Multiple sclerosis among patients with rheumatoid arthritis: nationwide, pilot study from Poland based on healthcare claims (2009–2021)

INTRODUCTION: Multiple sclerosis (MS) and rheumatoid arthritis (RA) are common, chronic immune‑mediated diseases that share pathogenetic pathways, but their epidemiologic relationship is uncertain. OBJECTIVES: We aimed to describe the association between MS and RA in the Polish population. PATIENTS AND METHODS: This pilot, nationwide study was based on administrative health care claims sourced from the Polish National Health Fund (NHF). Between 2009 and 2021, MS and RA cases were identified through a combination of the International Classification of Diseases, Tenth Revision claims, in conjunction with the Anatomical Therapeutic Chemical codes for medications or drug program records. Annual prevalence and incidence trends were assessed. Within the study timeframe, a retrospective cohort of RA patients and a general population-based comparator were recruited, with year‑ by year assessment of MS burden. The controls were matched 1:1 for age, sex, and calendar year of RA diagnoses, from individuals with any NHF contact in the corresponding year. RESULTS: In the years 2009-2021, 464 cases of concurrent MS were identified among patients with RA, whereas 813 MS cases were found in the matched control group (n = 307 519 per group). While the overall population trends suggest increased MS prevalence, annual incidence declined (P