--I think the difference is in the location of the problem.

Small fiber syndromes, through skin biopsy, will often show damage to the actual peripheral, unmyelinated fibers--excesive branching and swelling, frayed fibers, reduced intrepidermal nerve fiber density. I've yet to see that come up as a finding in people complaining of fibromyalgia-type symptoms--though it's certainly possible, if people are inspecific in describing symptoms, for doctors to confuse one with the other, or for people to have evidence of both.

I tend to go with Mrs. D's theory that Fibro is a more central condition, and the mechanism is more a disorder of signalling having to do with neurotransmitter dysregulation and neuronal sensitization. There has not yet been a demonstration in fibro that there is actual mechanical breakdown of nerve fiber itself as there usually is with small-fiber neuropathy. I also think this reflects the difference in symptoms--most people with small-fiber neuropathy will report that particular dysethetic burning pain, or other parastheses, and allodynia (pain without stimulus to cause it). The pain patterns in fibro tend to be more nociceptive in perception, e.g., more like "regular" pain dialed up too high.