[Diagnosis of Idiopathic Inflammatory Myopathy: A Muscle Pathology Perspective]

Idiopathic inflammatory myopathies are historically classified into polymyositis and dermatomyositis based on the presence or absence of skin lesions. Recently, however, a more histology-oriented classification into 6 subtypes has been proposed. The subtypes include dermatomyositis, polymyositis, inclusion body myositis, immune-mediated necrotizing myopathy, anti-synthetase syndrome, and non-specific myositis. With strict criteria applied, polymyositis is now extremely rare, while immune-mediated necrotizing myopathy is the most common among all inflammatory myopathies and is often associated with autoantibodies including those for signal recognition particle (SRP) or 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR).