Rheumatology
Polymyositis
다발근염
Autoimmune muscle inflammation with symmetric proximal weakness
- How common
- Rare (2–5 per 100,000)
- Typical age
- Ages 30–60
What is it?
CD8+ T cells directly attack MHC-I-expressing muscle fibers causing muscle damage (unlike DM, not complement/B-cell mediated).
Commonly affected: Proximal muscles (shoulder/pelvic girdle)
How it develops
- Upregulated MHC-IAberrant MHC-I expression on muscle fibers
- CD8 T-cell infiltrationCD8 T cells invade endomysium
- Direct muscle attackPerforin/granzyme cause myofiber death
- Proximal weaknessSymmetric proximal weakness progresses
Symptoms
- Symmetric proximal weaknessDifficulty with stairs, standing, lifting
- DysphagiaPharyngeal muscle involvement
- ILD (especially anti-Jo1+)Exertional dyspnea, antisynthetase syndrome
- MyalgiaMild–moderate tenderness
- ArthralgiaNon-erosive arthritis
How it is examined
- Muscle strength (MRC)Proximal weakness pattern (≤4/5)
- Muscle enzymes + autoantibodiesCK >10x, aldolase, anti-Jo1, anti-SRP
- EMG + muscle biopsyMyopathic EMG; biopsy shows endomysial CD8 infiltrate
Imaging
X-ray low yield; muscle MRI preferred.
- Nonspecific
Muscle MRI (T2/STIR) shows edema; HRCT for ILD.
- Muscle edema (T2 high)
- NSIP/UIP pattern ILD
Non-surgical care
- High-dose corticosteroidsPrednisone 1mg/kg/day initial
- Steroid-sparing ISMethotrexate, azathioprine
- IVIG (refractory)Effective for dysphagia and severe weakness
- RehabilitationStrength recovery and prevent atrophy
- Cancer screeningScreen for lung, breast, ovarian, etc.
Conservative options are generally tried first. Medications listed here can have side effects — discuss them with your prescriber.
When surgery is considered
Refractory disease or progressive ILD
Procedures that may be discussed
- Rituximab
- MMF
- Cyclophosphamide (severe ILD)
- Tofacitinib (JAK inhibitor)
Outlook
5-year survival 75–85%. Worse with ILD, dysphagia, malignancy.