Rationale and Background:
Idiopathic inflammatory myopathies (IIM) are rare and diverse autoimmune disorders characterized by muscle inflammation, weakness, and extramuscular manifestations affecting organs like skin, lungs, heart, and joints (Lundberg 2021, Sasaki 2018). The subgroups include dermatomyositis, antisynthetase syndrome, immune-mediated necrotizing myopathy, inclusion body myositis, polymyositis, and overlap myositis (Lundberg 2021). Despite their rarity, understanding the epidemiology of these disorders is essential to identify patterns and determinants.
Currently, there are no approved specific therapies for dermatomyositis (DM) and polymyositis (PM) based on randomized controlled trials. These diseases are challenging because of their associated morbidity and mortality (Aggarwal et al., 2017). Classification criteria developed by the European League Against Rheumatism (EULAR) and the American College of Rheumatology (ACR) help identify major IIM subgroups. Diagnostic tools involve elevated muscle-derived enzymes in serum, antinuclear antibodies, muscle biopsy, electromyography, and MRI (Lundberg 2021, Papadopoulo 2023).
The pathogenesis, treatment responses, and organ involvement vary among IIM subtypes, necessitating a deeper understanding of molecular pathways and auto-antigens (Lundberg 2021, Findlay 2015). Glucocorticoids are commonly used as first-line treatment, often combined with immunosuppressive agents like methotrexate, azathioprine, and others (Oldroyd 2022, Sasaki 2018). Rituximab shows promise in refractory cases (Valiyil R 2010-, Mok 2007). Although TNF’s role is implicated, anti-TNF treatments’ efficacy is limited (Lundberg 2021).
Pediatric cases require special consideration. Juvenile idiopathic inflammatory myopathies affect children and young individuals, involving muscles, skin, and other organs. Differences exist between juvenile and adult forms in terms of pathogenesis, autoantibody profiles, and treatment responses. Consensus guidelines help guide diagnosis and management (Belluti et al.).
In conclusion, idiopathic inflammatory myopathies encompass a spectrum of rare autoimmune disorders affecting muscles and various organs. Understanding their epidemiology, classification, diagnostic criteria, and treatment approaches is essential for improving patient outcomes and tailoring treatments, especially in paediatric cases.
Research question and Objectives:
The overall objective of this study is to describe and characterise dermatomyositis (DM), polymyositis (PM) and their juvenile forms (JDM and JPM), in terms of prevalence, natural history of the disease, disease severity, and treatment.
The specific objectives of this study are:
1. To estimate the yearly prevalence of DM and PM in adult (18+ years) and paediatric populations (less than 2 years, 2 to less than 6 years, 6 to less than 12 years, 12 to less than 18 years) overall and by sex.
2. To characterise patients and describe age at disease onset, for DM, PM, JDM and JPM.
3. To describe the occurrence in adults and children of biomarker measurements (e.g. creatinine kinase, tests for myositis auto-antibodies – INF-b levels, INF type I gene signature) before, at the time, and after a diagnosis of DM, PM, JDM and JPM.
4. To describe the occurrence of clinical manifestations (muscle inflammation, muscle weakness, connective tissue disease overlap, presence of calcinosis in children) before, at the time, and after a diagnosis of DM, PM, JDM and JPM.
5. To describe disease severity including organ involvement (skin, joints, lung, heart, GI tract) before, at the time, and after a diagnosis of DM, PM, JDM and JPM.
6. To describe treatment administered (including combinations and sequences) after a diagnosis of DM, PM, JDM and JPM
All results will be reported by database, overall, and by study periods (2006-2013, 2013-2020, and 2020-2022), and stratified by age and sex when possible.
Research Methods:
-Study design
Cohort study. We will include cohorts of first diagnosed DM, PM, JDM, JPM and new user cohorts of their treatments (for objective 6).
-Population
The source population will include all individuals eligible in the database between 01/01/2006 and end of the available date in each database. For objective 1, all patients active in the database at the start of all calendar year will be included. For objectives 2-5, two cohorts with be characterised, one with a 90-day prior history requirement from diagnosis date, and one without this requirement. For objective 6, a washout period of 365 days at the treatment ingredient level will be applied to capture new users of DM, PM, JDM and JPM treatment.
-Variables
DM, PM, JDM and JPM will be assessed as first occurrence of the codes specified in Annex 1. Additional age criteria, <18 year old at time of first diagnosis will be applied for JDM and JPM and >18 at time of first diagnosis for DM and PM. Co-morbidities and co-medications will be used for large-scale patient characterisation, identified as concept/code and descendants. A list of pre-specified co-morbidities, measurements, clinical manifestations, and severity markers will also be characterised and is included in Annex 1. Treatments of DM, PM, JDM, JPM will be identified using the codes included in Annex 1.
-Data sources
1. IQVIA Disease Analyzer Germany (IQVIA DA Germany), Germany
2. Sistema d’Informació per al Desenvolupament de la Investigació en Atenció Primària (SIDIAP), Spain
3. Clinical Data Warehouse of Bordeaux University Hospital (CDW Bordeaux), France
4. Clinical Practice Research Datalink (CPRD) GOLD, United Kingdom (UK)
5. Estonian Biobank (EBB), Estonia
-Sample size
No sample size has been calculated as this is a descriptive Disease Epidemiology Study where we are interested in the characteristics of all incident DM, PM, JDM and JPM patients. Based on a preliminary feasibility assessment the expected number of patients in the included databases for this study will be approximately 6,000 for DM, and 5,000 for PM. We will define JDM and JPM based on codes and age at diagnosis as they are likely to not be coded specifically as juvenile, so we are not able to determine a concrete sample size for the juvenile forms (we expect around 3-10% of them to be juvenile).
-Data analyses
Point prevalence of each outcome of interest (DM, PM, JDM, JPM), with every individual deemed to have the diagnosis from first occurrence until end of follow-up calculated on an annual basis as of the 1st January for each year, estimated overall and stratified by age and sex.
Age and sex at time of DM, PM, JDM, JPM diagnosis (index date) will be described for each of the generated study cohorts (Objective 2). Large-scale patient-level characterisation will be conducted for objectives 3 to 5. Occurrence of co-morbidities, measurements, clinical manifestations, and severity markers will be assessed for anytime –and up to 365 days before index date, for 364 to 91, for 90 to 31, and for 30 to 1 day before index date, and at index date. We will also report them for 1 to 90, 91 to 180, 181 to 365 days, 366 to 1095, 1096 to 1825 days, and 1826 days to any time post index date.
The number and percentage of patients receiving each of a pre-specified list of DM, PM, JDM and JPM treatments (see Appendix 1) and treatment combinations will be described at index date, 1 to 90, 91 to 180, 181 to 365 days, 366 to 1095, 1096 to 1825 days, and 1826 days to any time post index date. Additionally, sunburst plots and Sankey diagrams will be used to describe treatment patterns and sequences over time (objective 6).
For all continuous variables, mean with standard deviation and median with interquartile range will be reported. For all categorical analyses, number and percentages will be reported. A minimum cell count of 5 will be used when reporting results, with any smaller counts reported as “<5”. All analyses will be reported by country/database, overall and stratified by age groups and sex when possible (minimum cell count reached). Additionally, to capture treatments availability and changes over time, analyses will be further stratified by study periods (2006-2013, 2013-2020, and 2020-2022).