I'm not sure of a stenosis in the neck, but I have had many a fistulogram done to fix a stenosis in my graft, itself. It's not so bad. Basically they put dialysis sized needles in and insert a wire like thing into the needles. I've had a few where they had to balloon out my graft and also had to used this weird suction device to suck out clots. The place I go to have this done gives me pain relief drugs and anti-anxiety drugs before and during the procedure, so it doesn't hurt too bad.
Good luck to you!