Distinguishing Disease Activity and Damage in Myositis

Among the idiopathic inflammatory myopathies (IIM), disease activity relates to the inflammatory manifestations of illness in the muscles, skin, lungs, joints, and other systems that are reversible with treatment, whereas disease damage relates to irreversible changes and long-term side effects of medication and comorbid conditions that have accumulated over time. Most patients with IIM have a combination of active disease and damage. Disease activity and damage may be discordant in different organs at any point in time. There are multiple modalities to assess disease activity and damage including muscle strength and functional testing, serum muscle enzymes, imaging, muscle biopsy, electromyographic findings, extra-muscular symptoms, and skin findings. In the assessment of IIM, muscle strength and physical function are key components but, by themselves, cannot distinguish active disease and damage. While serum levels of muscle enzymes can discriminate active disease from damage or remission and correlate relatively well with disease activity in some IIM subgroups, such as polymyositis and necrotizing myopathies, they do not correlate well with muscle strength, endurance, or physical function in many patients. Magnetic resonance imaging of proximal muscles can demonstrate areas of muscle edema, suggesting muscle inflammation, and areas of muscle atrophy, fatty replacement, or fibrosis, reflecting chronic changes. Occasionally further testing that reveals an irritable electromyogram or muscle biopsy findings of inflammation is required to demonstrate active disease. Dyspnea may be related to pulmonary and non-pulmonary etiologies with difficulty ascertaining active lung inflammation from chronic fibrosis. A comprehensive evaluation utilizing echocardiography, high-resolution chest computed tomography, and respiratory physiology at rest and during exercise is often required. Most skin manifestations are signs of either disease activity (e.g., erythema) or disease damage such as cutaneous scarring. A careful skin examination can distinguish active disease from damage. Tender and swollen joints are often a sign of active arthritis that can be seen in some subsets of myositis like the anti-synthetase syndrome. Often a combination of modalities is most appropriate to assess disease activity and disease damage in individual IIM patients.