Polymyositis & Dermatomyositis
A high-yield USMLE lesson on polymyositis and dermatomyositis covering the proximal-weakness pattern, CK/EMG findings, DM skin signs, myositis-specific antibodies, malignancy and ILD associations, biopsy/mechanism, key mimics, and treatment. Seven blocks: two prose, one highYield, two clinical vignettes, and two tables (autoantibodies plus a PM-vs-DM comparison). Verified accurate; the highYield now includes the top-tested DM-vs-PM biopsy/immunology contrast.
Overview
Polymyositis (PM) and dermatomyositis (DM) are idiopathic inflammatory myopathies defined by symmetric, proximal muscle weakness that develops insidiously over weeks to months. Patients struggle to climb stairs, rise from a chair, or comb their hair; muscle pain is often mild or absent. Serum creatine kinase (CK) is typically markedly elevated (often >10x normal), reflecting ongoing myofiber injury — though CK may be normal or only mildly raised in amyopathic DM. DM adds characteristic cutaneous findings and carries the strongest link to occult malignancy. Both can involve the pharynx (dysphagia), the diaphragm and lungs (interstitial lung disease), and the heart (myocarditis, arrhythmia), which drive morbidity and mortality. Recognizing the weakness pattern, the DM rash, and the antibody and biopsy signatures is the core of the exam question.
- Weakness: symmetric, proximal > distal; painless or mildly sore; spares eye/facial muscles (unlike myasthenia gravis)
- Labs: markedly high CK, plus aldolase, AST/ALT, LDH; ANA frequently positive
- EMG: myopathic - short, small, polyphasic motor units with fibrillations
- Biopsy: DM = perifascicular atrophy (complement/MAC on capillaries; CD4+ T & B cells); PM = endomysial CD8+ T cells invading MHC-I+ fibers
- DM skin: heliotrope rash (violaceous eyelids +/- periorbital edema), Gottron papules (violaceous plaques over MCP/PIP knuckles), shawl / V sign, mechanic's hands, periungual telangiectasias
- Anti-Jo-1 -> antisynthetase syndrome: ILD, mechanic's hands, Raynaud, arthritis, fever
- Malignancy: DM >> PM - perform age-appropriate cancer screening (highest risk with anti-TIF1-gamma / NXP2)
- ILD is a leading cause of death; screen with PFTs and HRCT
- Treatment: high-dose corticosteroids first line; add methotrexate or azathioprine (steroid-sparing), or IVIG for refractory disease
Myositis-Specific Antibodies
| Autoantibody | Association / clinical clue |
|---|---|
| Anti-Jo-1 (anti-synthetase) | Antisynthetase syndrome: ILD, mechanic's hands, Raynaud, arthritis, fever |
| Anti-Mi-2 | Classic DM skin (Gottron, heliotrope); good steroid response, better prognosis |
| Anti-MDA5 | Amyopathic / hypomyopathic DM; rapidly progressive ILD, skin ulcers |
| Anti-TIF1-gamma (p155) | DM with the highest malignancy risk in adults |
| Anti-NXP2 | DM with malignancy; calcinosis (especially juvenile DM) |
| Anti-SRP | Immune-mediated necrotizing myopathy - severe, treatment-resistant |
| Anti-HMGCR | Statin-associated necrotizing myopathy (persists after statin stopped) |
Stem: A 58-year-old woman reports 3 months of difficulty rising from chairs and lifting objects overhead. Exam shows symmetric proximal weakness, a violaceous rash on the eyelids, and scaly violaceous papules over the knuckles. CK is 3,200 U/L; ANA positive.
Diagnosis: Dermatomyositis - heliotrope rash + Gottron papules + proximal weakness + high CK.
Next steps: Support with EMG and confirm with muscle biopsy (perifascicular atrophy); send myositis-specific antibodies. Because DM strongly associates with occult malignancy, pursue age-appropriate cancer screening - mammography, Pap/pelvic exam plus transvaginal ultrasound (ovarian), colonoscopy, and chest/abdomen/pelvis imaging - especially with anti-TIF1-gamma. Begin high-dose corticosteroids with an early steroid-sparing agent.
Polymyositis vs Dermatomyositis
| Feature | Polymyositis (PM) | Dermatomyositis (DM) |
|---|---|---|
| Skin findings | None | Heliotrope, Gottron, shawl / V sign |
| Weakness | Proximal, symmetric | Proximal, symmetric |
| Inflammation site | Endomysial | Perimysial / perivascular |
| Effector cells | CD8+ T cells invade non-necrotic fibers (MHC-I up) | CD4+ T cells, B cells; complement (MAC) on capillaries |
| Biopsy hallmark | Endomysial CD8 infiltrate | Perifascicular atrophy |
| Malignancy link | Modest | Strong |
| Mechanism | T-cell-mediated (cellular) | Humoral / microangiopathy |
Stem: A 72-year-old man has 6 months of slowly worsening weakness. He struggles with fine finger movements and knee extension; weakness is asymmetric, involving finger flexors and quadriceps. CK is only mildly elevated, and steroids given elsewhere produced no benefit.
Diagnosis: Inclusion body myositis (IBM) - older men, insidious, distal + proximal and asymmetric, finger-flexor/quadriceps predilection, rimmed vacuoles on biopsy, poor steroid response.
Key mimics to separate:
- Polymyalgia rheumatica: proximal pain/stiffness without true weakness, normal CK, high ESR, dramatic response to low-dose steroids.
- Statin / anti-HMGCR necrotizing myopathy: proximal weakness with very high CK that persists after stopping the statin.
- Hypothyroid myopathy: proximal weakness with high CK - check TSH.
Mechanism & Management
Pathophysiology. DM is a humorally mediated microangiopathy: complement (MAC) deposition on endomysial capillaries causes ischemia and the characteristic perifascicular atrophy. PM is cell-mediated - CD8+ T cells attack non-necrotic fibers expressing MHC class I.
Diagnosis integrates the weakness pattern, high CK, myopathic EMG, myositis-specific antibodies, and biopsy (MRI can guide the biopsy site).
Treatment. First-line is high-dose glucocorticoids, tapered alongside an early steroid-sparing agent (methotrexate or azathioprine); IVIG helps refractory disease and is favored in DM. Monitor CK and strength to gauge response, and watch for glucocorticoid-induced (steroid) myopathy, which paradoxically worsens weakness with a normal or falling CK. Continue to screen for malignancy and interstitial lung disease - the principal drivers of mortality.
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