Some cohort studies have reported similar onset age and disability accrual in primary and secondary progressive MS (PPMS, SPMS); others have reported later onset and faster disability accrual in SPMS. Comparisons are complicated by differences in baseline disability and exposure to disease-modifying therapies (DMT), and by lack of a standardized definition of SPMS.
We compared hazards of disability accrual in PPMS and SPMS patients from the MSBase cohort using multivariable Cox models, applying validated diagnostic criteria for SPMS (Lorscheider et al., Brain 2016).
Inclusion required adult-onset progressive MS; ≥ 3 recorded Expanded Disability Status Scale (EDSS) scores; and, for SPMS, initial records with EDSS ≤ 3 to allow objective identification of SPMS conversion. Phenotypes were subgrouped as active (PPMS-A, SPMS-A) if ≥ 1 progressive-phase relapse was recorded, and inactive (PPMS-N, SPMS-N) otherwise. Disability accrual was defined by sustained EDSS increases confirmed over ≥ 6 months. Hazard ratios (HR) for disability accrual were obtained using Andersen-Gill Cox models, adjusted for sex and time-varying age, disability, visit frequency, and proportion of time on DMT or immunosuppressive therapy. Sensitivity analyses were performed using (1) PPMS and SPMS diagnosed since 1995, and (2) physician-diagnosed SPMS. Cumulative probability of reaching EDSS ≥ 7 (wheelchair required) was assessed (Kaplan-Meier).
5461 patients were included (1257 PPMS-N; 1308 PPMS-A; 1731 SPMS-N; 1165 SPMS-A). Age at progression onset was older in SPMS than PPMS (47.2 ± 10.2, vs. 41.5 ± 10.7 [mean ± SD]), and in the inactive subgroups of each phenotype. Hazard of disability accrual was decreased in SPMS relative to PPMS (HR 0.85; 95% CI 0.78–0.92); decreased by proportion of time on DMT (HR 0.99 per 10% increment; 0.98–0.99); and higher in males (1.18; 1.12–1.25). Relative to PPMS-N, hazard was decreased in SPMS-A (0.79; 0.71–0.87) but similar for PPMS-A (1.01; 0.93–1.10) and SPMS-N (0.94; 0.85–1.05). Sensitivity analyses corroborated these results. However, patients with SPMS-A reached EDSS ≥ 7 at younger ages (cumulative probability 30% by 57, vs. 64–66 for SPMS-N, PPMS-A, PPMS-N).
Progressive phase onset is later in SPMS than PPMS. Hazard of disability accrual during the progressive phase is lower in SPMS than PPMS. However, patients with SPMS-A reach wheelchair requirement younger than other progressive phenotypes, reflecting earlier progression onset versus SPMS-N, and greater disability at onset versus PPMS