Pathological basis of diffusely abnormal white matter: insights from magnetic resonance imaging and histology

BACKGROUND: The pathological basis of diffusely abnormal white matter (DAWM) in multiple sclerosis (MS) has not been elucidated in detail, but may be an important element in disability and clinical progression. METHODS: Fifty-three subjects with MS were examined with T₁, multi-echo T₂ and magnetization transfer (MT). Twenty-three samples of formalin-fixed MS brain tissue were examined with multi-echo T₂ and subsequently stained for myelin phospholipids using luxol fast blue, for axons using Bielschowsky, immunohistochemically for the myelin proteins myelin basic protein (MBP) and 2',3'-cyclic nucleotide 3' phosphohydrolase (CNP) and for astrocytes using glial fibrillary acidic protein (GFAP). Regions of interest in DAWM were compared with normal appearing white matter. RESULTS: Fourteen of 53 subjects with MS in the in vivo study showed the presence of DAWM. Subjects with DAWM were found to have a significantly lower Expanded Disability Status Scale (EDSS) and shorter disease duration (DD) when compared with subjects without DAWM (EDSS: 1.5 versus 3.0, p = 0.031; DD: 5.4 versus 10.3 years, p = 0.045). DAWM in vivo had reduced myelin water and MT ratio, and increased T₂ and water content. Histological analysis suggests DAWM, which shows a reduction of the myelin water fraction, is characterized by selective reduction of myelin phospholipids, but with a relative preservation of myelin proteins and axons. CONCLUSIONS: These findings suggest that the primary abnormality in DAWM is a reduction or perturbation of myelin phospholipids that correlates with a reduction of the myelin water fraction.