Polymyositis (PM), dermatomyositis (DM), and inclusion body myositis (IBM) form part of the idiopathic inflammatory myopathies (IIM), a heterogeneous group of rare autoimmune diseases characterized by an acquired proximal muscle weakness, raised muscle enzymes (including creatine kinase), inflammatory cell infiltrates in muscle biopsy tissue, electrophysiological abnormalities, and presence of circulating myositis-specific/myositis-associated autoantibodies. The underlying aetiology of IIM is poorly understood, but likely involves interactions between environmental and genetic risk factors. Myositis may also manifest in association with other connective tissue disorders. The predominant clinical presentation of IIM is skeletal muscle weakness, but many extramuscular features can also occur.
Access to good neuropathological support is essential in securing an accurate IIM diagnosis and excluding non-inflammatory myopathies, although IBM is often difficult to distinguish from PM. Antibody testing can help define IIM clinical subtypes, including cancer-associated myositis, predict prognosis, and help in optimizing treatment decisions. MRI can be invaluable for differentiating disease activity from damage, and detecting treatment-induced interval changes. Therapeutic effectiveness of new and existing treatments (where the evidence base remains poor) depends on making a prompt diagnosis and initiating early and appropriately aggressive treatment to prevent establishment of muscle damage.
This chapter attempts to summarize the salient features of IIM and update the reader about currently used diagnostics and treatment paradigms in this rare and understudied disease.