Myositis Basics/Who Gets Myositis

The idiopathic inflammatory myopathies (IIM), often referred to collectively but less specifically as “myositis,” have an estimated prevalence of ~14 per 100,000 with an annual incidence of ~8 per 1,000,000 population, which appears to be increasing over time. Any age group can be affected, but myositis commonly presents between 30 and 60 years of age. Differences are seen based on autoantibody subtypes as, for example, anti-signal recognition particle (SRP) seems more common in younger adults. Females are affected about twice as often as males, but again there are important differences when considering specific clinical subtypes as inclusion body myositis (IBM) is more common in males. Current research is focussed on understanding the complex interactions between genetic susceptibility and exposures to environmental triggers. The majority of genetic risk factors identified to date lie within the major histocompatibility complex (MHC), a region which contains genes responsible for antigen presentation, but a small number of genetic risk factors have been found outside this region and implicated in innate and adaptive immune functions. Important environmental associations have been already identified, including exposures to ultraviolet radiation, smoking and certain medications such as statins.