Epidemiology

Due to changing classification criteria, the epidemiologic study of idiopathic inflammatory myopathy (IIM) is challenging, with biases in patient identification and heterogeneity within the subtypes of IIMs.[1]

IIMs are rare, with a reported incidence rate of 0.2 to 2 per 100,000 person-years, and prevalence rate of 2 to 25 per 100,000 people. The incidence and prevalence of IIM varies with age, gender, and geographic regions. There is sparse data on ethnic differences, particularly in indigenous populations in South America, Africa, and Asia.[1]

There is evidence to suggest that the incidence of dermatomyositis and polymyositis is bimodal, with peaks in childhood and/or adolescence, and again in later life.[1] In adults, the median range of reported age of onset is wide, but most cases occur between 50 and 60 years, with a female preponderance of 60% to 80%.[1] Evidence suggests that polymyositis and dermatomyositis have a female to male ratio of 2:1, and are more commonly found in black people compare with white people with a ratio of 2.8:1.[8][9][10][11]​ Immune-mediated necrotizing myopathy as a subtype occurs more frequently in females, although gender bias is less marked in anti-HMGCR-positive immune-mediated necrotizing myopathy.[1]

The risk of cancer-associated myositis increases in the older age groups.[1]

Patients with inclusion body myositis have characteristics distinct from the other IIM subtypes. Typical onset of symptoms occurs after the age of 50 years, with a male preponderance of 60% to 70%, and has been demonstrated to occur more frequently in white people, although the exact racial differences are not clear.[1][12] The prevalence of inclusion body myositis has been reported as low as 0.07 per 100,000 people to as high as 19.2 per 100,000 people.[13][14]​​​

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