Do everything you can to get her a neurosurgeon that specializes in chiari malformations....really....especially a pediatric neurosurgeon if your daughter is still that young. My 12 year old daughter was diagnosed April 2009 and had a decompression/duraplasty in July ..did awful..then had a repeat procedure in October...doing wonderfully. The second procedure was at UCLA with Dr. Lazareff. I would highly recommend him and his staff, which includes an education specialist I wish I had the first time around...she took care of the home disability teacher request, etc. It was also very, very important to have her in a pediatric
ICU after surgery....or at least a neuro ICU. We have an HMO and going to UCLA was covered just as well as our local community hospital!
I don't think it's so much how many mm the herniation is ...so much as the symptoms and the degree of syringomyelia/scoliosis. If your daughter has headaches and dizziness then she might have CSF flow blockage and you just don't want to mess with the brainstem being squashed.
I'm so sorry this came up for your daughter, also.
Learn everything you can about the surgery....what type of replacement material is planned for the duraplasty...how they monitor for scar tissue, etc.....followup is ver important and neurosurgeons and their offices are not famous for that.