Inflammatory Myopathies¶
Greg Jackson
Background¶
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Inflammation and damage of skeletal muscle primarily leading to weakness
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Subtypes: Dermatomyositis, polymyositis, antisynthetase antibody syndrome associated myositis, immune-mediated necrotizing myositis, inclusion body myositis
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Additional myopathies: statin-induced myopathy, metabolic (hypothyroid, electrolyte), viral/infection myositis, diabetic myonecrosis
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Important point: PMR= painful, preserved strength vs inflammatory myopathy = painless weakness
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Hypomyopathic (or amyopathic) variants: skin and/or lungs are affected >> muscles
Presentation¶
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Proximal, symmetric muscle weakness in upper and lower extremities in all but inclusion body myositis which is typically more distal and asymmetric
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Dermatomyositis: skin involvement can include heliotrope rash of upper eyelids, Gottron’s papules (on MCP, PIP), “mechanic’s hands,” and shawl sign
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Raynaud’s and arthralgias can also be present
Evaluation¶
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Labs: BMP, TSH, CK level, anti Jo-1 antibody
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EMG: findings indicate myopathy
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MRI extremity/affected muscle group: muscle edema
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Skin biopsy in dermatomyositis: “interface dermatitis.”
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Muscle biopsy (do not do biopsy in same muscle as EMG done)
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Polymyositis, inclusion body myositis: T cell mediated muscle injury
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Dermatomyositis: immune complex/B cell mediated muscle injury
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Management¶
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Prednisone 1 mg/kg/day for 4-6 weeks with subsequent taper
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Consult Rheumatology
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MTX or AZA used if refractory to steroids, may need IVIG, rituximab
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If statin induced: stop statin or switch to lower intensity
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If dermatomyositis: evaluate for underlying malignancy