To define the histopathological substrates of diffusivity abnormalities in cortical normal-appearing gray matter (cNAGM) and cortical lesions (CLs) of patients with multiple sclerosis (PwMS) by combining postmortem diffusion tensor (DT) MRI and histopathology.

Brain 3DT1 and DT postmortem in situ 3T MRI were obtained from 16 PwMS and 10 non-neurological controls (nNC). At subsequent autopsy,  110 tissue blocks (54 from PwMS, 56 from nNC) were dissected from 6 standardized cortical regions. Tissue regions were matched to 3DT1 and co-registered to DT sequence to obtain regional FA. Cortical density of myelin and microglia, axon number and orientation, number and size of neurons and glial cells were evaluated. Histopathological correlates of diffusivity alterations were assessed through linear mixed models for nested data.

CLs (subpial=77%) were found in 27/54 cortical regions from PwMS. cNAGM of PwMS had a significantly lower FA vs nNC (p=0.04), whereas in PwMS CLs vs cNAGM had a higher FA (p=0.003). Compared to nNC, cNAGM in PwMS showed a lower density of perpendicular and total axons (p≤0.02). In PwMS, CLs vs cNAGM had a lower density of myelin, parallel and total axons (p≤0.02). Regarding the pathological substrate, nNC cNAGM FA was positively associated with density of perpendicular, parallel and total axons (p≤0.005). In PwMS, cNAGM FA was positively associated with density of myelin, glial and total cells and perpendicular, parallel and total axons (p≤0.05), while CLs FA was positively associated with neuron and total cell density and negatively with microglia density (p≤0.05).

Demyelination, inflammation and variable loss of axons with different orientations, determine cortical microstructural abnormalities contributing to explain the different patterns of FA changes occurring in cNAGM and CLs of PwMS.