Several statistical surveys have revealed that cases diagnosed as non-specific myositis (NSM) account for 15–25% of cases of inflammatory myopathy based on muscle biopsy. A moderate degree of muscle fiber degeneration with or without inflammatory change and aberrant expression of MHC class I antigen suggest inflammatory myopathy; however, collectively this pattern does not match any specific diagnostic criteria established for other types of myositis. Notably, this disease group is important clinically and pathologically. An autoantibody that remains unknown or an overlapping condition that remains undetected to date could be etiopathogenetic contributors in these patients. Various types of myositis, which were previously classified as PM can be histopathologically classified as NSM. Primary biliary cirrhosis with anti-mitochondrial antibodies is occasionally associated with myositis and cardiomyopathy (Maeda, 2012). Furthermore, NSM may erroneously include non-inflammatory myopathies with secondary non-specific inflammation.
