White matter microstructure of sensorimotor projection fibers in chronic low back pain

Chronic low back pain (cLBP) is linked to altered cortical sensorimotor structure and function, yet white matter microstructure in sensorimotor projection fibers remains poorly characterized. Objective: To address this gap, we used diffusion MRI to comprehensively characterize sensorimotor projection fibers in 44 patients with cLBP and 41 matched healthy controls.

We estimated an array of microstructural measures, including fractional anisotropy (FA), extracellular free water fraction (FW), free water-corrected FA (fwcFA), and neurite orientation dispersion and density imaging (NODDI) metrics.

General linear models identified significantly higher FA and fwcFA and lower orientation dispersion index (ODI) in cLBP vs HC within projection fibers emanating from the primary somatosensory and motor cortices (all corrected P-values <0.05). Conjunction analyses indicated spatial overlap between increased anisotropy and reduced ODI, suggesting a reduction in geometric complexity of neurites underlies this group difference. Partial correlations revealed significant relationships between diffusion metrics and anxiety and depressive symptoms (FA: r = -0.41, P < 0.01; fwcFA: r = -0.38, P = 0.01; ODI: r = 0.38, P = 0.01). The relationships between anxiety and depressive symptoms and diffusion MRI metrics are consistent with work outside of chronic pain.

Our results align differences in white matter microstructure with previous neurophysiological evidence that identifies the primary sensorimotor cortices as relevant in cLBP and emphasizes the interrelatedness of anxiety and depressive symptoms and sensorimotor structure.