Merrilee Needham

The autoimmune myopathies are a heterogeneous group of disorders involving an immune attack on skeletal muscle and in some cases, also involving other body systems including skin, joints, heart, lungs, and the gastrointestinal system. There has been significant progress in our understanding of the aetiopathogenesis of these disorders in the new millennium, as well as the identification of several novel myositis-specific antibodies, allowing more precise classification into subgroups that helps to predict organ involvement, risk of underlying malignancy, prognosis and response to increasingly precise therapies targeting specific immune pathways. The term “autoimmune myopathies” discussed in this chapter includes the following major subgroups: (1) dermatomyositis (DM); (2) immune-mediated necrotising myopathy (IMNM); (3) inclusion body myositis (IBM); (4) overlap myositis (OM), including the antisynthetase syndrome (ASS); and (5) polymyositis. This chapter focuses on summarising our current understanding of the aetiopathogenesis of these disorders, their clinical presentations and investigation with an emphasis on autoantibodies, imaging and muscle biopsy findings, and approach to treatment.

Keywords: inclusion body myositis; sporadic inclusion body myositis (sIBM) - common myopathy in Caucasians over 50 years of age; epidemiology and genetic susceptibility; genetic susceptibility, linked to HLA-DR3 and 8.1 MHC ancestral haplotype (AH) in West Australian cohort; serum CK level - normal or mildly elevated (up to 10 times normal); no therapy currently - for stopping disease progression; intravenous immunoglobulin therapy (IVIG) - short-term benefit in selected cases; glucocorticoids and cytotoxic agents