Hi Megan, do you have a neuro that is a PN specialist ?, if you have great, but if not i would strongly urge you to find one, if this neuro throws the towel in after a few tests and if you live in Victoria i can put you onto a top one in Melbourne, as a good PN specialist will do all the tests available to find the cause.
I would check out Liza's charts http://www.lizajane.org/ against the tests that this neuro has done so far, if the neuro says " it's idopathic no more tests ", then you will have questions as to why other test's haven't been done, if any.
good luck, i hope your neuro has found the cause anyway.
Brian