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Essentials of Diagnosis
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Bilateral proximal muscle weakness
Characteristic cutaneous manifestations in dermatomyositis (Gottron papules, heliotrope rash)
Elevated creatine kinase, myositis-specific antibodies, diagnostic muscle biopsy
Increased risk of malignancy, particularly in dermatomyositis
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General Considerations
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An autoimmune disease of unknown cause characterized primarily by inflammation of muscles
Five clinically defined subsets
Juvenile dermatomyositis
Dermatomyositis
Polymyositis
Myositis associated with malignancy
Myositis may overlap other connective tissue diseases, especially
Malignancies most commonly associated with dermatomyositis in descending order of frequency
Ovarian
Lung
Pancreatic
Stomach
Colorectal
Non-Hodgkin lymphoma
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Progressive muscle weakness of the proximal muscle groups of the upper and lower extremities as well as neck over weeks to months
Leg weakness (eg, difficulty in rising from a chair or climbing stairs) typically precedes arm symptoms
No facial or ocular muscle weakness
Pain and tenderness of affected muscles (25%)
Initiation of swallowing may be difficult
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The characteristic rash is dusky red and may appear in malar distribution mimicking the classic rash of SLE
Erythema also occurs over other areas of the face, neck, shoulders, and upper chest and back ("shawl sign")
Periorbital edema and a purplish (heliotrope) suffusion over the eyelids are typical signs
Periungual erythema, dilations of nailfold capillaries, Gottron papules (raised violaceous lesions overlying the dorsa of DIP, PIP, and MCP joints) and Gottron sign (erythematous rash on the extensor surfaces of the fingers, elbows, and knees) are highly suggestive
A subset of patients with polymyositis and dermatomyositis develops the "antisynthetase syndrome," a group of findings including
Inflammatory nonerosive arthritis
Fever
Raynaud phenomenon
"Mechanic's hands" (hyperkeratosis along the radial and palmar aspects of the fingers)
Interstitial lung disease
Often, severe muscle disease associated with certain autoantibodies (eg, anti-Jo-1 antibodies)
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Differential Diagnosis
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OTHER CAUSES OF PROXIMAL MUSCLE WEAKNESS
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Polymyalgia rheumatica (PMR)
Endocrine
Hypothyroidism
Hyperthyroidism
Cushing syndrome
Alcoholism
Drugs
HIV myopathy
Hyperparathyroidism
Spinal stenosis
Osteomalacia
Mitochondrial myopathy
Limb-girdle muscular dystrophy
Peripheral and central nervous system disorders (eg, chronic inflammatory polyneuropathy, multiple sclerosis, myasthenia gravis, Eaton-Lambert disease, and amyotrophic lateral sclerosis) can produce similar weakness