The diagnosis and classification of polymyositis

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 CD8+ 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.