DDx
Diagnostic Considerations
Conditions to consider in the differential diagnosis of polymyositis include the following:
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Hypokalemia
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Hypophosphatemia
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Hypothyroidism
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Myasthenia gravis
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Myopathies
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Inclusion body myositis
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Eosinophilic myositis
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Myositis ossificans
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Focal myositis
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Giant cell myositis
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Diabetic polyradiculopathy
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Metabolic myopathy
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Muscular dystrophy
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Myasthenia gravis
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Overlap connective-tissue diseases
Drug-induced myopathy may result from the following:
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Ethanol
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Antimalarials
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Clofibrate
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Colchicine
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Ketoconazole and other azole antifungal agents
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Statins
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D-penicillamine
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Vincristine
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Zidovudine (AZT)
Differential Diagnoses
Media Gallery
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Polymyositis. MRI of thighs showing increased signal in the quadriceps muscles bilaterally consistent with inflammatory myositis.
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Polymyositis. Histopathology slide shows endomysial mononuclear inflammatory infiltrate and muscle fiber necrosis.
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Polymyositis. Close view of muscle biopsy, showing chronic inflammatory infiltrate consisting of T lymphocytes, especially CD8+ T lymphocytes.
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Polymyositis. Hematoxylin and eosin frozen section shows polymyositis. Endomysial chronic inflammation is present among intact myofibers, which are remarkable only for increased variability of fiber size. Image courtesy of Roberta J. Seidman, MD.
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Polymyositis. Hematoxylin and eosin paraffin section shows polymyositis. Patient had dense endomysial inflammation that contains an abundance of plasma cells, which can be observed in patients with chronic polymyositis. Two necrotic myofibers, characterized by dense eosinophilic staining, are observed. Focal fatty infiltration of the muscle is present in the lower left quadrant of the photomicrograph. Image courtesy of Roberta J. Seidman, MD.
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Polymyositis. Hematoxylin and eosin paraffin section shows polymyositis. Photomicrograph illustrates attack on a nonnecrotic myofiber by autoaggressive T lymphocytes. On the left, the central myofiber is intact. On the right, it is obliterated by a segmental inflammatory attack. If immunohistochemistry were performed, expected findings would include an admixture of CD8 T lymphocytes and macrophages in the inflammatory process. Image courtesy of Roberta J. Seidman, MD.
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Polymyositis. Hematoxylin and eosin paraffin shows dermatomyositis. In dermatomyositis, inflammation is characteristically perivascular and perimysial. Vessel oriented approximately vertically in the center has a mild perivascular chronic inflammatory infiltrate. The endothelium is plump. The wall is not necrotic. A few lymphocytes in the wall of the vessel are probably in transit from the lumen to the external aspect of the vessel. Some observers may interpret this finding as vasculitis, but it is certainly neither necrotizing vasculitis nor arteritis. Image courtesy of Roberta J. Seidman, MD.
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Polymyositis. Hematoxylin and eosin paraffin section shows polymyositis. Longitudinal section shows a dense, chronic, endomysial inflammatory infiltrate. Image courtesy of Roberta J. Seidman, MD.
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