Dermatomyositis is an inflammatory myopathy that affects children and adults, and involves skin and muscle, and sometimes other tissues. Treatment with corticosteroids and other immunosuppressant agents control the disease. The cause is unknown.
Dermatomyositis has been viewed as a humorally mediated autoimmune disease, in which autoantibodies are directed against the endothelium of muscle capillaries and that ensuing muscle damage is the result of insufficient blood supply (ischemia) to muscle. Major problems with this theory exist and are discussed in the following publications:
- Proposed immunologic models of the inflammatory myopathies and potential therapeutic implications
- Interferon-alpha/beta Mediated Innate Immune Mechanisms in Dermatomyositis
- Uncertainties in the Pathogenesis of Adult Dermatomyositis
Historically, much of the understanding of the mechanisms of disease in muscle in dermatomyositis has come from the study of muscle pathology under the microscope. Some of the characteristic features of dermatomyositis pathology are:
- Perivascular inflammation: The inflammatory cells seen in muscle biopsies with standard hematoxylin and eosin (H&E) staining are frequently accumulated around blood vessels in the spaces around muscle fascicles (perimysial vessels).
- Perifascicular atrophy: Perifascicular atrophy refers to the presence of small muscle fibers around the edges of fascicles (bundles of muscle fibers). The nature of perifascicular small fibers remains uncertain. Whether they are previously healthy fibers that have gotten smaller (“atrophy”), new fibers that are regenerating, or a mixture of the two is uncertain. They have been attributed to ischemia, a lack of blood supply, but we doubt this and have provided other evidence regarding their nature in the publications linked to above. Both perivascular inflammation and perifascicular atrophy are shown in the figure above.
- Capillary abnormalities: Abnormalities of muscle capillaries are frequent in dermatomyositis.
- Tubuloreticular inclusions are another characteristic feature of dermatomyositis muscle pathology. These are only seen on electron microscopy studies and are found within capillaries in muscle.