A 60-year-old man has a 3-month history of proximal muscle weakness (difficulty rising from chair, climbing stairs), elevated CK 4200 U/L, positive anti-Jo-1 antibody, and HRCT chest showing bilateral basal ground-glass opacities. The MOST likely diagnosis and the MOST important systemic association to screen for is:
- A Polymyalgia rheumatica; giant cell arteritis
- B Dermatomyositis without skin rash; screen for cardiac involvement only
- C Polymyositis with anti-synthetase syndrome; interstitial lung disease (already present) and screen for occult malignancy ✓
- D Inclusion body myositis; no systemic associations
Explanation
Anti-Jo-1 antibody is the most common anti-synthetase antibody (targeting histidyl-tRNA synthetase) and defines the anti-synthetase syndrome: inflammatory myopathy (polymyositis/dermatomyositis), interstitial lung disease (already manifest as ground-glass opacities), arthritis, Raynaud's, and mechanic's hands. ILD is the major cause of morbidity and mortality. All patients with idiopathic inflammatory myopathies (IIM) require screening for occult malignancy, which occurs in ~15–25% of adult IIM (highest risk with dermatomyositis and older age). Treatment includes high-dose steroids plus steroid-sparing agents (azathioprine, MMF).
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.