Inflammatory myopathies: A new landscape

Greater accuracy in clinical descriptions combined with advances in muscle histology and immunology have established that inflammatory myopathies (IMs), similarly to inflammatory rheumatic diseases, constitute a highly heterogeneous group of conditions. The topographic distribution, severity, and tempo of onset of the myopathy vary widely, and the histological findings distinguish at least five different profiles, which may reflect different pathophysiological processes. Most IMs are connective tissue diseases that can affect multiple organs, among which the most common targets are the skin, joints, and lungs. The extramuscular manifestations may antedate the muscular involvement and should therefore suggest a diagnosis of IM even in the absence of obvious muscle disease. About 20 different autoantibodies have been identified in patients with IM. Some are mutually exclusive and associated with specific combinations of clinical manifestations. Following the model of antisynthetase syndrome, about 10 syndromes associated with autoantibodies specific of IM have been identified. Thus, polymyositis is now emerging as a rare entity that is often mistaken for more recently described patterns of IM. No consensus exists to date about the classification of IMs. Nevertheless, the clinical manifestations, autoantibody profile, and muscle histology can be used to distinguish patient subgroups with fairly homogeneous patterns of complications, treatment responses, and outcomes. These subgroups are also characterized by specific genetic and environmental factors. The advances made in the nosology of IMs have benefited the diagnosis, personalization of treatment strategies, and understanding of pathophysiological mechanisms. They can be expected to assist in the development of specific treatments.