Walsh & Hoyt: Topical Diagnosis of Lesions of the Lateral Geniculate Body

Lesions affecting the lateral geniculate body (LGB; also lateral geniculate nucleus [LGN]) are less commonly diagnosed than those of the optic tract. At the beginning of the 20th century, it was accepted that incongruity of homonymous field defects is most pronounced with lesions near the junction of the optic chiasm and tract and diminishes progressively with more posterior lesions. This diagnostic perimetric axiom was attributed to a gradual alignment in the optic tract of the crossed and uncrossed visual axons from corresponding retinal areas. However, this theory was challenged by several clinicoanatomic case studies and two clinical reports suggesting that lesions of the lateral geniculate body produce significantly incongruous homonymous field defects. Gunderson and Hoyt subsequently reported quantitative perimetric studies in two patients with lesions of the lateral geniculate body. One patient had a progressive hemianopia that began as an incongruous, relative wedge-shaped homonymous defect and ultimately resulted in a complete homonymous hemianopia with splitting of the macula. At autopsy, the patient had a diffuse pilocytic astrocytoma in the right temporal lobe, thalamus, and brain stem that completely replaced the right LGB. The second patient had a small arteriovenous malformation localized to the LGB. Hoyt later described these two patients and two others in a more extensive discussion of perimetric findings in patients with lesions of the LGB. The most striking perimetric feature in these patients was the extensive incongruity of their visual field defects.