Polymyositis-dermatomyositis (PM-DM) is a group of acquired inflammatory myopathies with subacute or chronic onset, and its main pathological features are muscle fiber necrosis, regeneration and inflammatory cell infiltration in the interstitial tissue. The etiology of PM-DM is unknown and it is still an exclusionary diagnosis. All inflammatory myopathies that cannot find a clear infectious factor (such as viruses, bacteria, parasites, etc.) fall into this category, so it is also called idiopathic inflammatory myopathy. Since this group of diseases responds well to corticosteroid treatment, it is speculated that its pathogenesis may be related to autoimmune abnormalities. The main clinical manifestation of polymyositis (PM) is diffuse inflammatory myopathy with weakness and pain in the proximal limbs, neck and pharyngeal muscles. If combined with a typical rash, it is called dermatomyositis (DM). About 1/3 of patients may have other connective tissue diseases, and 1/10 patients may have tumors. Renal damage is seen in a minority of patients with polymyositis-dermatomyositis.