Transhemispheric optic pathway degeneration following unilateral post-geniculate lesions.

A unilateral lesion in a post-geniculate section of the retino-geniculo-striate pathway (hereafter: optic pathway) leads to binocular vision loss in the contralateral visual hemifield. The extent of additional damage following such a lesion is not fully understood. Although degeneration of the ipsilesional optic tract and both retinas has been reported, potential degeneration in the contralesional post-chiasmal optic pathway has largely been overlooked. We aimed to investigate the presence and extent of contralesional degeneration in individuals with post-geniculate optic pathway lesions. In this case-control study, we examined the optic pathways of study cohorts with 10 (dataset 1) and 22 (dataset 2) individuals with unilateral post-geniculate lesions and 12 (dataset 1) and 17 (dataset 2) neurologically healthy controls. For both datasets, we applied a higher-order analysis framework, i.e. fixel-based analysis, to diffusion-weighted imaging data to evaluate the white matter of optic pathway tracts. Fixel-based analysis showed reduced fibre density and fibre-bundle cross-section in the ipsi- and contralesional optic pathways. Post-hoc analysis and observations further demonstrated reduced fibre density and fibre-bundle cross-section in the forceps major. In individuals with unilateral post-geniculate optic pathway lesions, degeneration extends beyond their primary site to the optic pathway tracts, including contralesional ones. This pattern of widespread transhemispheric degeneration suggests that it spreads more extensively than previously recognized and highlights the need for understanding its implications for visual function.