Hi.,..sounds like the tonsils of ur cerebellum herniated ...this is a condition called chiari malformation. We do have a support forum here on MedHelp
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http://www.medhelp.org/forums/Chiari-Malformation/show/257?camp=msc&personal_page_id=862904]
The tonsils can herniated quit a ways down onto the spinal cord and compress the brainstem.
AS far as why ur tonsils r herniated now as to ur previous MRI, chiari is a congenital condition in which the skull is too small....and nething from a MVA, to a fall can trigger the growth of the tonsils down onto the cord...and that causes increased symptoms.
U do need to locate a true chiari specialist to have ur condition reviewed. also to check for related conditions.
I hope u decide to join us in the chiari forum for more info and support.
Good luck
"selma"