Pathogenesis of primary inflammatory myopathies

Common elements Primary inflammatory myopathies consist of dermatomyositis (DM), polymyositis (PM) and inclusion body myositis (IBM). They have certain characteristics in common: progressive muscle weakness and mononuclear inflammatory infiltrates in the muscle. Different mechanisms They may be distinguished by their histological features which also reflect their different underlying pathogeneses. The mechanism of DM would be complement-mediated microangiopathy, the inflammatory infiltrate of which would be secondary to ischemic phenomena, whereas in PM the muscle fibres are damaged by cytotoxic CD8 T lymphocytes. The factors triggering-off these two forms of myositis remain unknown. IBM may be a degenerative disease with accumulation of a variety of proteins within the fibres. The inflammatory infiltrate, which is similar to that seen in PM, may be a reaction to accumulated proteins.