💪 Polymyositis = inflamed striated muscle with proximal weakness. If the skin is involved, it is termed Dermatomyositis (see topic). ⚠️ Both may be associated with underlying malignancy.

📘 About

Characterised by inflammation and necrosis of skeletal muscle fibres.
Polymyositis: Muscle inflammation only.
Dermatomyositis: Muscle inflammation + distinctive skin changes.

🧬 Aetiology

Possible viral trigger.
Genetic associations: HLA-B8, DR3.
Anti-Jo-1 antibody (anti–tRNA synthetase) → linked to interstitial lung disease, arthritis, Raynaud’s.
Most common age 40–60; female:male = 3:1.

🩺 Clinical Presentation

Systemic: fever, malaise, weight loss.
Progressive proximal muscle weakness (shoulders, hips) → difficulty climbing stairs, rising from a chair, lifting arms.
Can involve: hands, feet, face, bulbar muscles, pharynx, upper oesophagus → dysphagia.
Respiratory muscle involvement → risk of respiratory failure.
Rare: cardiac involvement.

✨ Key distinction: Dermatomyositis shows heliotrope rash, Gottron’s papules, “shawl sign”, and has a stronger cancer association.

🔬 Investigations

FBC: may show anaemia; ESR often normal.
Muscle enzymes: CK ↑↑ (up to 50-fold), LDH, AST elevated.
Muscle biopsy: chronic inflammation, fibre necrosis.
EMG: fibrillation potentials, low-amplitude polyphasic potentials, high-frequency discharges.
MRI: localises inflamed muscle.
Autoantibodies: ANA positive (~80%), Anti-Jo-1 positive in ~30% (poor prognosis, lung involvement).
Screen for malignancy (especially in older patients).

📋 Differential Diagnosis

🧾 Comparison of Inflammatory Myopathies


Feature	💉 Polymyositis	🌸 Dermatomyositis	🧓 Inclusion Body Myositis
Sex	♀ ≥ ♂	♀ ≥ ♂	♂ ≥ ♀
Age	Usually adults	Any age (children & adults)	> 50 years
Onset	Acute / insidious	Acute / insidious	Slow, insidious
Distribution of Weakness	Proximal ≥ distal	Proximal ≥ distal	Selective → finger flexors & quadriceps
Course	Often rapid	Often rapid	Gradual, progressive
Serum CK	↑↑ Very high	↑↑ Very high	Normal / mild ↑ (≤12-fold)
EMG	Myopathic ± neurogenic	Myopathic ± neurogenic	Myopathic ± neurogenic
Response to Tx	Good 👍	Good 👍	Poor 👎
Skin Changes	No ❌	Yes ✅ (heliotrope rash, Gottron’s papules)	No ❌
Malignancy Risk	No ❌	Yes ✅ (paraneoplastic association)	No ❌
Biopsy	Intrafascicular CD8+ T cell infiltrates	Perifascicular atrophy + CD4+/B-cell infiltrates	Endomysial CD8+ T cells + rimmed vacuoles

💡 Clinical Pearls

🌸 Dermatomyositis: Think skin + cancer risk (do a malignancy screen).
💉 Polymyositis: Proximal weakness, raised CK, responds well to steroids.
🧓 Inclusion Body Myositis: Older males, selective weakness (finger flexors, quadriceps), poor response to therapy.

💊 Management

Steroids: Prednisolone 0.5–1 mg/kg × 1 month, then gradual taper. CK falls quickly; strength recovery takes weeks.
Immunosuppressants: azathioprine, methotrexate, ciclosporin for resistant disease.
Some cases may be self-limiting over time.

Cases — Polymyositis

Case 1 — Classic proximal weakness 💪: A 49-year-old woman presents with gradual onset of difficulty climbing stairs and lifting shopping bags. No rash. Exam: symmetric proximal muscle weakness, especially in thighs and shoulders. Bloods: CK 3500 IU/L, aldolase raised, ANA positive. EMG: myopathic pattern. Diagnosis: polymyositis. Managed with high-dose corticosteroids and physiotherapy.
Case 2 — Malignancy-associated 🎗️: A 62-year-old man reports 3 months of proximal weakness and dysphagia. No cutaneous signs. Investigations: CK elevated, anti–TIF1-γ antibody positive. CT chest/abdomen/pelvis: bronchogenic carcinoma. Diagnosis: paraneoplastic polymyositis. Treated with immunosuppression and cancer-directed therapy.
Case 3 — Interstitial lung disease link 🫁: A 40-year-old woman with polymyositis presents with progressive exertional dyspnoea and dry cough. HRCT: interstitial lung disease. Serology: anti-Jo-1 antibody positive. Diagnosis: anti-synthetase syndrome with polymyositis. Managed with corticosteroids, azathioprine, and respiratory follow-up.

Teaching Point 🩺: Polymyositis is an idiopathic inflammatory myopathy causing proximal muscle weakness without the skin signs of dermatomyositis. Key associations include malignancy and interstitial lung disease. Always check autoantibodies (e.g. anti-Jo-1, anti-SRP, anti-TIF1-γ).

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Polymyositis