Inflammatory Muscle Diseases

The idiopathic inflammatory myopathies (IIMs) are a group of rare systemic diseases and include polymyositis (PM), dermatomyositis (DM), inclusion body myositis (IBM), and immune-mediated necrotizing myopathy. Similar to other autoimmune diseases, patients with IIM often have serum autoantibodies. Some of these antibodies are specific for myositis and are not seen in other rheumatic disorders. The IIMs usually present with muscle weakness and elevated muscle enzymes. In patients with DM, skin manifestations may be the initial presentation. The differential diagnosis of the IIMs is broad and includes drug-induced myopathies, neuromuscular disorders, muscular dystrophies, and metabolic and endocrine myopathies. To add to the complexity of the classification, some DM patients do not develop muscle weakness, referred to as clinically amyopathic DM. Muscle biopsy can be critical for the diagnosis of IIMs. The presence of perifascicular atrophy is strongly suggestive of DM, whereas the finding of rimmed vacuoles in the appropriate context suggests IBM. Muscle biopsy may also help differentiate between the IIMs and other myositis clinical mimics. Treatment of the IIMs is based largely on experience. Corticosteroids remain the mainstay of treatment. Steroid-sparing agents are frequently initiated at presentation while a steroid taper is attempted. In patients with refractory disease, intravenous immunoglobulin and biological medications may be tried. Inclusion body myositis is the most treatment resistant subset of the IIMs. Finally, an iatrogenic form of myositis has recently been described, precipitated by treatment with immune checkpoint inhibitors (ICS).