Inflammatory myopathies are disorders of muscle in which there is clinical and laboratory evidence of an inflammatory process:
These are acquired muscle disorders as opposed to the inherited dystrophies and may be classified as follows:
Polymyositis Adult form
Dermatomyositis Inclusion body myositis
Inflammatory myopathy associated with malignant disease
Inflammatory myopathy associated with collagen vascular disorders, e.g. lupus erythematosus, systemic sclerosis, rheumatoid arthritis
Infective - viral, e.g. Coxsackievirus, echovirus, parasitic: e.g. toxoplasmosis, schistosomiasis.
Drug-induced - penicillamine, cholesterol lowering agents e.g. clofibrate, bezafibrate. POLYMYOSITIS/DERMATOMYOSITIS
There are two principal forms of inflammatory myopathy - polymyositis and dermatomyositis which are separated clinically by the dermatological findings in the latter. All age groups are affected. Annual incidence is 8 per 100000. These disorders are sporadic though familial cases are described.
An autoimmune basis for these disorders is supported by:
- response to immunosuppressive therapy.
- association with other known immunological disorders, e.g. collagen vascular disorders.
- elevated IgG in blood and presence of circulating autoantibodies, e.g. antinuclear antibody in some cases.
an increased incidence of ccrtain histocompatibility antigens (IILA antigens)-B8, DR3.
- the reproduction of a similar disorder in laboratory animals by injection of muscle extract with Freund's adjuvant.
Humoral and cell mediated immune mechanisms seem responsible for these disorders but the trigger factor(s) remain unknown.
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