What measures can be taken to maximize muscle biopsy yield? How can muscle magnetic resonance imaging (MRI) help?
• Biopsy a muscle that is clearly weak, but not severely so.
• Biopsy the muscle contralateral to one that is abnormal by EMG (i.e., perform neurodiagnostic studies unilaterally, and biopsy the contralateral side based on EMG results). Do not biopsy a muscle that has undergone recent (<2–4 weeks) EMG evaluation to avoid spurious results (i.e., EMG artifact).
• Do not biopsy a muscle within 3 months of an episode of rhabdomyolysis.
• MRI scan of the muscle can be helpful to direct muscle biopsy with a sensitivity of 96% to 100%. Areas of inflamed muscle demonstrate increased signal on T2-weighted images with fat suppression (short tau inversion recovery [STIR] images) but not on T1-weighted images, denoting areas of edema/inflammation. In chronic disease, MRI can also show fatty degeneration on T1-weighted images, which is unlikely to improve with medications