I had the exact same thing happen for a procedure I had back in December of last year. The nerve ablation was denied because it was considered experimental (even though it was pre-approved...as is the policy of the PM practice). I'm still fighting it because of the pre-approval, but I'm afraid I'm going to be stuck in the long run. I swear I even called the Dr's office the day before to make certain that I was OK with the pre-approval and they assured me that they NEVER do any procedure without pre-certification and that is why I had to have the "test" procedures (which were covered in full). I'm kind of waiting on the shoe to fall on this one and to be hit with a serious bill because I'll be done paying the last 2000.00 on the brain MRI I had done at the beginning of January in July. They would only allow a six month payment plan without a credit application and all that nonsense. I refuse to apply for credit for a procedure I have already had done....that's insane!! Of course, I do pay my bills....but a credit app for a hospital....give me a break!
Best to you on this one. Appeal as quickly as possible in every way. Make sure to have everything in writing to the insurance company and keep copies. They tend to give an impossible timeline for dispute and if you don't turn it in by that date, they assume you agree with the decision made.