They don't usually pay for IVIG for small fiber neuropathy unless an inflammatory component is involved, such as, perhaps, if your ANA is elevated. Mine was prescribed because they thought it was inflammatory, still may be...that said, after a few years, IVIG didn't help my in case. That is not to say it won't in yours. I guess you need to file a formal appeal and reject their premise. Can your doc help?

I have a friend who is literally going to die if she doesn't get Remicade for her Sarc, and the insurance rejected that! Insurers are not exactly benevolent.

Just make sure you file your appeal in their time limits.