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.
