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Inclusion Body Myositis — RACP Adult Medicine MCQ

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HardRheumatologyInclusion Body MyositisRACP Adult Medicine

A 65-year-old man presents with slowly progressive asymmetric weakness affecting finger flexors and knee extensors over 3 years. He has difficulty gripping objects and frequent falls. CK is mildly elevated (500 U/L). EMG shows mixed myopathic and neurogenic features. Muscle biopsy shows rimmed vacuoles with inflammatory infiltrate and inclusion bodies. He does not respond to prednisolone. What is the most likely diagnosis?

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Correct answer: C — Inclusion body myositis

Slowly progressive asymmetric weakness of finger flexors and knee extensors in an elderly male with mildly elevated CK, rimmed vacuoles and inclusion bodies on biopsy, and steroid-resistance is inclusion body myositis. IBM is the most common acquired myopathy over age 50 in men. Unlike PM/DM, IBM does not respond to immunosuppression. Anti-cN1A antibody is associated (~60%). No proven effective treatment exists; exercise therapy may slow decline.

Reference: ENMC – 2024 – IBM Criteria; eTG Neurology 2024