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Inflammatory Myopathies

Tina Arkee

Background

  • Heterogenous group of disorders that classically present with proximal muscle weakness, although some may present with systemic features (ILD, rashes, inflammatory arthritis, mechanics hands, Raynaud phenomenon)
  • Subtypes: dermatomyositis (DM), polymyositis (PM), antisynthetase antibody syndrome associated myositis, immune-mediated necrotizing myopathy (IMNM), inclusion body myositis (IBM)
  • Additional myopathies: statin-induced myopathy, metabolic (hypothyroid, electrolyte), viral/infection myositis, diabetic myonecrosis
  • Important point: PMR= painful, preserved strength vs inflammatory myopathy = painless weakness
  • Consider screening patients for underlying malignancy – up to 15-30% of cases are associated with malignancy

Presentation

  • Acute vs. Chronic
    • Acute or subacute: DM, PM, IMNM
    • Gradual and progressive: IBM
  • Distribution of weakness
    • Proximal and symmetric: DM, PM, IMNM
    • Distal and asymmetric: IBM (often involves weakness of finger flexor muscles)
  • Systemic signs or symptoms: fever, rash, dyspnea, dysphagia, arthritis
    • DM: Gottron’s papules, heliotrope rash, shawl sign, mechanics hands
    • Anti-synthetase syndrome: cough/dyspnea, arthritis, Raynaud’s or mechanics hands
    • IBM: can present with dysphagia; advanced cases may present with muscle atrophy
  • Consider patient’s medications including statins and chronic prednisone

Evaluation

  • Labs: CMP, TSH, CK level, LDH and aldolase
  • Extended myositis panel (screens of antibodies that include but are not limited to anti-Jo1 and anti-Mi2)
    • MDA5 Ab is associated with rapidly progressing ILD, which may present with AHRF
    • TIF1 and NXP2 Abs are associated with underlying malignancy
  • EMG: useful for ruling out neuromuscular etiologies
  • MRI extremity/affected muscle group: can help guide biopsies and evaluate edema on T2 and STIR (high signal intensity on these sequences may reflect active inflammation) and fatty replacement (reflects muscle damage) on T1.
  • Skin biopsy in dermatomyositis: “interface dermatitis,” notably the same findings as skin biopsy of lupus rashes
  • Muscle biopsy: gold standard for diagnosis
    • Do not do biopsy in same muscle as EMG done
    • Dermatomyositis: perifascicular and perivascular inflammatory infiltrate (CD4 T cells and pDCs)
    • Polymyositis: endomysial inflammatory infiltrate (CD8 T cells)
    • IMNM: muscle fibers in various stages of necrosis, scant inflammatory infiltrate
    • IBM: endomysial inflammatory infiltrate, intracellular vacuoles, protein aggregation

Management

  • Consult Rheumatology
  • 1st line for DM and PM: high dose steroids (1mg/kg/day prednisone) for 4-6 weeks PLUS steroid sparing treatment (methotrexate, azathioprine, or mycophenolate mofetil)
    • Can add IVIG for severe cases if cost is not an issue
  • 1st line for IMNM: high dose steroids for 4-6 weeks PLUS IVIG and rituximab
  • If statin induced: stop statin or switch to lower intensity
  • Refractory cases: abatacept or tofacitinib