Emergencies in Idiopathic Inflammatory Myopathies

Idiopathic inflammatory myopathies (IIMs) are a group of disorders with autoimmune-mediated inflammation of the skeletal muscle and other distinct symptoms. The main five subgroups include dermatomyositis (DM), anti-synthetase syndrome (ASS), overlap myositis (OM), immune-mediated necrotizing myopathy (IMNM), and inclusion body myositis (IBM). Most subsets start with sub-acute, symmetrical muscle weakness of proximal arms and legs, except IBM, which presents with a slowly progressive weakness of proximal and distal muscles. Symptoms leading to emergency situations include severe muscle weakness, interstitial lung disease, myocarditis or arrhythmias, arthritis, skin ulcers, cancer, and dysphagia. The diagnosis is established by careful clinical workup including medical history, careful inspection of the skin, muscle strength assessment, laboratory testing including serum creatine kinase (CK) activity, and myositis-immunoblot for myositis-specific, myositis-associated, and anti-HMGCR antibodies, muscle imaging, and muscle biopsy. Depending on the myositis subtype and antibody profile, there may be an association with cancer. The standard treatment consists of high-dose corticosteroids and immunosuppressants, as well as add-on or escalation therapy (e.g. in the presence of interstitial lung disease), in case of severe weakness and unresponsiveness to steroids. Usually, immunosuppressants are added after 1 month of treatment in IMNM (Allenbach et al., Neuromuscul Disord 28(1):87–99, 2018). No effective treatment is available for IBM. Delay of treatment may cause irreversible damage to muscles and lungs. Therefore, an adequate workup and diagnosis is particularly important in patients with severe and rapidly progressive symptoms. This chapter provides a comprehensive overview of emergencies in IIM and how to treat them.