Lesions posterior to the optic chiasm produce which type of visual field defects?

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

Answer: (C)

Focusing on the pathway after the optic chiasm shows why this is the right pattern. Once fibers have crossed at the chiasm, the information from each visual field is carried in the same posterior pathways to the brain. A lesion behind the chiasm disrupts the corresponding half of the visual field in both eyes, creating a homonymous visual field defect. For example, damage to the right optic tract or right optic radiations will knock out the left half of the visual field in both eyes. Quadrantanopia is a specific, partial form that can occur with selective involvement of parts of the radiations (producing loss in a quadrant), but the overall, defining change with posterior lesions is homonymous field loss. A bitemporal defect comes from chiasm involvement, and a monocular deficit comes from an optic nerve lesion before the chiasm.