What aspects on history and physical exam could help differentiate genetic muscular dystrophy from immune-mediated myopathy?  

Great question, and one that comes up all the time in clinical practice. Unfortunately, the question gets a little bit complicated because of the heterogeneity of the musculodystrophies, metabolic myopathies, and neuropathies that can mimic myositis (not to mention the phenotypic variation between t...