--of skin punch biopsy for confirmation of small-fiber neuropathy--this, by the way, should be considered in those with neural pain or numbness symptoms but with fundamentally normal EMG/NCV testing and no other "smoking gun" test results--there are relatively few instances in which sural nerve fiber biopsy should still be performed. (A lot of neuros who suggest it are not aware of the skin biopsy option, or don't have access to the microscopy needed to analyze it.)

Diagnostic yield is not very high in sural nerve biopsy (though, to be fair, it's almost zilch in skin punch biopsy--that is used more to confirm small-fiber nerve damage in those with consonant symptoms, and rarely will reveal a cause). But, a few instances in which it might be useful, in that it might not only reveal a cause but might have an effect on proposed treatment, are when neuropathy secondary to vasculitis is suspected, when amyloid (either familial or secondary to blood cancer) is a possibility (in the latter case, when neuropathy may be a presenting symptom, a major blood dyscrasia work-up, including immunofixation electrophoresis, serum ionic calcium, and possible even bone marrow biopsy should be done first), or when certain infectious or hereditary neuropathies are suspected.

See:

http://www.neuro.wustl.edu/neuromuscular/nother/bx.html

As is indicated here, for many inflammatory processes which may produce asymmetrical symptoms, myopathy may also be present and a muscle biopsy should be performed at the same time.