A lesion at the optic chiasm produces which visual field defect, and what is the anatomic reason?

A lesion at the optic chiasm produces which visual field defect, and what is the anatomic reason?

• A. Monocular vision loss due to destruction of the optic nerve before the chiasm.
• B. Homonymous hemianopia due to loss of fibers after the chiasm.
• C. Bitemporal hemianopia due to loss of nasal retinal fibers crossing at the chiasm.
• D. Central scotoma due to chiasm lesion.

Answer: (C)

A lesion at the optic chiasm disrupts the crossing (decussating) fibers from the nasal retina of both eyes. Those nasal-retinal fibers carry information from the temporal visual fields. When they’re damaged at the chiasm, the temporal halves of the visual field are lost in both eyes, producing bitemporal hemianopia. The central (foveal) vision can be relatively preserved if the lesion spares those fibers. This differs from lesions before the chiasm (optic nerve) that cause monocular loss, or lesions after the chiasm (optic tracts, radiations) that cause a homonymous hemianopia. Central scotoma isn’t a typical result of a chiasm lesion.

A lesion at the optic chiasm disrupts the crossing (decussating) fibers from the nasal retina of both eyes. Those nasal-retinal fibers carry information from the temporal visual fields. When they’re damaged at the chiasm, the temporal halves of the visual field are lost in both eyes, producing bitemporal hemianopia. The central (foveal) vision can be relatively preserved if the lesion spares those fibers. This differs from lesions before the chiasm (optic nerve) that cause monocular loss, or lesions after the chiasm (optic tracts, radiations) that cause a homonymous hemianopia. Central scotoma isn’t a typical result of a chiasm lesion.