Anti-HMGCR Myopathy — SCE Rheumatology MCQ

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HardMyositisAnti-HMGCR MyopathySCE Rheumatology

A 60-year-old woman with inflammatory myopathy has anti-HMGCR antibodies confirmed. She took Rosuvastatin for 2 years before symptom onset. She has never had skin features of dermatomyositis. Biopsy shows necrosis with minimal inflammation. Her CK is 12000 U/L. What is this subtype of myositis?

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Correct answer: B — Immune-mediated necrotising myopathy

The best answer is “Immune-mediated necrotising myopathy”. Anti-HMGCR immune-mediated necrotising myopathy (IMNM) is an autoimmune myopathy triggered by statin exposure. Unlike self-limiting statin myopathy it persists and progresses after statin withdrawal because the autoimmune process becomes self-perpetuating. Biopsy shows prominent myofibre necrosis and regeneration with minimal or absent inflammatory infiltrate distinguishing it from polymyositis (which shows endomysial lymphocytic infiltrate). CK is typically very high. Treatment requires immunosuppression (glucocorticoids IVIg Methotrexate) and statins must never be restarted. The competing options represent related diagnoses, tests or treatments, but they do not match the decisive phenotype, safety constraint or management point in this stem.

Reference: BSR idiopathic inflammatory myopathy guideline: https://academic.oup.com/rheumatology/article/61/5/1760/6555980