Basilar Migraines or Chiari Malformation

Patient: My 9 year old has suffered over the past year almost daily with migraines along with tingling and numbness in her legs and sometimes arms, popping sensations in her knees and feet, a few bouts of vertigo etc. the neurologist diagnosed her with basilar migraines and she is now on a preventative medication, topomax. She still gets them around 5 times a week and perhaps a little less severe. My concern is that her MRI of the brain showed that her cerebral tonsil is 4mm below level- is this chiari malformation?