Idiopathic Inflammatory Myopathies: an Update on Classification and Treatment with Special Focus on Juvenile Forms

Juvenile inflammatory myopathies represent a heterogeneous group of rare and potentially fatal disorders of unknown aetiology, characterised by inflammation and proximal and symmetric muscle weakness. Beyond many similarities, specific clinical, laboratoristic and histopathologic features underlie different subsets with distinguishing demographic, prognostic and therapeutic peculiarities. Over time, several forms of inflammatory idiopathic myopathies have been described, including macrophagic myofascitis, immune-mediated necrozing myopathy and the spectrum of amyopathic dermatomyositis that include hypomyopathic dermatomyositis, inclusion body myositis and cancer-associated myositis occurring almost exclusively in adults. However, juvenile dermatomyositis is the most frequent in childhood, whereas polymyositis is relatively more frequent in adults. The aetiology is nowadays widely unclear; however, current theories contemplate a combination of environmental triggers, immune dysfunction and specific tissue responses involving muscle, skin and small vessels endothelium in genetically susceptible individuals. Myositis-specific autoantibodies, found almost exclusively in patients with myositis and myositis-associated autoantibodies, detectable both among patients with myositis and in subjects suffering from other autoimmune diseases, have an important clinical role because of their relation to specific clinical features, response to therapy and prognosis. The gold standard treatment for juvenile dermatomyositis is represented by corticosteroids, along with adjunctive steroid-sparing immunosuppressive therapies, which are used to counteract disease activity, prevent mortality, and reduce long-term disability. Further treatment approach such as biologic agents and autologous stem cell transplantation are emerging during the last years, in particular in patients difficult to treat and with poor prognosis. Therefore, a highly medical specialised approach is required for diagnosis and management of these conditions. This review comprehensively examines juvenile inflammatory myopathies focusing on clinical and laboratory classifications as well as on the current treatment approaches, referring in particular on biologic agents and latest therapeutic opportunities.