# Transhemispheric optic pathway degeneration following unilateral post-geniculate lesions

## TL;DR

This study shows that brain damage from a single optic pathway lesion can cause widespread degeneration across both hemispheres, affecting vision more than previously thought.

## Contribution

The study reveals transhemispheric degeneration in the optic pathway following unilateral post-geniculate lesions, challenging prior assumptions.

## Key findings

- Degeneration occurs in both ipsi- and contralesional optic pathways, including the forceps major.
- Fixel-based analysis detected reduced fibre density and cross-section in white matter tracts.
- The findings suggest more extensive damage than previously recognized, with implications for visual function.

## Abstract

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.

Halbertsma et al. report widespread and transhemispheric white matter degeneration following unilateral post-geniculate optic pathway lesions, evidenced by axonal loss and fibre bundle atrophy. Their findings challenge the assumption that the contralesional hemisphere remains unaffected and underscore the need to investigate functional and rehabilitative implications of this degeneration.

Graphical Abstract