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For further information, see CMDT Part 20-12: Immune-Mediated Inflammatory Myopathies

Key Features

Essentials of Diagnosis

  • Progressive muscle weakness

  • Characteristic cutaneous manifestations in dermatomyositis (Gottron papules, heliotrope rash)

  • Elevated creatine kinase, myositis-specific antibodies, diagnostic muscle biopsy

  • Mimics include infectious, metabolic, or drug-induced myopathies

General Considerations

  • An autoimmune disease of unknown cause characterized primarily by inflammation of muscles

  • Immune-mediated inflammatory myopathies include

    • Polymyositis

    • Dermatomyositis

    • Myositis resulting from a rheumatic disease or overlap syndrome

    • Inclusion body myositis (IBM)

      • Affects older men

      • Characterized by more distal weakness in the upper extremities and is generally less symmetric

    • Immune-mediated necrotizing myopathy

  • These disorders are characterized by progressive muscle weakness, and all but IBM demonstrate an inflammatory infiltrate in muscle tissue

  • Myositis may also overlap with other connective tissue diseases, especially

    • Systemic sclerosis (scleroderma)

    • Systemic lupus erythematosus

    • Mixed connective tissue disease

    • Sjögren syndrome

  • Immune-mediated necrotizing myopathies include those associated with the signal recognition particle (SRP) or with anti-3-hydroxy-3-methylglutaryl coenzyme A reductase (anti-HMGCR) autoantibodies in the setting of statin use

  • Malignancies most commonly associated with dermatomyositis

    • Lung

    • Ovarian

    • Breast

    • Colorectal

    • Cervical

    • Bladder

    • Nasopharyngeal

    • Esophageal

    • Pancreatic

    • Renal

Demographics

  • Peak incidence: fifth and sixth decades

  • Women are affected twice as commonly as men

Clinical Findings

Symptoms and Signs

POLYMYOSITIS

  • 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

  • Respiratory muscle weakness can be severe enough to cause CO2 retention and respiratory failure

DERMATOMYOSITIS

  • 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)

INCLUSION BODY MYOSITIS

  • Tends to mimic polymyositis

  • A common cause of "treatment-resistant polymyositis"

  • Typical patient is White, male, and over the age of 50

  • Onset is ...

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