[New classifications and pathophysiology of the inflammatory myopathies]

Review on new classifications of myositis linked with their different pathophysiology.

The classification of myositis refined recently, taking into account clinical (such as isolated muscle involvement or not, association with cancer...), immunological (presence or absence of auto-antibodies) and pathological criteria. This new classification has the ability to separate different clinical and physiopathological entities, having actually different prognosis factors. The most common inflammatory myopathies include dermatomyositis (DM), polymyositis (PM), inclusion body myositis (IBM), but also, overlap myositis (defined, among others, by the presence of auto-antibodies), and myositis associated to cancers. These myopathies may be also distinguished by their histological features which also reflect their different underlying pathogeneses. The mechanism of DM is complement-mediated microangiopathy, the inflammatory infiltrate being secondary to ischaemic damage. In PM the muscle fibres are damaged by cytotoxic CD8 T lymphocytes. 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 secondary to accumulated proteins.

These diseases with different pathogeny and prognosis should be treated by specific approaches. That is the reason why we initiated specific clinical trials for respectively inclusion body myositis and overlap myositis.