Joanne such is the state of diagnosis today for PN for many patients. There is a lot of trial and error and educated guessing. Im surprised the doctor wouldnt want to do a lumbar puncture to help maybe nail down or eliminate possible causes. It can show presence of markers for autoimmune antibodies, lyme, meningitus, bacterial infections, white cells, and more. I had it done and it really is not as big of a deal or as arduous as it is made out to be. A nerve biopsy on the other hand is invasive and usually leaves permanent numbness and or pain in the area cut.

A third opinion might not be a bad idea either. sometimes you have to get a few, research yourself and then decide. When my wife broke her leg and it wouldnt heal after 2 years, she had to go to many different orthopedists and get opinions on what to do which often contradicted each other. They ranged from continuing to wait to heal, to external fixators, to plates, to rods to amputation. Medicine to a large degree is still an art rather than just a science.