Sorry you are in so much pain. I used to have this too. I don't know the answer to this I'm afraid because mine were negative too but we're only taken from my upper calves - one on each side. I live in the Scotland/UK where testing for SFN is very rudimentary as far as I can see. So I have little faith in the punch biopsy results I had taken last year - especially because they had to be transported from my island home to the mainland.

My SFN has still been clinically diagnosed by a neurologist so I guess it must be possible to have it despite a negative punch biopsy - although none of my doctors seem terribly committed to finding out more I have to admit.

When the soles of my feet were very hot and swollen a few years ago, just as you describe, I was told by a podiatrist that my Rheumatoid Arthritis was flaring. But I actually think it may have been the SFN. Now my feet are mostly quite numb and the burning affects them less - but it affects my pelvis, hands and wrists and my gums more. My feeling is that this is because it's slowly killing off the tiny nerves in my toes and feet and progressing to other parts. This is depressing so I am constantly pushing to get the cause identified and acknowledged.