--it is currently thought that non-length dependent forms of small-fiber neuropathy, especially, may more likely result from neuronopathy, which involves damage to the cell bodies in the dorsal root ganglia, thereby damaging the whole nerve cell. Length dependent neuropathy, which is more common, refers to the end fibers farthest from the center of the body being affected first, often with gradual "die-back" towards the center over time.

Length dependent neuropathy often involves an ischemic or circulatory component; it's harder to get nutrients and oxygen to, and waste products away from, nerve fibers that are farther from the seat of circulation if there is circulatory compromise, so those far away areas tend to be affected first. Diabetic neuropathy, for example, usually presents in a length dependent manner, as the deleterious actions of high blood sugar involve compromise of blood vessels (among other things).

Non-length dependent neuropathies show a greater percentage of toxic or autoimmune causes, as these are systemic and don't often just congregate at the extremities. But, if the autoimmune attack is on blood vessel walls, as in many antinuclear antibody related conditions, some of these may present first at the extremities, too.

It's really a complicated issue that it may take a lot of research to clarify.