High prevalence of inclusion body myositis in Norway; a population‐based clinical epidemiology study

Abstract

Knowledge about the occurrence of sporadic inclusion body myositis (sIBM) in the general population is limited. Here, our aim was to identify and characterize every sIBM patient living in southeast Norway (population 2.64 million) from 2003 to 2012. Two sIBM case finding strategies were applied. First, all hospital databases in southeast Norway were screened to identify cases with sIBM-compatible International Classification of Diseases 10 (ICD-10) codes. These cases were then manually chart reviewed. Secondly, all muscle histology reports encoded with inflammation were independently reviewed. Finally, cases were classified according to the 1997 and the 2011 European Neuro-Muscular Centre (ENMC) Research Diagnostic Criteria for sIBM. The combined case finding strategy identified 3160 patients with sIBM compatible ICD-10 codes, and a largely overlapping cohort of 500 patients having muscle biopsies encoded with inflammation. Detailed retrospective review of chart and histology data showed that 95 patients met the 2011 ENMC sIBM criteria and 92 met the 1997 criteria. Estimated point prevalence of sIBM was 33/1000000, equal with both criteria sets. Mean age at diagnosis was 66.9years and mean diagnostic delay was 5.6years. Chart review revealed higher frequencies of dysphagia (94% vs. 65%) and anti-Sjøgren syndrome A antibodies (39% vs. 12%) in female sIBM patients (n=40) than in males. Coexisting rheumatic diseases were present in 25% of sIBM cases, with Sjøgren's syndrome in 10%. An estimated point prevalence of sIBM seven times higher than previously observed in Europe is reported. Our data show considerable diagnostic delay, a major challenge with new sIBM treatments in the pipeline.