Help! is mg right diagnosis?

well at the age of 17 started with weekness in the arms and legs and I had to stop working.  the first neurologist diagnosed me with limb-girdle muscular dystrophy and we left it at that.  but 5 years later i saw a different neurologist who saw that i looked normal i just couldn't lift my arms over my head nor take more that 8 steps without my legs giving out!  i have NO weakness in my face or eyes and no respiratory problems. he did a single fiber emg and there were fluctuations so he diagnosed mg.  No antibodies.  No response, at all, to 12 IVIG's and no Improvement with 5 treatments of plasma exchange. been on azathioprine for a year and prednisone for 6 months with no improvement.  Mestinon seems to help a little, it gives me allows me to take a few more steps sometimes.and some days are better than others. i am frustrated and i ask if there is any other disease that this could be? and since i've had it for 8 years, does this affect the effectiveness of the treatment? the only weakness or problems are the weakness in my legs and arms. when i am sitting i am strong. .Thank you in advance