Review: immune-mediated necrotizing myopathies--a heterogeneous group of diseases with specific myopathological features.

Immune-mediated necrotizing myopathies (IMNMs) are now well recognized among the so-called idiopathic inflammatory myopathies (IIMs), which also comprise dermatomyositis (DM), polymyositis (PM), sporadic inclusion body myositis (sIBM) and non-specific myositis. All of these conditions are defined on the basis of distinct clinical symptoms, in combination with results derived from muscle biopsy and additional data, such as measurement of the serum creatine kinase (CK) level as well as myositis-associated and myositis-specific autoantibodies, electromyography (EMG) and modern imaging techniques. Importantly, diagnosis of one of the above mentioned myositis forms implies a specific clinical syndrome or a distinct disease. However, there is considerable clinical heterogeneity, and overlap requiring further diagnostic precision. Classification and subclassification of IIMs are highly debated and the subjects of intense research, especially as clinical trials with anti-inflammatory agents should follow universally defined and accepted criteria. This review focuses on the description of the spectrum of immune-mediated necrotizing myopathies with an emphasis on their myopathological features.