5Adult inflammatory myopathies
Section snippets
Epidemiology
The inflammatory myopathies are rare. The incidence rate of PM is estimated to be between 2.18×10−6 and 7.7×10−6. Oddis and Medsger found an increasing incidence in two separate studies performed 20 years apart, although this may reflect better diagnostic accuracy rather than a true increase.6., 7. The incidence appears to increase with age, highest rates being seen in the 35–44 and 55–64-year-old age groups. There is a paucity of epidemiological studies that address the incidence among
Pathogenesis
The pathogenesis of the autoimmune inflammatory myopathies remains elusive and the mechanism of cell injury in inflammatory myopathy unclear. Although there are cases of myositis in which frank inflammation is missing on muscle biopsy, it is generally accepted that inflammation is the major cause of muscle damage and that autoimmunity lies at the root of the inflammation. Autoantibodies—albeit not directed at muscle specific antigens—are found in the majority of cases of PM and DM, but not
Diagnosis
The definitive diagnosis of an inflammatory requires a muscle biopsy. A biopsy should be performed on every patient in whom the diagnosis of an inflammatory myopathy is considered, in order not only to confirm the diagnosis, but also to rule out the many conditions that resemble myositis clinically. In addition to a careful history, family history and laboratory tests, both electromyography and imaging studies, particularly magnetic resonance imaging (MRI), may serve as useful adjuncts to
Treatment
It is well known that there is a high morbidity and substantial mortality associated with the inflammatory myopathies so the prompt recognition and therapy of these diseases is essential. One series performed a retrospective chart review and determined the overall mortality rate to be as high as 22%, largely as a result of cancer and pulmonary complications.55 DM has to date proved the most treatable inflammatory myopathy, responding to steroids, intravenous immunoglobulin or immunosuppressants
Summary
The inflammatory myopathies of adults-principally DM, PM and IBM-are uncommon, have a high morbidity; are not infrequently the first sign of an associated malignancy; may be part of another connective tissue disease and resemble a number of rarer non-inflammatory diseases from which it is essential to distinguish them. Biopsy is an essential part of the work-up of every patient in whom an inflammatory myopathy is considered, as is a careful search for a possible associated malignancy. The use
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