--that can imply small-fiber disruption: there is quantitative sensory testing, in which an electrode that can assume various temperatures is used to determine one's sensitivity to hot and cold (those this relies on self-reporting), and there are also autonomic tests such as the sudomotor axon reflex (sweat) test. But skin biopsy is currently the most definitive.

Essential fatty acids are the type found in fish oil, flax, etc.--Omega 3's primarily (we tend to get not enough omega 3 and too much omega 6 in standard diets), and these are necessary for proper maintenance of the myelin sheathing of large nerve fibers.