[Idiopathic inflammatory myopathies from the viewpoint of a neurologist]

Idiopathic inflammatory myopathies (IIMs) are a heterogeneous group of systemic autoimmune disorders characterized by inflammation of skeletal muscle. In Japan, patients with IIMs usually visit a dermatologist, rheumatologist, or neurologist depending on the main symptom. Because most of the patients with IIMs have muscle weakness as a main symptom, muscle biopsy is usually performed to differentiate these from other non-inflammatory myopathies. Thus, neurologists in Japan tend to consider mostly the pathological findings in the diagnosis and classification of IIMs. From this background, IIMs have been classified into four pathologically distinct subsets: polymyositis, dermatomyositis, necrotizing autoimmune myositis, and sporadic inclusion body myositis. However, in clinical practice, the percentage of patients with typical pathological findings is generally not high. Therefore, other clinical factors, including rash or clinical complications (malignancy, collagen diseases, or interstitial pneumonitis), have been used along with pathological classification. With the recent discovery of new myositis-specific autoantibodies (MSAs) by rheumatologists and dermatologists, it has been suggested that the presence of a MSA is another important factor for classification. To develop useful methods of classification and to reveal the pathological mechanisms of IIMs, further collaborative studies by dermatologists, rheumatologists, and neurologists are necessary.