Associations of local white matter geometry with network efficiency, macrostructural abnormalities, and clinical...

Behavioural variant frontotemporal dementia (bvFTD), marked by profound changes in behaviour and personality, is the most common subtype of frontotemporal dementia, driven by neurodegeneration in frontotemporal regions. This neurodegeneration pattern is partially shaped by white matter abnormalities arising from the spread of protein aggregates along axonal pathways. While prior studies mainly focused on diffusion tensor imaging metrics such as fractional anisotropy and mean diffusivity, the alteration in local white matter geometry remains largely unexplored. Using a novel Director Field Analysis (DFA) method, 51 patients with bvFTD and 51 healthy controls were studied to examine alterations in the local geometry of white matter fibres in bvFTD, and their associations with macrostructural morphology, global network parameters, and clinical manifestations. Unlike the unidirectional decrease in fractional anisotropy and increase in mean diffusivity, we identified significant bidirectional alterations in white matter local geometry, characterized by increased geometric distortion in the forceps minor and dorsal cingulum and decreased distortion in widespread frontotemporal association tracts, including the inferior fronto-occipital fasciculus, superior longitudinal fasciculus, uncinate fasciculus, frontal aslant tract, and arcuate fasciculus. Patients with bvFTD also showed reduced cerebral white and grey matter volumes (both P < 0.0026), enlarged lateral ventricles and choroid plexus (both P < 0.0001), decreased global network efficiency (P = 0.0010), and increased local efficiency (P = 0.0014). Importantly, decreased white matter geometric distortion across affected tracts was strongly associated with greater clinical severity, as reflected by higher Clinical Dementia Rating scores (r = -0.68, P < 0.0001). Mediation analyses further demonstrated that white matter geometric distortion significantly mediated the effects of macrostructural atrophy and reduced global network efficiency on clinical severity. Furthermore, neuroimaging-transcriptional association analysis on the group differences in nodal efficiency of the white matter networks identified several biological processes/pathways critical for the formation and propagation of TAR-DNA-binding protein 43/microtubule-associated protein tau pathologies along axonal pathways, as well as processes related to cellular homeostasis and oligodendrocyte-related pathways that may exacerbate these proteinopathies. Our findings advance understanding of the neural bases of the functional impairments in bvFTD and suggest potential mechanistic pathways for developing novel treatment strategies.