Hello!
I had the same problem with my insurance when my son had to go out-of-network for treatment of his seizure condition. Although it took awhile, the insurance company did finally approve his treatment.
Here's a few helpful things:
1) STAY CALM! Remember, the insurance company is in this to make a profit - the more they pay out, the less they make! You are just a voice on the phone. If you can stay calm, and even friendly, you smooth the way.
2) The first person you talk to at the insurance company will almost certainly NOT be empowered to make this decision. If that person is not able to help, request a supervisor. For subsequent phone calls, always request that same supervisor.
3) Send a detailed fax about your condition. AN is a relatively rare thing, so the insurance company may not have much experience in its treatment. Mention the expertise of House for surgery, and Barrow, or Stanford, or Pittsburgh, etc., for radiosurgery.
4) Find out what the process is for appealing a decision should your treatment not be approved. Follow it to the letter, providing as much detail as possible. Find out if there's a neurosurgeon or radiosurgeon in-plan and speak to them. A good doc will give you the names of other docs to get second opinion.
5) Talk to the folks at House - they deal with this issue all the time, and can help communicate with your insurance. (That is, if you choose the surgery route.)
6) Talk to an attorney. Most insurance companies are used to hollow statements like "I'm calling my lawyer!". However, when the insurance company is actually contacted by an attorney, the gears can often become magically unstuck.
7) Be persistent! This is your health, and your insurance is there to assist you in maintaining it.
Document everything: every phone call, name, time, response, etc.
9) REMAIN CALM!
Remember, the squeeky wheel gets the oil - if you remain silent, they'll ignore you.
Hopefully, you won't have to much more than present your case well. But if that doesn't work (as it didn't for me), these are the steps I took to get the ball rolling.
Good luck!
Josh