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The aetiology of amyotrophic lateral sclerosis (ALS) is poorly understood, and it is unclear if autoimmunity plays a role or if ALS is associated with inflammatory/autoimmune diseases such as rheumatoid arthritis (RA). There have been very few reports of patients with RA who develop ALS, and no epidemiological study examining the co-occurrence of the two diseases has been conducted.1–6 In December 2012, a cluster of ALS cases following treatment with tumour necrosis factor inhibitor (TNFi) was reported using data from the European system for reporting suspected adverse drug reactions, the EudraVigilance database.7 We sought to estimate the ALS rate in RA exposed and unexposed to TNFi using a nationwide, prospective, population-based cohort study design.
We used Swedish national registers to identify patients with RA (National Patient Register inpatient care 1969–2011 and outpatient care 2001–2011 and the Swedish Rheumatology Quality Register 1996–2011) and TNFi exposures (Swedish Biologics Register ARTIS 1999–2011). General population comparators were selected from the total population register matched on age, sex, county and date of the matched diagnosis of patients with RA …