Hi Mat,
I also have RA and SFN. The SFN was diagnosed before the RA,

I have been on Methatrexate ( also due to treating a combination of vasculitis and RA), and I am now on Leflunamide to treat the RA, and on Amitriptyline and Lyrica to treat the awful symptoms of SFN.

My symptoms of RA are flaring, sometimes over periods of weeks, and then goes away for periods of weeks, but it is still VERY active. ( having a flare now)

The symptoms of SFN is more constant, but do flare. Symptoms include "burning feeling, like acid/salt, mostly all over, all 4 extremities, torso and including face mouth and gums.

Regarding the treatment with IVIG, I would think it would qualify as treatment for you due to positive Spinal Tap? Perhaps when you also have had the Skin Bx for evaluation of SFN that would help to qualify you ( if it is positive.) But also bear in mind, sometimes the skin bx might turn out negative. I think, at least here in US, the ratio of accuracy is about 89 %, if I am not wrong.

I was worked up for treatment with Rituxan, which has here been approved for treatment of RA, and it is said to have an excellent side benefit to also treat SFN.
It turned out after my VectraDA test, that my RA disease was active but in a mild disease activity.

I can so easily understand your frustration regarding diagnosis and treatment!!!
I have a question to you: What kind of adverse effect did you experience from Amitriptyline?

I wish you all the best.