Objectives. The aims of this study were to determine the incidence and prevalence of inflammatory myopathies (IMs), their epidemiological tendencies over time and their possible key determinants.
Methods. All original articles in English or French regarding the prevalence and/or incidence of IMs were searched. The methods of case ascertainment, epidemiological analysis and diagnostic criteria were systematically analysed.
Results. Forty-six articles published between 1966 and 2013 were found in which the incidence of IMs as a whole ranged from 1.16 to 19/million/year and their prevalence ranged from 2.4 to 33.8 per 100 000 inhabitants. Methodological heterogeneities limited comparisons, although certain epidemiological tendencies were highlighted. The relative incidence of DM may follow a latitudinal gradient in the northern hemisphere that may be explained by the immunomodulatory action of ultraviolet radiation. The prevalence of sporadic inclusion body myositis (sIBM) was correlated with the frequency of HLA-DR3. Juvenile myositis onset was non-random over seasons and/or time, consistent with a role of infectious diseases, although other environmental factors may be involved. Disparities according to sex, age and geographical origin were also found. The frequency of IM increased over time, which may reflect progress in diagnostic performance, although there is still a need to increase the level of awareness with regard to these diseases, especially sIBM, as attested by its considerably delayed diagnosis.
Conclusion. This first systematic literature review confirms the rarity of IM and may highlight certain genetic and environmental determinants of IM. There is a need for uniformity in diagnostic and classification criteria as well as more exhaustive case ascertainment to refine IM epidemiology.