INFLAMMATORY MYOPATHIES is a topic covered in the Harrison's Manual of Medicine.

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The most common group of acquired and potentially treatable skeletal muscle disorders. Three major forms: dermatomyositis (DM), inclusion body myositis (IBM), and polymyositis (PM). Usually present as progressive and symmetric muscle weakness; extraocular muscles spared but pharyngeal weakness (dysphagia) and head drop from neck muscle weakness are common. Respiratory muscles may be affected in advanced cases. IBM is characterized by early involvement of quadriceps (leading to falls) and distal muscles; IBM may have an asymmetric pattern. Progression is over weeks or months in PM and DM, but typically over years in IBM. Skin involvement in DM may consist of a heliotrope rash (blue-purple discoloration) on the upper eyelids with edema, a flat red rash on the face and upper trunk, or erythema over knuckles (Gottron’s sign). A variety of cancers are associated with DM. Features of each disorder are summarized in Table 195-1.

CharacteristicPolymyositisDermatomyositisInclusion Body Myositis
Age at onset>18 yearsAdulthood and childhood>50 years
Familial associationNoNoYes, in some rare cases
Extramuscular manifestationsYesYesYes
Associated conditions
 Connective tissue diseasesYesaScleroderma and mixed connective tissue disease (overlap syndromes)Yes, in up to 20% of casesa
 Systemic autoimmune diseasesbFrequentInfrequentInfrequent
 MalignancyNoYes, in up to 15% of casesNo
 DrugsdYesYes, rarelyNo
 Parasites and bacteriaeYesNoNo
aSystemic lupus erythematosus, rheumatoid arthritis, Sjögren’s syndrome, systemic sclerosis, mixed connective tissue disease.
bCrohn’s disease, vasculitis, sarcoidosis, primary biliary cirrhosis, adult celiac disease, chronic graft-versus-host disease, discoid lupus, ankylosing spondylitis, Behçet’s syndrome, myasthenia gravis, acne fulminans, dermatitis herpetiformis, psoriasis, Hashimoto’s disease, granulomatous diseases, agammaglobulinemia, monoclonal gammopathy, hypereosinophilic syndrome, Lyme disease, Kawasaki disease, autoimmune thrombocytopenia, hypergammaglobulinemic purpura, hereditary complement deficiency, IgA deficiency.
cHIV (human immunodeficiency virus) and HTLV-1 (human T cell lymphotropic virus type 1).
dDrugs include penicillamine (dermatomyositis and polymyositis), zidovudine (polymyositis), statins (necrotizing, toxic, or autoimmune myositis), and contaminated tryptophan (dermatomyositis-like illness). Other myotoxic drugs may cause myopathy but not an inflammatory myopathy (see text for details).
eParasites (protozoa, cestodes, nematodes), tropical and bacterial myositis (pyomyositis).

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