The diagnosis and classification of polymyositis.

[inclusion body myositis]

Polymyositis is classified as a separate entity among idiopathic inflammatory myopathies but it is considered as the least common since it is an exclusion diagnosis . This myopathy usually presents with subacute-chronic symmetric proximal limb weakness , although some extramuscular manifestations are common . Creatine kinase values may be increased up to 50 -fold in active disease . Muscle biopsy is characterized by endomysial inflammatory infiltrate consisting predominantly of CD 8 + T cells that invade healthy muscle fibres expressing the MHC-I antigen . Although serum autoantibodies , EMG and imaging techniques can help in diagnosis , muscle histopathology is a pivotal value . The clinical picture together with the pathological findings confers the also called PM pattern . A broad differential diagnosis is needed before concluding a diagnosis of pure PM . Sporadic inclusion-body myositis , toxic , endocrine and metabolic myopathies as well as muscular dystrophies are the major categories to be ruled out . Finally , a diagnostic algorithm for suspected cases of PM is also proposed .