--are too thin to be evaluated by standard imaging techniques, which is why they are biopsied and looked at under the microscope--in fact, they are so small that electron microscopy is used, which is why examination of small fibers is only done at specialized centers that have this capability.

There are a few experimental modalities being played with in an attempt to better image small fiber and other nerves, but nothing yet that's made it to the standard pathology lab.