Approach to Diagnosis and Management of an Idiopathic Inflammatory Myopathy

Idiopathic inflammatory myopathies are a group of autoimmune diseases characterized by proximal muscle weakness and inflammation of skeletal muscle. Specific forms of idiopathic inflammatory myopathy include polymyositis, dermatomyositis, and inclusion body myositis. Inflammatory myopathies have a bimodal distribution, occurring in patients between 10 and 15 years and 45 and 60 years of age, except for inclusion body myositis and malignancy-associated myositis, which occur in patients older than 50 years of age. The inflammatory myopathies are rare, with an annual incidence of 5 to 10 new cases per million population worldwide [1]. Like other autoimmune diseases, they occur predominantly in women (2:1), except for inclusion body myositis [1]. Persons of any race can be affected. Patients with an idiopathic inflammatory myopathy usually present with symmetric proximal muscle weakness. The differential diagnosis of muscle weakness is extensive, and careful evaluation of patients is essential for proper diagnosis and treatment. Prognosis differs with each specific disease; however, untreated inflammatory myopathies have a 5-year mortality rate of 50% [2]. Prompt diagnosis and treatment are important to limit morbidity and mortality associated with these diseases. This review discusses the diagnosis and treatment of polymyositis and dermatomyositis.