(BSPOT33) Natural history of dermatomyositis and polymyositis in adults and paediatric populations: a network cohort study part of DARWIN EU®

Background: Dermatomyositis (DM) and polymyositis (PM) are rare disorders that cause muscle inflammation, weakness, and affect other organs. Juvenile forms, (JDM and JPM) are even rarer. There is little information on DM, PM, JDM and JPM, biomarkers, clinical manifestations, and treatments used in clinical practice.

Objectives: To describe and characterise dermatomyositis (DM), polymyositis (PM) and their juvenile forms (JDM and JPM), in terms of patient characteristics, occurrence of biomarkers, clinical manifestations, disease severity and treatments used.

Methods: Cohort study. We included incident patients between 2006 and 2022 in primary care databases from the UK (CPRD GOLD), Germany (IQVIA DA Germany) and Spain (SIDIAP); secondary care from France (CDW Bordeaux), and a biobank linked to primary and secondary care data from Estonia (EBB). We identified primary cases based on SNOMED codes and age at onset. We described patients’ demographics, comorbidities, tests performed, clinical manifestations and treatments.

Results: We identified 3969 DM, 2541 PM, 333 JDM and 32 JPM patients, predominantly women (~70%). Median age for DM and PM was 50-60 years old. JDM median age was between 9-13 years old, JPM was 10-13 years old.
In most databases, biomarkers such as C-reactive protein (CRP), erythrocyte sedimentation rate (ESR) and aspartate aminotransferase (AST) showed higher testing in the months before and after diagnosis of DM and PM. Autoantibodies were only visible in hospital databases.
As for clinical manifestations, they were low in all databases except CDW Bordeaux, the most common were muscle pain (around 15% for DM and PM), interstitial lung disease (8% in DM, 6% in PM), dysphagia (7% in DM, 11% in PM), Raynaud’s (3% in DM, 4% in PM) and thrombosis (2% in DM, 4% in PM).
Primary care databases only recorded disease-modifying anti-rheumatic drugs (DMARDs) and corticosteroids. The most used drugs in DM and PM before cohort entry were Glucocorticoids. Their use increased in the 3 months after diagnosis date and decreased afterwards. Use of DMARDs was low before index but increased in the months following diagnosis and for up to 3 years after. JDM and JPM had similar patterns of use, but with very low use of all medicines before diagnosis. In outpatient databases, we were able to see other treatments such as biologics (2-7%) or immunoglobin (14-16%).

Conclusions: Occurrence of biomarker testing showed great variability, with poor record of clinical manifestations and complications. As for treatment, primary care databases captured mostly DMARDs and corticosteroids; hospital databases included records of other treatments too. This information could be used to understand and improve the care of DM and PM patients in Europe.