It seems that anti-sulfatide IGM, is cause of the neuropathy, i googled, it and it does cause it. This would make an autoimmune disease, that cause this particular igm to cause neuropathies.
I think the insurance denied you based on cost. according to wikipedia(of all places)
IVIG cost is stable but over $75/g. ($15,000 for a 100 kg (220 lbs) person at 2g/kg).
I know they wouldnt pay something like that.