Inclusion Body Myositis — SCE Rheumatology MCQ

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ModerateMyositisInclusion Body MyositisSCE Rheumatology

A 65-year-old man has a 4-year history of slowly progressive, mildly asymmetric weakness. Examination shows marked weakness of the deep finger flexors and knee extensors, with quadriceps wasting. There is no rash or sensory loss, thyroid function is normal, and creatine kinase is 600 U/L. There has been no objective improvement after 6 months of prednisolone and methotrexate. What is the most likely diagnosis?

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Correct answer: C — Inclusion body myositis

The diagnosis is inclusion body myositis. The highly discriminating pattern is slowly progressive, often asymmetric weakness of the deep finger flexors and knee extensors in an older man, accompanied by quadriceps wasting and only a modest CK rise. Failure to improve with glucocorticoid and methotrexate further supports IBM, although muscle biopsy is required for diagnostic confirmation. Polymyositis usually causes predominantly symmetric proximal weakness and is generally more treatment-responsive; many historical cases of treatment-resistant “polymyositis” represent IBM or another myopathy. Dermatomyositis would be supported by characteristic cutaneous features. Hypothyroid myopathy is excluded by normal thyroid function. Motor neuron disease may cause wasting and mild CK elevation but would usually have neurogenic features such as fasciculations or upper motor neuron signs rather than this characteristic selective muscle pattern.

Reference: Naddaf E, Roy B. Updates on Diagnostic Criteria of Inclusion Body Myositis. Rheumatic Disease Clinics of North America. 2025;51(4):595-607. https://pubmed.ncbi.nlm.nih.gov/39843353/