I don't know what instrument you could switch to that would not require some left-arm movement. A dobro might be an option if you can deal with the motion of the slide up and down the neck -- at least it would not require you to use your arm and hand muscles to exert pressure on the strings against the fretboard.

Or maybe it's time to invest in a music-recording software program, and learn to create music electronically, combine tracks, etc.

Good luck! Hope the medical examinations will lead to some permanent correction of the problem.