Dermatomyositis and polymyositis are autoimmune disorders in which the skeletal muscle is damaged by an inflammatory process dominated by lymphocytic infiltration. The term polymyositis is applied when the condition spares the skin, and the term dermatomyositis when polymyositis is associated with a characteristic skin rash. One-third of cases are associated with various connective tissue disorders, such as rheumatoid arthritis, lupus erythematosus, mixed connective tissue disorder, and progressive systemic sclerosis, and one-tenth with a malignancy. Inclusion body myositis is a distinct clinicopathologic entity characterized by the presence of vacuolated inclusions containing tubulofilaments in muscle.
The precise cause of these diseases is unknown, but interplay between host genetic factors, viral infection of muscle, and autoimmune mechanisms is probably contributory. Familial occurrence of these diseases, and the increased frequency of HLA-DR3 and -DRw52 antigens in patients, suggest an underlying genetic and immunologic predisposition. Experimental viral myositis can be induced in animals by coxsackievirus. A mild inflammatory myopathy can occur with influenza and coxsackieviruses in humans. However, the several electron-microscopic observations of virus-like particles in muscle fibers in dermatomyositis or polymyositis have not been confirmed by virus isolation or demonstration of rising viral antibody titers, and the disease has not been passed into animals by injection of extracts of skeletal muscles. Nevertheless, the presence of serum antibodies to several cytoplasmic ribonucleoproteins involved in translation [especially histidyl tRNA synthetase or Jo-1 and signal recognition particle (SRP)] may result from an immune response to an altered virus that serves as an immunogen in polymyositis. These antibodies probably represent a cross-reactive phenomenon.
A lymphocyte-mediated disease resembling polymyositis has been reported in laboratory animals injected with muscle antigens together with Freund’s adjuvant (experimental allergic myositis). Immunohistochemical and muscle co-culture studies indicate that muscle fiber necrosis in polymyositis and inclusion body myositis probably derive from activation of CD8+ T lymphocytes, accompanied by CD4+ T lymphocytes and macrophages present in the inflammatory infiltrates. In dermatomyositis, deposition of immunoglobulins and the C5b-9 complement membrane attack complex has been demonstrated on intramuscular blood vessels even in unaffected or minimally involved regions of muscle, suggesting that humorally mediated blood vessel damage initiates the angiopathy that precedes muscle destruction. The final pathway for muscle fiber damage in dermatomyositis may be T cell-dependent stimulation of B cells, with resultant antibody-mediated cytotoxicity. In inclusion body myositis, the accumulation of amyloid, paired helical filaments, and several other proteins typically seen at autopsy in brain specimens of patients with Alzheimer’s disease suggest some overlap of pathogenic mechanisms in these disorders.
A widely used classification of the dermatomyositis-polymyositis group. Other diseases uncommonly associated with polymyositis are sarcoidosis, giant cell myositis with thymoma, and myositis in systemic infections due to viruses, toxoplasma, or parasites. A focal infective myositis due to streptococcal or staphylococcal infection is mostly seen in the tropics. Focal nodular myositis is a variant of polymyositis in which focal areas of myositis cause hot, often painful, multifocal muscle masses.
Classification Of Polymyositis-Dermatomyositis
- Group I Primary idiopathic polymyositis
- Group II Primary idiopathic dermatomyositis
- Group III Dermatomyositis (or polymyositis) associated with neoplasia
- Group IV Childhood dermatomyositis (or polymyositis) associated with vasculitis
- Group V Polymyositis (or dermatomyositis) with associated collagen vascular disease
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Skeletal Muscle Pathology
The principal changes in muscle consist of infiltrates of inflammatory cells (lymphocytes, macrophages, plasma cells, ...