Clinical Features of Myositis: Muscular Manifestations

The main muscular symptom of inflammatory myopathies is muscle weakness. It is mostly symmetrical and proximally located in patients with polymyositis (PM), dermatomyositis (DM), and immune mediated necrotising myopathy (IMNM). In IBM it is usually asymmetric and more often distal. In amyopathic DM weakness may not be present or clinically obvious and sometimes can only be found when tested for. Muscle pain may be present, but not as a main symptom, and if predominant, it should raise suspicion for other diagnosis. Muscular weakness is usually caused by disease activity, but also acquired muscle damage or drug-induced myopathy could be the reason. Dysphagia is a frequent symptom and is due to active inflammation of upper esophagus muscles. Treatment-resistant dysphagia may be the consequence of muscle fibrosis. MRI is a useful technique to visualize inflammation, which manifests as edema on MR images. Atrophy, fibrosis, and fatty infiltration can also be observed. A number of conditions may mimic inflammatory myopathy. The presence of ocular or facial muscle involvement, scapular winging, second-wind phenomenon, prominent distal weakness, exercise-induced cramps, fasting-associated symptoms, fasciculations, myotonia, and dominant pain but also nonresponse to treatment, absence of myositis autoantibodies, lack of any extramuscular symptoms, and possibly also history of the use of myopathic drugs should lead to suspicion of neuropathy, muscle dystrophy, metabolic myopathy, endocrinopathy, toxic myopathy, or fibromyalgia.