As a Radiology Nurse, I don't read MRIs. I don't even know HOW to read MRIs. You might ask your questions of your Neurologist (hopefully you DO trust her knowledge/understanding of neurological disease processes), or of the Radiologist who read the MRI. Or you could get a second reading (you may have to get copies of the original MRI and submit to the Radiologist you choose for a second opinion), OR, a second opinion from another Neurologist. You don't mention how long you've had Arnold Chiari's, or what symptoms you have/had, or how long you've been with your present Neuro doc.
Again, I refer you to your Neurologist (for the disease process question) or to a Radiologist (for MRI interpretation). Good luck!