The biological correlates of a more rapidly disabling disease course in AA patients are not clear 
. A recent MRI study comparing patients from the two ethnic groups has shown that AA patients with MS suffered more severe diffuse tissue damage, as measured by magnetization transfer ratio, and higher lesion volumes compared to CA patients 
. Our study adds to previous work by clearly showing higher T2- and T1-LV in the more disabled AA MS group than in the CA MS patients, further supporting the role of lesion accumulation and lesion evolution into black holes as correlates of the more aggressive clinical course in AA MS patients.
We did not find here any significant difference in global and regional brain volume between the two ethnic groups. This is somehow surprising considering the significant higher lesion burden in AA patients and might be explained by ethnic differences in brain volumes between AA and CA subjects 
. Admittedly, since the high-resolution 3D T1-weighted sequence was not available for the entire cohort of patients, the volumetric analysis was performed in a subgroup of 67 AA and 64 CA patients (see and ). Therefore, the lack of any difference in terms of brain atrophy could be due to the low number of patients analyzed. It is also possible, that our volumetric approach including global measures of whole brain, and WM and GM atrophy is not sensitive enough to detect regional differences in WM and GM volume between the two groups. However, when all patients, regardless of ethnic background, were stratified according to clinical status, those who required ambulatory assistance had significant lower NBV, NGMV and NWMV than those who did not require any assistance. This is consistent with the notion that not only lesion accumulation but also brain tissue loss contributes to development of irreversible disability 
Prospective longitudinal studies with large samples will be needed to track the development of atrophy and its clinical impact in AA patients with MS.
Limitations of our study lie in its retrospective design, in the assessment of disability on the requirement of ambulatory assistance without the inclusion of more sophisticated measures of physical and cognitive decline and, finally, in a routine MRI protocol, which did not include modern sequences for the evaluation of cortical lesions and damage in the normal-appearing tissue. Nonetheless, by studying a quite large sample of well demographically and clinically matched CA and AA patients with a homogeneous MRI protocol we were able to provide further insights into the mechanisms of disease progression in AA patients with MS.