The range of clinical and histopathological features suggest that there may be different disease mechanisms in different subgroups of myositis.​ There are at least four major histopathological features:

​i). One is predominated by endomysial inflammatory cellular infiltrates composed of CD8+ T-lymphocytes, plasma cells, macrophages and dendritic cells suggesting that the muscle fibres are targets of the immune reaction.​

ii). The other histopathological phenotype is predominated by perivascular inflammatory cellular infiltrates predominated by CD4+T cells, macrophages, dendritic cells (and sometimes B cells), suggesting that the blood vessels constitute the target of the immune system (Figure 1). This pattern is often seen in patients with dermatomyositis and patients who have a myositis associated with a connective tissue disorder.​

iii). The latter histopathological phenotype is that of sporadic IBM in which there are endomysial cell infiltrates as described in polymyositis (i). However, IBM is further characterised by rimmed vacuoles.​

iiii). Another feature is that of a necrotising myopathy in which necrotic muscle fibres are the main feature with no signs of inflammatory infiltrates.


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