We've just been having a rather long (and technical) discussion of small fiber syndromes over at the peripheral neuropathy forum.

The most common cause is diabetes or impaired glucose tolerance--there's a lot of info now that painful small-fiber neuropathies can occur before a diagnosis of frank diabetes. However, small-fiber neuropathy can also result from vasculitis, gluten sensitivity (everyone with small-fiber de-ennervation should get a gluten panel done immediately, in my estimation), amyloid, and some toxins. Fully a quarter to a third remain stubbornly "idiopathic", though in many of these (such as mine) molecular mimicry autoimmune processes are suspected.

If you haven't yet, take a LONG look through the Useful Websites part of that forum:

http://neurotalk.psychcentral.com/thread177.html