Hi and welcome to the Chiari Forum.
We are not medical professionals here, so to read an MRI is not in our abilities.....
However, I do have a few questions for you, since you know when the headaches started....was she in a MVA, or a trauma to the head or neck prior? Even a flu can cause symptoms to flare up.
It is possible to develop acquired Chiari , which blunt force trauma can cause the cerebral tonsils to descend and cause a blockage of CSF flow.
Chiari Malformation is an abnormality of the skull in which we are born with.
It is highly possible that she had other symptoms prior to the time frame indicated but since we are born with this condition we consider how we feel to be "normal" when in fact it is not.
Does she ever have head pains when moving her bowels, sneezes or laughs?
Does she refrain from sports/ physical activities because how it makes her feel?
Not everyone is affected the same way, so it can be difficult to pinpoint.
Make sure you have a TRUE Chiari Specialist. Many Drs will offer surgery, and many of those Drs DO NOT have the experience to know for related conditions prior to doing the surgery and can leave a patient worse off then before the surgery.
Do your homework and make sure ALL related conditions are ruled out, and it is the right Dr for your Daughter. I had to travel out of state in order to get to the right Dr for me.
Know you are not alone.