--I'd check on the completeness of the work-up (I suspect I know who you saw at NYU) with the Liza Jane spreadsheets:

www.lizajane.org

Unfortunately, unless there's a screamingly obvious cause of small-fiber neuropathy, such as diabetes, it can be especially hard to pin down the cause (predominantly small-fiber neuropathies have a higher "idiopathic" rate on first and second-line investigation than any other type).