Necrotising Myopathy — SCE Rheumatology MCQ

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HardMyositisNecrotising MyopathySCE Rheumatology

A 55-year-old man develops progressive, symmetrical proximal limb weakness while taking atorvastatin. Six months after the statin is withdrawn, his weakness continues to worsen and his creatine kinase remains 5600 U/L. Thyroid function is normal. MRI demonstrates muscle oedema, and muscle biopsy shows numerous necrotic and regenerating fibres with only sparse inflammatory infiltrates. What is the most likely diagnosis?

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Correct answer: E — Anti-HMGCR immune-mediated necrotising myopathy

This is anti-HMGCR immune-mediated necrotising myopathy. The discriminating features are progressive symmetrical proximal weakness, marked CK elevation that persists long after statin withdrawal, and a necrotising biopsy with relatively little inflammation. Anti-HMGCR antibodies should be sought and strongly support the diagnosis. Self-limiting toxic statin myopathy ordinarily improves after the drug is withdrawn. Hypothyroid myopathy is undermined by normal thyroid function. Inclusion body myositis usually progresses more slowly, often with asymmetric finger-flexor and quadriceps weakness, and generally causes a less striking CK elevation. McArdle disease presents from a younger age with exercise-induced cramps, intolerance and possible rhabdomyolysis rather than persistent progressive proximal weakness. Immune-mediated disease generally requires immunosuppressive or immunomodulatory treatment rather than statin withdrawal alone.

Reference: Sandoz Limited. Atorvastatin 10 mg film-coated tablets, Summary of Product Characteristics, section 4.4: Skeletal muscle effects. Text revised 14 May 2025. https://www.medicines.org.uk/emc/product/15739/smpc