Epidemiology of the idiopathic inflammatory myopathies

The idiopathic inflammatory myopathies (IIMs) are a heterogeneous group of systemic autoimmune diseases that affect the skeletal muscles and can also involve the skin, joints, lungs and heart. The epidemiology of IIM is obscured by changing classification criteria and the inherent shortcomings of case identification using healthcare record diagnostic coding. The incidence of IIM is estimated to range from 0.2 to 2 per 100,000 person-years, with prevalence from 2 to 25 per 100,000 people. Although the effects of age and gender on incidence are known, there is only sparse understanding of ethnic differences, particularly in indigenous populations. The incidence of IIM has reportedly increased in the twenty-first century, but whether this is a genuine increase is not yet known. Understanding of the genetic risk factors for different IIM subtypes has advanced considerably. Infections, medications, malignancy and geography are also commonly identified risk factors. Potentially, the COVID-19 pandemic has altered IIM incidence, although evidence of this occurrence is limited to case reports and small case series. Consideration of the current understanding of the epidemiology of IIM can highlight important areas of interest for future research into these rare diseases. In this Review, the authors present epidemiological data relating to the idiopathic inflammatory myopathies and summarize the current understanding of the risk factors associated with these conditions, as well as highlighting areas that require further research. The idiopathic inflammatory myopathies (IIMs) are a heterogeneous group of rare, systemic autoimmune conditions with an incidence of 0.2-2 per 100,000 person-years, and a prevalence of 2-25 per 100,000 people.Changing classification criteria, biases in patient identification and heterogeneity within the group of IIMs affect the ascertainment of incidence and prevalence.The incidence and prevalence of IIM varies with age and gender, and between geographical regions, but is still unreported in many areas, particularly in South America, Africa and Asia.Further research is needed in non-white cohorts of IIM to better understand phenotypic, serological and genetic characteristics of ethnically diverse groups.Although mortality from IIM has improved with earlier diagnosis and treatment, substantial morbidity remains in association with malignancy, cardiovascular events, lung disease and infections, resulting in considerable healthcare costs.Understanding is growing of the unique genetic risk factors and gene-environment risk interactions in IIM, and of the influence of infections, medications (including statins and immune-checkpoint inhibitors), malignancy and geography.

Automated summary

Idiopathic inflammatory myopathies (IIMs) are a group of systemic autoimmune diseases that affect skeletal muscles and other body parts. The incidence and prevalence of IIM are influenced by various factors, including age, gender, and geographical location. Although genetic and environmental factors have been identified, more research is needed to understand the differences in different ethnic groups.