Idiopathic Inflammatory Myopathies (IIM)

Van Jones

Background

  • Characterized by skeletal muscle inflammation & symmetric proximal muscle weakness.
  • Differential diagnosis: statin-induced myopathy, electrolyte imbalance, thyroid dz,
    viral/infectious myositis, diabetic myonecrosis.
  • Incidence: 40-60 y, F>M (2:1).

Subgroups

  • Dermatomyositis (DM): skin (Gottron papules, heliotrope rash), malignancy in up to 25% depending on age and antibody status.
  • Polymyositis (PM): proximal muscle weakness.
  • Immune-mediated necrotizing myositis (IMNM): rapidly progresses, CK may be in 10k-50k range.
  • Anti-synthetase syndrome: rapidly progressive ILD, mechanic’s hands, Raynaud’s, arthritis.
  • Anti-MDA5: rapidly progressive ILD often resulting in ICU admission at diagnosis.
  • Inclusion body myositis (IBM): predominantly men with proximal hip weakness and finger flexor weakness. Poorly responsive to therapy.

Presentation

  • Typically chronic (esp IBM), but IMNM and MDA5 often acute.
  • Constitutional: fever, fatigue, weight loss.
  • MSK: symmetric painless muscle weakness (IBM may be asymmetric, prox. & dist.), non-erosive polyarthralgia of small joints.
  • Skin: Gottron papules & heliotrope rash (DM), mechanic’s hands, poikiloderma.
  • Pulmonary: ILD (rapidly progressive in anti-synthetase syndrome).
  • GI: dysphagia.
  • Vascular: Raynaud’s.

Evaluation

  • Diagnosis: clinical but can use 2017 EULAR/ACR Classification Criteria for Adult and Juvenile Myositis; weighted point system including age of onset, distribution, skin involvement, elevated muscle enzymes, EMG, muscle biopsy, myositis-specific antibodies.
  • CK (elevated), CMP, TSH, LDH, aldolase + extended myositis panel (screens commonly associated antibodies including anti-Jo1, anti-Mi2, etc.)
    • MDA5 Ab: for rapidly progressing ILD.
    • TIF1 and NXP2 Abs: for underlying malignancy.
    • EMG: rule neuromuscular etiologies.
    • MRI extremity/affected muscle group.
    • Skin biopsy in dermatomyositis: “interface dermatitis."
  • Muscle biopsy: gold standard - do not biopsy the same muscle where EMG is done!
    • DM: perifascicular & perivascular inflammatory infiltrate (CD4 T cells).
    • PM: endomysial inflammatory infiltrate (CD8 T cells).
    • IMNM: variable stage necrotic fibers, scant inflammatory infiltrate.
    • IBM: endomysial infiltrate, intracellular vacuoles, protein aggregates.

Management

  • If suspected, consult Rheumatology!
  • Prednisone 1mg/kg/day for 4-6w + MTX, AZA, or MMF.
  • If refractory: IVIG, rituximab, abatacept, tofacitinib.
  • IBM: supportive care (poor response to above).
  • Rapidly progressive ILD: IV methylprednisolone 1mg/kg + steroid-sparing agent.
  • Lifestyle: physical therapy, statin avoidance in IMNM, light avoidance in DM.
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