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Myositis

Also Known As: Idiopathic Inflammatory Myopathy (IIM), Inflammatory myositis, Autoimmune myopathy, Polymyositis (PM), Dermatomyositis (DM), Inclusion Body Myositis (IBM), Immune-Mediated Necrotizing Myopathy (IMNM)
System: Immune System, Musculoskeletal System, Integumentary System, and Respiratory System
Autoantibodies:

  • ANA (Antinuclear Antibodies)

  • Anti-Jo-1 (most common, seen in antisynthetase syndrome)

  • Anti-Mi-2 (specific for dermatomyositis)

  • Anti-SRP (associated with IMNM)

  • Anti-TIF1-Îł (linked to cancer-associated dermatomyositis)

  • Anti-HMGCR (seen in statin-related necrotizing myopathy)

Primary Organ or Body Part Affected:

  • Skeletal muscles

  • Skin (in dermatomyositis)

  • Lungs (in interstitial lung disease, a common complication)

Acceptance as Autoimmune: Confirmed
Myositis

What is Myositis?

Myositis refers to a group of rare autoimmune diseases characterized by chronic inflammation of the muscles. This inflammation leads to muscle weakness, fatigue, and sometimes pain. The main types include polymyositis, dermatomyositis, inclusion body myositis, and immune-mediated necrotizing myopathy. Each type has slightly different symptoms and underlying immune activity, but all involve the immune system mistakenly attacking muscle tissue.

What are the signs and symptoms of Myositis?

Common features across types include:

  • Progressive muscle weakness (especially in the hips, thighs, shoulders, and neck)

  • Muscle fatigue or pain

  • Difficulty climbing stairs, lifting arms, or rising from a seated position

  • Trouble swallowing (dysphagia)

  • Shortness of breath (if respiratory muscles are affected)

Additional symptoms by type:

  • Dermatomyositis: red or purple rash (often on eyelids, knuckles, or chest), skin thickening

  • Inclusion Body Myositis (IBM): asymmetrical weakness, particularly in forearms or quadriceps, common in older adults

  • Polymyositis: symmetrical proximal weakness without skin involvement

  • IMNM: sudden, severe muscle weakness with high muscle enzyme levels and minimal inflammation on biopsy

What are the causes of Myositis?

Myositis is caused by the immune system mistakenly attacking healthy muscle tissue. Specific triggers are often unknown but may include:

  • Viral infections (e.g., HIV, HTLV-1, Coxsackie virus)

  • Genetic predisposition

  • Certain medications (e.g., statins in IMNM)

  • Cancer (in paraneoplastic dermatomyositis)

  • Environmental exposures

Diagnosis

Diagnosis often involves a combination of clinical evaluation, lab tests, and imaging:

  • Blood tests: Elevated creatine kinase (CK), aldolase, AST/ALT

  • Autoantibody panel: Myositis-specific and associated antibodies (see below)

  • Electromyography (EMG): Shows muscle irritation

  • MRI: Identifies inflamed muscles

  • Muscle biopsy: Confirms inflammation, necrosis, or inclusion bodies

  • Skin biopsy: In cases of dermatomyositis

  • Cancer screening: Particularly in older adults or dermatomyositis patients

Treatment

First-line treatments:

  • Corticosteroids (e.g., prednisone)

  • Immunosuppressive drugs (methotrexate, azathioprine)

Other therapies:

  • Intravenous immunoglobulin (IVIG)

  • Rituximab or other biologics (for refractory cases)

  • Physical and occupational therapy

  • Sun protection and topical steroids (for dermatomyositis rash)

IBM is typically resistant to treatment, and management focuses on physical therapy and fall prevention.

Prognosis

  • Dermatomyositis and polymyositis: Good prognosis with early treatment; some achieve remission

  • IMNM: Varies, but often responds to aggressive immunotherapy

  • Inclusion Body Myositis: Progressive and often treatment-resistant, though progression is slow

  • Respiratory and swallowing difficulties can be life-threatening if not managed

Prevalence

  • Estimated 5–10 cases per 100,000 people

  • More common in women and people aged 30–60 (except IBM, which affects older men more often)

  • Dermatomyositis and polymyositis are most common in middle-aged adults and children (juvenile dermatomyositis)

Citations

  • Dalakas MC. Inflammatory Muscle Diseases. N Engl J Med. 2015;372:1734-47.

  • Tiniakou E, Mammen AL. Idiopathic Inflammatory Myopathies and Autoantibodies. Clin Rev Allergy Immunol. 2017.

  • Lundberg IE, et al. 2017 European League Against Rheumatism/American College of Rheumatology Classification Criteria for Adult and Juvenile Idiopathic Inflammatory Myopathies. Arthritis Rheumatol. 2017.

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