Idiopathic inflammatory myositis (IIM) is classified into four subtypes based on clinical and histopathological features. Polymyositis and dermatomyositis (PM/DM) are 2 of those subtypes in addition to immune-mediated necrotizing myopathy and inclusion body myositis. Apart from symmetric proximal muscle weakness, PM and DM have several clinical manifestations in common when compared to inclusion body myositis and necrotizing myopathy. The estimated prevalence of polymyositis and dermatomyositis (PM/DM) is 5 to 22 per 100,000 persons, and the incidence is approximately 1.2 to 19 million persons at risk per year. The incidence of myositis is increasing over time due to an increase in the detection rate. In the United States, the African American to white ratio of incidence is 3 to 4:1. In Europe, the prevalence significantly increases from the north to south, and this may be due to either environmental or genetic reasons.

Figure 1: Dermatomyositis and polymyositis

As for treatment, glucocorticoids are used empirically as the first-line treatment despite their various adverse effects. Concomitant treatment with steroid-sparing immunosuppressive agents reduce successfully initial glucocorticoid doses for the remission induction, the relapse risk during glucocorticoid tapering, and adverse effects of glucocorticoids. Treatment with biologics, seems promising in some IIM patients. Multi-target treatment with glucocorticoids and several steroid-sparing immunosuppressive agents is effective in refractory IIM patients.