Background The idiopathic inflammatory myopathies (IIM) are a rare, heterogeneous group of autoimmune muscle disorders, including dermatomyositis (DM), polymyositis (PM), and inclusion body myositis. Data on the epidemiology of IIM are limited and quite variable with the reported incidence and prevalence ranging from 0.4 to 5.8 per 100,000 person-years (PY) and 3.5 to 25.0 per 100,000 population, respectively. Differences in methods of case identification, small study sizes, geographical differences, and changes in coding and reporting practices may contribute to the variation.
Objectives To describe the epidemiology of IIM patients in England using medical record databases.
Methods By linking the General Practice Research Database (GPRD) and the Hospital Episode Statistics (HES) database, we identified IIM patients with a diagnosis code of myositis (DM, PM, juvenile DM, and unspecified dermatopolymyositis) in either database and retrospectively measured epidemiologic indicators of IIM.
Results From 2000-2009, 326 incident IIM cases were identified of which 62% were female and 81% were in those 45 years of age or older. The overall incidence of IIM was 1.9/100,000 PY (95% CI: 1.7-2.1/100,000 PY) ranging from 1.6 in 2000 to 2.0 in 2009. Among the incident cases, the 5- and 10-year survival was 77% and 74%, respectively. There were 465 prevalent IIM cases identified in 2009 corresponding to the overall prevalence of 27.2/100,000 people (95% CI: 24.8-29.8/100,000 people). The prevalence was positively associated with female gender and older age. Among prevalent IIM cases, 14% had a diagnosis of other autoimmune conditions including rheumatoid arthritis (9%), systemic lupus erythematosus (3%), systemic sclerosis (4%), or Sjogren’s disease (2%). In 2009, the hospitalization rate for IIM was 5.6 per 100,000 in the general populations.
Conclusions We found a higher IIM prevalence in England than previously reported in other countries. Studies that confirm the electronic diagnoses are needed to validate the findings.
Disclosure of Interest T. Tran Employee of: MedImmune LLC, A. Steffey Employee of: MedImmune LLC, H. Caspard Employee of: MedImmune LLC