Inflammatory Myopathies

Abstract

Idiopathic inflammatory myopathies (IIMs) represent a heterogeneous group of muscular disorders characterized by acquired muscle weakness and inflammatory infiltrates in skeletal muscle tissue. The three main disease entities in this group are sporadic inclusion body myositis (IBM), polymyositis (PM), and dermatomyositis (DM). These diseases differ strongly from each other in terms of clinical and pathological features. Since 1975, the Bohan and Peter criteria have been applied, allowing only a distinction between dermatomyositis and polymyositis based on the presence or absence of skin abnormalities. Histopathologically, there was no difference between PM and DM. At that time, IBM was not generally recognized as a separate disease entity and usually was not differentiated from PM. It was considered a “treatment-resistant” polymyositis. In 1984, IIMs were looked at from a histopathological point of view, which allowed PM, DM, and IBM to be discriminated particularly on histopathological criteria. A cytotoxic CD8-positive T-cell reaction is the main feature of PM and IBM, whereas in DM a B-cell-mediated microangiopathy is most prominent. Recently, this classification has been challenged as numerous cases of histopathologically defined PM have been described that were nonresponsive to immunosuppressants and immunomodulating therapies, eventually mimicking the clinical picture of IBM. However, differentiation between PM and IBM is crucial because of the different therapeutic approaches required. This chapter focuses on the three most relevant IIMs: IBM, PM, and DM.