Diffusely abnormal white matter converts to T2 lesion volume in the absence of MRI-detectable acute inflammation

Diffusely abnormal white matter, characterised by biochemical changes of myelin in the absence of frank demyelination, has been associated with clinical progression in secondary progressive multiple sclerosis. However, little is known about changes of diffusely abnormal white matter over time and their relation to focal white matter lesions. The objectives of this work were: (i) to characterize the longitudinal evolution of focal white matter lesions, diffusely abnormal white matter and diffusely abnormal white matter that transforms into focal white matter lesions; and (ii) to determine whether gadolinium enhancement, known to be associated with the development of new focal white matter lesions, is also related to diffusely abnormal white matter voxels that transform into focal white matter lesions. Our data included 4220 MRI scans of 689 secondary progressive multiple sclerosis participants, followed for 156 weeks, and 2677 scans of 686 relapsing-remitting multiple sclerosis participants, followed for 96 weeks. Focal white matter lesions and diffusely abnormal white matter were segmented using a previously validated, automatic thresholding technique based on normalized T2 intensity values. Using longitudinally registered images, diffusely abnormal white matter voxels at each visit that transformed into focal white matter lesions on the last MRI scan as well as their overlap with gadolinium-enhancing lesion masks were identified. Our results showed that the average yearly rate of conversion of diffusely abnormal white matter to focal white matter lesions was 1.27 cm3 for secondary progressive multiple sclerosis and 0.80 cm3 for relapsing-remitting multiple sclerosis. Focal white matter lesions in secondary progressive multiple sclerosis participants significantly increased (t = 3.9; P = 0.0001) while diffusely abnormal white matter significantly decreased (t = -4.3 P < 0.0001) and the ratio of focal white matter lesions to diffusely abnormal white matter increased (t = 12.7; P < 0.00001). Relapsing-remitting multiple sclerosis participants also showed an increase in the focal white matter lesions to diffusely abnormal white matter ratio (t = 6.9; P < 0.00001) but without a significant change of the individual volumes. Gadolinium enhancement was associated with 7.3% and 18.7% of focal new T2 lesion formation in the infrequent scans of the relapsing-remitting multiple sclerosis and secondary progressive multiple sclerosis cohorts, respectively. In comparison, only 0.1% and 0.0% of diffusely abnormal white matter to focal white matter lesions voxels overlapped with gadolinium enhancement. We conclude that diffusely abnormal white matter transforms into focal white matter lesions over time in both relapsing-remitting multiple sclerosis and secondary progressive multiple sclerosis. Diffusely abnormal white matter appears to represent a form of pre-lesional pathology that contributes to T2 lesion volume increase over time, independent of new focal inflammation and gadolinium enhancement.