Abstract
Patients with relapsing-remitting multiple sclerosis (RRMS) may experience disability progression independent of relapse activity (PIRA), which can be an early sign of secondary progressive MS (SPMS). We defined persistent PIRA as ongoing sustained disability over the entire available follow-up period. However, PIRA events can regress over time. Identifying factors that predict PIRA persistence is of great interest as they can refine the definition of RRMS to SPMS transition. Equally, factors associated with the non-persistence of PIRA have potential treatment implications for patients suffering from a PIRA event. We conducted a study to examine risk factors for PIRA persistence and risk differences in long-term disability progression between persistent and non-persistent PIRA. In this cohort study, we included only patients who had already experienced a PIRA event and investigated the persistence of disability progression following their first PIRA event. Therefore, PIRA occurrence time was set as the baseline. Data were collected from the MSBase registry between April 1995 and January 2024, with a median follow-up of 8.7 years. The primary outcome was time to 6-month confirmed non-persistence of PIRA. Secondary outcomes comprised time to 6-month confirmed Expanded Disability Status Scale (EDSS) 6 and time to SPMS. A stratified Cox regression model was used to identify risk factors associated with non-persistent PIRA. We then matched persistent PIRA patients with non-persistent PIRA patients in a 1:1 ratio using propensity scores, and compared their risk of reaching EDSS 6 using the Cox regression model. We re-matched patients with complete Kurtzke Functional Systems Scores to compare their risks of reaching SPMS. We included 4713 RRMS patients with PIRA, of whom around one-third experienced a post-PIRA disability improvement, over a relatively long period (median of 2.6 years to improvement). Use of high-efficacy disease-modifying therapies (DMT) at baseline [hazard ratio, 1.22; 95% confidence interval, (1.08-1.38); P = 0.0015], lower baseline EDSS [hazard ratio, 0.73 (0.69-0.78); P < 0.0001] and younger age [per 10 years; hazard ratio, 0.84 (0.80-0.89); P < 0.0001] were associated with non-persistent PIRA. Patients with non-persistent PIRA had a hazard ratio of 0.19 [95% confidence interval, (0.15-0.25); P < 0.0001] for reaching EDSS 6 and 0.18 [(0.11-0.29); P < 0.0001] for reaching SPMS compared to patients with persistent PIRA. PIRA events slowly regress in one-third of patients. Patients with persistent PIRA had a substantially higher risk of reaching EDSS 6 and SPMS than those with non-persistent PIRA. Younger age, lower baseline EDSS, and use of high-efficacy DMT during PIRA events were associated with PIRA regression.