Introduction to Myositis

Abstract

To understand the myositis syndromes, also called the idiopathic inflammatory myopathies (IIMs), one must first understand the terminology used to describe the entities within them and how those entities are defined. Although what we know now as the IIMs were recognized as early as 1863, defining them is difficult because their definitions have changed over time, and new entities that comprise them have been discovered. Muscle diseases as a group are called myopathies, and myositis refers to a myopathy that has muscle inflammation as a prominent feature. When the cause of the myositis is unknown, it is classified as an idiopathic inflammatory myopathy. The IIMs have been subdivided in many ways. They were originally separated into dermatomyositis (DM) and polymyositis using the classic criteria proposed by Bohan and Peter that focused on muscle biopsy, electromyographic findings, laboratory tests (i.e., muscle enzyme elevation), physical exam features of proximal muscle weakness, and the classic rash(es) of DM. These diseases were further subdivided by their association with cancer (cancer-associated myositis), the age at onset (juvenile vs. adult versions of disease), and the presence of other autoimmune diseases (overlap connective tissue disease myositis). Later, additional IIM subgroups were added, including inclusion body myositis, clinically amyopathic dermatomyositis, and immune-mediated necrotizing myopathy. New findings relating to the importance of myositis autoantibodies have led to a parallel classification of the IIMs based on autoantibody status. Although there remains disagreement in the field as to the optimal ways to define and classify the IIM, there are currently concerted efforts to develop global consensus in this area by integrating all that has been learned about these evolving complex diseases.