Hi and welcome to the Chiari forum.
First, Chiari is typically a congenital condition, symptoms do not always raise their heads until a trigger occurs..a trigger could be anything from a laugh, cough,, or a fall.....sometimes it could be getting the flu.....
Acquired Chiari is something you can have due to a trauma to the head or neck area....or a LP that was drawn too fast......
May I ask, since your MRI have your Drs suggested you have a CINE MRI? AND have any other related conditions been ruled out or testing ordered to rule them out?