Impact of paramagnetic rim lesions on disability and race in multiple sclerosis: mediation analysis.

Objective: Black American (BA) multiple sclerosis (MS) patients experience greater disability compared to White American (WA) patients. Here, we investigated the role of paramagnetic rim lesions (PRLs), a subset of chronic active lesions, on race-related disability in MS.

Methods: We conducted a retrospective observational study comparing BA and WA MS patients. PRLs were identified through Quantitative Susceptibility Mapping (QSM) MRI. A causal mediation analysis explored the impact of PRLs on the relationship between race and disability, as measured by the Expanded Disability Status Scale (EDSS).

Results: The prevalence of PRLs in BA patients with MS was higher at 55% compared to WA patients at 39% (p = 0.022). A higher percentage of PRLs among all white matter lesions was observed with BA (8.01%) patients compared to WA (3.4%) patients (p = 0.003). In a regression analysis, controlling for significant patient-level covariates and income-level demographics, the percentage of PRLs was, on average, 4.61 points higher for BA patients than for WA patients (p = 0.003). In a separate regression analysis, accounting for covariates, BA patients exhibited significantly higher EDSS scores (p < 0.001). Further analysis demonstrated that the percentage of PRLs was a mediator in the association between BA patients and greater disability (p = 0.031). Higher proportion of PRLs in BA population accounted for 14% of the total effect of race on disability.

Interpretation: BA patients exhibit greater disability, in part, due to their higher proportion of PRLs. This study underscores the substantial impact of chronic active lesions on disability outcomes in this specific minority MS patient population.