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Anti-HMGCR Statin-Triggered IMNM — RACP Adult Medicine MCQ

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HardRheumatologyAnti-HMGCR Statin-Triggered IMNMRACP Adult Medicine

A 40-year-old woman on atorvastatin 80 mg for 2 years presents with progressive proximal weakness over 6 months. CK is 12000 U/L. Weakness does not improve despite statin cessation for 3 months. Anti-HMGCR antibody is positive. Muscle biopsy shows necrotising myopathy. What is the most likely diagnosis?

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Correct answer: A — Immune-mediated necrotising myopathy

Persistent proximal weakness and elevated CK despite statin cessation with positive anti-HMGCR antibody and necrotising myopathy on biopsy is statin-triggered IMNM (not simple statin myopathy, which resolves on cessation). Anti-HMGCR IMNM is an autoimmune condition triggered by statin exposure that persists after drug withdrawal. Treatment requires immunosuppression (corticosteroids + IVIG ± methotrexate or rituximab).

Reference: ENMC – 2024 – IMNM; AMH 2025 – Statin Myopathy