which structure?

• A. Optic nerve
• B. Optic chiasm
• C. Temporal optic radiation
• D. Optic tract

Answer: (B)

Bitemporal field loss points to a problem at the optic chiasm, where the nasal retinal fibers from each eye cross to the opposite side. A lesion here disrupts these crossing fibers, so the temporal visual fields from both eyes are lost. Central vision being spared helps fit this location because the fibers subserving the central (macular) vision are often less affected by a midline chiasmal lesion, or can be relatively preserved depending on lesion size and pattern. If the damage were in the optic nerve, you’d see loss in only one eye; in the temporal optic radiation you’d get a contralateral superior quadrantanopia; in the optic tract you’d get a contralateral homonymous hemianopia.