I was just dx with SFN after spending almost 1.5 years with the Mayo here. My dx came from the neuro doc I was seeing before Mayo (I went back to him recently). But he just said I had SFN and adjusted my Gabapentin up to 3600mg a day and added 100mg per day of Tramadol. When I asked him why this happened, he said much of the time they cannot pinpoint the reason. My assumption is that due to insurance reimbursements, they don't want to spend the time trying to find out exactly why you have it. The insurance companies probably don't want to spend the cash. I guess I should have insisted on some further testing to find the origin, but I didn't. He stated that it is usually progressive and I have to learn to live with it and come back to see him every year. He worries about loading up more narcotics since I have a very high tolerance to narcotics.