Idiopathic inflammatory myopathies (IIMs), except for
sporadic inclusion body myositis (sIBM), present with subacute symmetrical weakness of the limb girdle muscles, an elevated serum
creatine kinase activity, and inflammatory cells in the muscle biopsy (necrotizing autoimmune
myopathy being an exception). In
dermatomyositis, additional
skin abnormalities are found. IIMs are nowadays subclassified into the following categories: (1)
dermatomyositis (DM), including (1a) classic
dermatomyositis, which may be associated with connective tissue disorders (
CTDs) and
malignancy, (1b)
juvenile dermatomyositis, and (1c) clinical
amyopathic dermatomyositis; (2)
polymyositis (PM) encompassing (2a) classical PM and (2b) nonspecific or overlap
myositis, associated with CTD; (3) autoimmune necrotizing
myopathy, associated with
malignancy,
statin use and CTD; and (4) sporadic IBM, sometimes associated with
CTDs. These conditions result from chronic immune activation after exposure to environmental risk factors in individuals with a predisposing genetic background. A strong association of
autoantibodies with distinct clinical phenotypes and prognosis is found in patients with
myositis.
Inflammatory myopathies, sporadic IBM excluded, are amenable to immunosuppressive and
immunomodulation therapies. The prognosis of IIM is not well known since long-term outcome and prognostic factors vary widely. Disease-related mortality rates in PM and DM are at least 10%. In DM mortality is attributed to
cancer and pulmonary complications.
Juvenile dermatomyositis has a low mortality rate. Because chronic immunosuppressive therapy is associated with significant side-effects, and many patients remain (partially) refractory to treatment, novel therapeutic agents that are safe and effective are needed.