--here are some good links to possible causes of small-fiber neuropathies:

http://www.neuro.wustl.edu/neuromusc...ory-small.html

http://www.neuro.wustl.edu/neuromuscular/time/hsn.htm

As you can see, there are a large number of hereditary/genetic conditions that involve axonal deterioration/loss. This includes things like celiac disease.

The most common acquired axonal neuropathy is from diabetes, but there are plenty of others from autoimmune causes, and a number from toxic exposures or infections. For the large number of people with idiopathic acquired syndromes, autoimmune mechanisms through post-infectious molecular mimicry are the strongest suspicion.