MRI show 6mm herniation...

Hi.  I had an mri for severe headaches in the back/base of my head.  My PCP's nurse called and said mri showed a chiari one malformation with 6mm tonsillar herniation.  I see a neurosurgeon next week but I have a couple questions that I can't find the answer to.  What EXACTLY is happening to cause such severe head pain?  Is it fluid buildup?  Is 6mm considered mild/borderline chiari?  What other tests might the doctor order?  The mri was done of cervical spine looking for disc herniation.  Should I not have a more specific brain image?  Is 6mm herniation with severe headache and mild numbness in fingers considered a surgical candidate?  I'll do anything at this point?  Any info from personal experience would be great!  Thanks