Right arm weakness

I am unable to use my right arm. It is so weak I can hardly lift it. It’s very difficult to do my hair, fold clothes, cook etc. My hand is ok, in order to eat I have to rest my elbow on the table. I don’t have any pain in my arm or shoulder, once in a while I get a pain in my neck. I’ve been to Mass General, had IVIG infusion treatments, prednisone infusions and physical therapy, multiple MRIs etc. The doctors seem puzzled. Does anyone have similar issue? I might add it started about 12 years ago with a slight weakness but got progressively worse.