PURPOSE. White matter (WM) degeneration of the visual pathways in primary open-angle glaucoma (POAG) is well documented, but its exact pathophysiology remains unclear. To date, glaucomatous WM degeneration has been exclusively studied using diffusion tensor imaging (DTI) only. However, DTI measures lack direct biological interpretation, and the approach itself suffers from multiple technical limitations. Fixel-based analysis (FBA) is a novel framework for studying WM degeneration, overcoming DTI's technical limitations and providing biologically meaningful metrics. FBA measures fiber density (FD), representing early microstructural changes, and fiber-bundle cross section (FC), representing late macrostructural changes. In this study, we use FBA to study glaucomatous degeneration of the pregeniculate optic tracts (OTs) and postgeniculate optic radiation (ORs) in POAG. METHODS. This was a cross-sectional case-control study with 12 POAG patients and 16 controls. Multi-shell diffusion-weighted images were acquired. FBA was used to produce a population template, and probabilistic tractography was used to track the OTs and ORs in template space. Finally, FD and FC of the tracts of interest were compared between the two groups. RESULTS. Compared with the controls, the OTs of the patients exhibited a significant (familywise error corrected P < 0.05) decrease in FD and FC, whereas their ORs exhibited a significant decrease in FD but not in FC. CONCLUSIONS. FBA provides sensitive measures to assess WM changes in glaucoma. Our findings suggest that the OTs of glaucoma patients exhibit signs of more advanced WM degeneration compared with the ORs. This potentially implicates anterograde trans-synaptic propagation as the primary cause of glaucomatous spread along the visual pathways.

Age-related hearing loss is the most prevalent sensory impairment in the older adult population and is related to noiseinduced damage or age-related deterioration of the peripheral auditory system. Hearing loss may affect the central auditory pathway in the brain, which is a continuation of the peripheral auditory system located in the ear. A debilitating symptom that frequently co-occurs with hearing loss is tinnitus. Strikingly, investigations into the impact of acquired hearing loss, with and without tinnitus, on the human central auditory pathway are sparse. This study used diffusion-weighted imaging (DWI) to investigate changes in the largest central auditory tract, the acoustic radiation, related to hearing loss and tinnitus. Participants with hearing loss, with and without tinnitus, and a control group were included. Both conventional diffusion tensor analysis and higher-order fixel-based analysis were applied. The fixel-based analysis was used as a novel framework providing insight into the axonal density and macrostructural morphologic changes of the acoustic radiation in hearing loss and tinnitus. The results show tinnitus-related atrophy of the left acoustic radiation near the medial geniculate body. This finding may reflect a decrease in myelination of the auditory pathway, instigated by more profound peripheral deafferentation or reflecting a preexisting marker of tinnitus vulnerability. Furthermore, age was negatively correlated with the axonal density in the bilateral acoustic radiation. This loss of fiber density with age may contribute to poorer speech understanding observed in older adults.

Background: Primary open-angle glaucoma (POAG) patients exhibit widespread white matter (WM) degeneration throughout their visual pathways. Whether this degeneration starts at the pre- or post-geniculate pathways remains unclear. In this longitudinal study, we assess the progression of WM degeneration exhibited by the pre-geniculate optic tracts (OTs) and the post-geniculate optic radiations (ORs) of POAG patients over time, aiming to determine the source and pattern of spread of this degeneration.Methods: Diffusion-weighted MRI scans were acquired for 12 POAG patients and 14 controls at two time-points 5.4 ± 2.1 years apart. Fiber density (FD), an estimate of WM axonal density, was computed for the OTs and ORs of all participants in an unbiased longitudinal population template space. First, FD was compared between POAG patients and the controls at time-point 1 (TP1) and time-point 2 (TP2) independently. Secondly, repeated measures analysis was performed for FD change in POAG patients between the two time-points. Finally, we compared the rate of FD change over time between the two groups.Results: Compared to the controls, POAG patients exhibited significantly lower FD in the left OT at TP1 and in both OTs and the left OR at TP2. POAG patients showed a significant loss of FD between the time-points in the right OT and both ORs, while the left OR showed a significantly higher rate of FD loss in POAG patients compared to the controls.Conclusions: We find longitudinal progression of neurodegenerative WM changes in both the pre- and post-geniculate visual pathways of POAG patients. The pattern of changes suggests that glaucomatous WM degeneration starts at the pre-geniculate pathways and then spreads to the post-geniculate pathways. Furthermore, we find evidence that the trans-synaptic spread of glaucomatous degeneration to the post-geniculate pathways is a prolonged process which continues in the absence of detectable pre-geniculate degenerative progression. This suggests the presence of a time window for salvaging intact post-geniculate pathways, which could prove to be a viable therapeutic target in the future.