I have axonal sensory motor long fiber polyneuropathy, caused by toxins. Every test i have had has been negative including a lumbar puncture except for the emg/ncs which showed what type of PN it was. I think a lumbar puncture is worthwhile to eliminate some possible causes and maybe uncover what is causing your PN. IT tests for a possible autoimmune, lyme, viral, bacterial as well as other causes. There are many autoimmune blood tests that can also be done.