The exact pathogenesis of<unk>  and<unk>  is not fully understood. However, it is understood that<unk>  and<unk>  are the result of autoimmune attack but triggering factors are not well-known.<unk>  is caused by inflammation and degeneration of the muscles. In<unk> , CD8-positive cytotoxic T cells invade muscle fibers that express MHC class I antigens which may leads to fiber necrosis via the perforin pathway. Hypoxia may reduce creatine phosphate and adenosine triphosphate (ATP) levels in muscle and lead to fatigue and muscle weakness.<unk>  is caused by skin inflammation. In<unk> , activation and deposition of complements may lead to lysis of endomysial capillaries and muscle ischemia. Genes including HLA DRB1*0301 and HLA DQA1*0501 alleles, and tumour necrosis factor 308A might be associated with development of<unk>  and<unk> , especially in familial cases. Different conditions associated with<unk>  and<unk>  include respiratory and cardiac diseases, other connective tissue diseases, and malignancies. On microscopic histopathological analysis, partial invasion of lymphocytes within the muscle fibers and regenerating fibers with enlarged nuclei are characteristic findings of<unk> .
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