What part of the body does myopathy affect?

Depends on the level: at which its affected. The spinal cord lesion, say is at the C4 level, then the affected body part would be the shoulder and side of the neck. So if the lesion is at L3-4 level, then affected side's knee area would be affected. See a Spine surgeon to explain it to you, which particular site and the cause of the lesion responsible for it. Good Luck.

Myopathy = muscle: disease or dysfunction. Skeletal muscles, heart muscle or smooth muscle of organs like the GI tract can be affected. There are a myriad of myopathies, most characterized by muscle weakness. For a list of them, see http://www.ninds.nih.gov/disorders/myopathy/myopathy.htm#Clinical_trials.