In all the years I’ve had MG - and that’s a lot of years, I found that more often than not, my back aches were caused by muscle fatigue. The middle section of your body,(waist to neck) does the most work. Nearly every move you make involves your back and abdomen muscles. Add to that, the fact that those muscle also help maintain balance when you walk and stand. So all of those muscles, at least for me, become fatigued the quickest.

Left side, from waist to upper back.

I get a sore upper back often and I assume it is trying to cover up other muscle problems.

Which part of your back hurts?

I agree. He/she will know your medical history or in case it's a new doctor, take a list of your usual meds. As was mentioned, some pain relief may interact with your myasthenia meds. I have been given amitriypyline at low dose 10mg from my pain consultant as I have concurrent spinal issue due to muscle weakness. I am not currently on mestinon however any central nervous system medication can affect and weaken breathing, which obviously is not welcome, especially with MG patients. Yours may be postural or a spinal issue such as a disc etc, which only a doctor can work out through tests and scans etc. and you may benefit from core strength exercise, after Doctor clearance. I hope it goes well, Diana.