Peak width of skeletonized mean diffusivity (PSMD) and cognitive functions in relapsing-remitting multiple sclerosis

Peak width of skeletonized mean diffusivity (PSMD) is a new MRI marker, which has shown clinical relevance in some neurological conditions and, in preliminary data, in multiple sclerosis (MS). We aimed here to investigate, in a group of relapsing-remitting MS (RRMS) patients, the relationship between PSMD and cognitive performances, in comparison with other MRI measures. RRMS patients (n = 60) and normal controls (n = 15) underwent a 3 T MRI examination. MRI-based white matter (WM) lesion volume, microstructural integrity (assessed with Tract-Based Spatial Statistics of diffusion tensor imaging [DTI] images) and brain volumes (i.e., total brain, grey matter [GM] and WM) were computed. In addition, PSMD was calculated through "skeletonization" of WM tracts and diffusion histograms. Cognition was evaluated with Rao's Brief Repeatable Battery (BRB), which incorporated tests of verbal and visual memory, attention, concentration, information processing speed and verbal fluency. PSMD closely correlated with symbol digit modalities test (SDMT) (r = -0.70, p < 0.001) and, to a lesser extent, with verbal and visual memory tests. Multiple regression analysis showed that PSMD explained SDMT variance (R2 = 0.54, p < 0.001) more than other MRI measures. Results point out the relevance of microstructural damage, as assessed by PSMD, as a reliable marker of cognition in MS, especially in explaining dysfunction in information processing speed.