Myopathies: Dermatomyositis & Polymyositis

For further information, see CMDT Part 20-12: Idiopathic Inflammatory Myopathies (Polymyositis & Dermatomyositis)

Key Features

Essentials of Diagnosis

• 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

General Considerations

• 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 associated with another connective tissue disease (especially systemic lupus erythematosus [SLE])

• Malignancies most commonly associated with dermatomyositis in descending order of frequency

  • Ovarian

  • Lung

  • Pancreatic

  • Stomach

  • Colorectal

  • Non-Hodgkin lymphoma

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

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)

Differential Diagnosis

MUSCLE INFLAMMATION

• Polymyositis

• Dermatomyositis

• SLE

• Scleroderma

• Sjögren syndrome

• Inclusion body myositis

• Statin-induced immune-mediated necrotizing myopathy

• Trichinosis

OTHER CAUSES OF PROXIMAL MUSCLE WEAKNESS

• Polymyalgia rheumatica (PMR)

  • PMR patients are age > 50 and—in contrast to patients with polymyositis—have pain but no objective weakness

• Endocrine

  • Hypothyroidism

  • Hyperthyroidism

  • Cushing syndrome

• Alcoholism

• Drugs

  • Corticosteroids

  • Alcohol

  • Hydroxychloroquine

  • Statins

  • Clofibrate

  • Colchicine

  • Chloroquine

  • Emetine

  • Aminocaproic acid

  • Bretylium

  • Penicillamine

  • Drugs causing hypokalemia

  • Immune checkpoint inhibitors

  • Tryptophan

• 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

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