How can the muscle weakness of juvenile dermatomyositis be differentiated from that of other causes of weakness

How can the muscle weakness of juvenile dermatomyositis be differentiated from that of other causes of weakness?

The muscle weakness of JDM predominantly involves the proximal musculature, and in general, the involvement is symmetric. The child gives a history of difficulty in climbing stairs or riding a bicycle. Some of the maneuvers detailed in the previous question (e.g., Gower maneuver) can discriminate true muscle weakness from, for example, inanition. The palate and swallowing musculature may be weak in JDM and may lead to choking, cough, or aspiration pneumonia. Serum muscle enzymes are elevated, but not to the degree seen in the muscular dystrophies. Assays for all muscle-derived enzymes are required (aldolase, creatine kinase, aspartate aminotransferase, and lactate dehydrogenase) because only one may be elevated. ANA is positive in 10% to 85% of patients. Myositis-specific antibodies are found in 66% patients including anti-p155/140 (TIF-1λ; 20%–30%), anti-NXP2 (MJ; 15%–25%), anti-Mi2 (5%–10%), anti-MDA5, and anti-HMGCR. In patients who have symptoms such as “classic” skin rash, muscle weakness, and elevated muscle enzymes, the diagnosis can be made clinically. A magnetic resonance imaging (MRI) scan of shoulder or thigh muscles can preclude the need for electromyography (EMG) and/or muscle biopsy if short tau inversion recovery or T2-weighted images of the muscle shows increased signal, indicative of muscle edema and active inflammation. In atypical cases, an EMG and muscle biopsy may be necessary and the MRI can determine the best site for biopsy. In JDM, the EMG will show spontaneous fibrillations, increased insertional activity, and small muscle unit action potentials, whereas the muscle biopsy will show inflammation and/or fiber necrosis, perifascicular atrophy, and small-vessel vasculitis.

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