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

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.

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.

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%).

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.