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IBM – Distinguishing Clinical Pattern — SCE Neurology MCQ

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ModeratePeripheral Neuropathy & NeuromuscularIBM – Distinguishing Clinical PatternSCE Neurology

A 55-year-old man presents with bilateral symmetrical progressive weakness affecting the proximal arms and distal legs (finger flexors and quadriceps most affected). He has been slowly progressive for 10 years. CK is mildly elevated (600 IU/L). Biopsy shows rimmed vacuoles and 15–18 nm tubulofilamentous inclusions. He has not responded to 2 years of prednisolone and IVIg. What key clinical feature distinguishes IBM from polymyositis?

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Correct answer: C — IBM has a characteristic pattern of weakness affecting finger flexors (grip weakness) and quadriceps (knee extension weakness) — this selective pattern, combined with treatment resistance and rimmed vacuoles on biopsy, distinguishes IBM from PM

IBM is the most common inflammatory myopathy in patients over 50. Key distinguishing features from PM: (1) selective pattern of weakness — finger flexors (deep finger flexor involvement causing weak grip) and quadriceps (knee buckling, falls) are affected early and prominently, (2) asymmetric weakness is common, (3) slow progression over years, (4) poor/no response to immunosuppressive therapy, (5) biopsy shows rimmed vacuoles with tubulofilamentous inclusions (in addition to inflammation). A: CK can be elevated in both. C: Skin rash distinguishes DM, not IBM from PM. D: Response distinguishes them — IBM is treatment-resistant. E: IBM has slow onset; PM can be subacute.

Reference: ENMC IBM Criteria (2011); ABN Inflammatory Myopathy Guidelines