Background: Diffusely abnormal white matter (DAWM) is associated with decreased axonal and myelin density, fibrillary gliosis, and inflammatory cell activation on histopathology, and has been linked to progression in secondary progressive MS (SPMS). However, few studies have focused on assessing DAWM in primary progressive MS exclusively. Hence, we aimed to characterize the longitudinal evolution of DAWM and its relationship with focal white matter lesions (FWML) and confirmed disability progression (CDP) in a PPMS population.
Objectives: 1) To automatically segment and characterize the longitudinal evolution of FWML and DAWM in PPMS. 2) To assess associations of voxels of DAWM that transform into FWML at last visit with CDP (CDP=sustained increase in EDSS that persisted for 24 weeks).
Methods: The data included 1753 MRI scans of 376 PPMS participants, followed for 122 weeks, scanned at screening, weeks 06, 48, 96, and 122. FWML and DAWM were automatically segmented using a previously validated automated 2-weighted-intensity thresholding technique. All gadolinium enhancing and new white matter lesion voxels were excluded from the FWML mask, to capture the chronic component of FWML. DAWM voxels at screening, weeks 06, 48 and 96 that transformed into FWML at the last MRI scan (w122) were segmented.
Results: As disease duration increased, PPMS participants showed volumes of chronic FWML that significantly increased (t=7.3; p<0.0001) but no significant changes in DAWM volumes. The voxels of DAWM in subsequent scans that transformed to FWML at the last visit significantly decreased as disease duration progressed (t=-8.8; p<0.0001) and were not significantly associated with CDP.
Conclusions: DAWM voxels show a dynamic transformation into FWML over time, with volumes of DAWM-to-FWML transformation in PPMS decreasing progressively as disease duration increases. These dynamic changes are similar to those observed in SPMS. However, unlike SPMS, where previous studies have shown an association between DAWM-to-FWML transformation and CDP, this group of PPMS participants did not show such an association. This finding would suggest that other factors, other than DAWM evolution, might have a stronger weight in disease progression in PPMS, compared to SPMS.