Postchiasmal

Postchiasmal refers to the portion of the visual pathway after the optic chiasm. It encompasses the optic tracts, the lateral geniculate nucleus of the thalamus, the optic radiations, and the occipital visual cortex. These structures preserve a retinotopic map, with fibers from the contralateral visual field projecting to the opposite side of the brain. Within the optic radiations, the temporal lobe carries the upper visual fields via Meyer's loop, and the parietal lobe carries the lower visual fields via the inferior radiations. Lesions at different points produce characteristic patterns of vision loss.

Clinically, postchiasmal lesions typically cause contralateral homonymous visual field defects. The exact pattern depends on the site: lesions of the optic radiations or occipital cortex can yield homonymous hemianopia, often with macular sparing in occipital lobe lesions due to dual blood supply from the posterior cerebral and middle cerebral arteries. Parietal lobe involvement tends to produce contralateral inferior quadrantanopia (a “pie on the floor”), while temporal lobe involvement (Meyer’s loop) tends to cause contralateral superior quadrantanopia (a “pie in the sky”). Larger or combined lesions can produce complete hemianopia.

Common etiologies include cerebrovascular accidents (strokes) affecting the optic radiations or occipital cortex, brain tumors, demyelinating disease, and trauma. Diagnosis relies on clinical visual field testing (perimetry) and neuroimaging, with MRI being preferred for detailing postchiasmal structures. Management focuses on treating the underlying cause, preventing progression, and, when possible, visual rehabilitation.