Makindo Medical Notes

Related Subjects: |Relapsing Polychondritis |Reactive Arthritis |Raynaud's Phenomenon |Polymyositis |Dermatomyositis |Polyarteritis nodosa |Osteoporosis |Rheumatoid Arthritis |Systemic Sclerosis (Scleroderma) |Rheumatology Autoantibodies |Overlap Syndrome |Inclusion Body Myositis |Inflammatory Myopathies

💪 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

💊 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).