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.

Common features: 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. Additional symptoms by type: Dermatomyositis: red or purple rash, skin thickening; IBM: asymmetrical weakness, particularly in forearms or quadriceps; Polymyositis: symmetrical proximal weakness without skin involvement; IMNM: sudden, severe muscle weakness.

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), or environmental exposures.

Blood tests: Elevated creatine kinase (CK), aldolase, AST/ALT; Autoantibody panel: Myositis-specific and associated antibodies; EMG: Shows muscle irritation; MRI: Identifies inflamed muscles; Muscle biopsy: Confirms inflammation, necrosis, or inclusion bodies; Skin biopsy in cases of dermatomyositis; Cancer screening in older adults or dermatomyositis patients.

First-line: Corticosteroids (e.g., prednisone); Immunosuppressive drugs (methotrexate, azathioprine). Other therapies: 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.

Dermatomyositis and polymyositis: Good prognosis with early treatment; some achieve remission. IMNM: Varies, but often responds to aggressive immunotherapy. IBM: Progressive and often treatment-resistant, though progression is slow. Respiratory and swallowing difficulties can be life-threatening if not managed.

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).