--is that it may be autoimmune, but not ANA autoimmune, and given Alan's general level of health now, and the long period of time he's had symptoms, it's probably not due to a monoclonal autoimmunity (gammopathy) either. He may have one of those persistent "idiopathic" neuropathies for which autoimmune mechanisms are suspected, but no known antibodies are found. (As Dr. Latov says, they are likely to one day be discovered--then neuropathies associated with them will no longer be classified idiopathic.)

Dr. Goldfarb may want to test him for some of the known antibodies specific to peripheral nerve, such as anti-MAG, or anti-sulfatide, if the other tests don't show anything amiss.

It's possible it's due to spinal issues--actually, more likely due to compression on nerve roots in the lumbosacral/cauda equina area--but this may not show up definitively, either.

The old standby causes are impaired glucose tolerance--it's now know that can cause neuropathy well before a frank diabetes diagnosis--or some sort of toxic or infectious exposure, but if the latter factors caused it initially, it may be impossible to find those agents now.