Survival and cancer risk in an unselected and complete Norwegian idiopathic inflammatory myopathy cohort

Objective: To utilise an exposed/unexposed cohort strategy for mortality and cancer analyses across unselected and complete cohorts of patients with idiopathic inflammatory myopathy (IIM) resident in south-east Norway (denominator population 2.6 million), between 2003 and 2012.

Method: IIM cases were identified by comprehensive searches through patient administrative databases followed by manual chart review. Polymyositis (PM) and dermatomyositis (DM) cases were classified by the Peter and Bohan and/or Targoff diagnostic criteria and sporadic inclusion body myositis (sIBM) by the European NeuroMuscular Centre (ENMC) criteria from 1997 and/or 2011. Every patient was matched for sex, age and residential area with 15 unexposed/non-IIM individuals drawn from the national population registry.

Results: Total mortality in the IIM cohort was 27% (87/326). Standardized mortality rate (SMR) was higher in DM (2.6) than PM (2.4) and sIBM (1.7). IIM-related causes of death were frequent (64%) and included cancer (all IIM subsets), aspiration (sIBM), pulmonary complications (PM/DM) and infections (PM/DM). Multivariate analyses identified age at diagnosis (PM and sIBM), positive anti-SSA (PM), cancer (DM), and DLCO < 60% (DM) as independent mortality risk factors. Cancer risk was increased in DM (standard incidence rate 2.0) and PM (SIR = 1.3), but not in sIBM (SIR = 0.9). Ovarian cancer was more prevalent in DM than in the general population (8.3% vs 1.1%).

Conclusion: Our results suggest that mortality rates and cancer risk remain elevated in DM, and to a lesser degree also in PM. Mortality rate was also increased in sIBM, but some deaths appeared to be due to potentially preventable causes.