Version July 2025
COX: cytochrome c oxidase; EMG: electromyography; IBM: sporadic inclusion body myositis; MR: magnetic resonance; US: ultrasound.
* An "irritable myopathy" demonstrates increased insertional activity, fibrillations, positive waves, and early recruitment of short-duration, small-amplitude polyphasic motor unit action potentials (MUAPs). Refer to topic for further discussion.
¶ A typical pattern of involvement includes edema, atrophy, and fatty replacement of the muscles of the anterior compartment of the thigh with relative sparing of the medial and posterior compartments. Completely normal imaging of the thigh muscles makes IBM highly unlikely.
Δ Refer to relevant topics for discussion of these tests.
◊ Confidence in the diagnosis is increased if the biopsy demonstrates other features of IBM (eg, rimmed vacuoles, protein aggregates, COX-negative fibers). Refer to topic for discussion.