Cervical Spine/Brain

  Explain this please......
  "The inferior cerebellar tonsills extend just below the opisthion-basion line by approx 4mm."
  This comment was on my 9 year old son's cervical/brain MRI.
  I am a knowledgeable Arnold Chiari patient myself but I am unfamiliar with the opisthion-basion line. The impression was a normal brain/cervical MRI but this comment has me concerned because Arnold-Chiari goes unseen by so many.
  Thank You Very Much!
  Trina Mattingly
  
Dear Trina:
The opisthion-basion line is an imaginary line drawn from the one side of the foramen magnum to the other using specific landmarks.  If the cerebellar tonsils fall far enough below this line then the diagnosis of Arnold-Chiari I is made.  As your son's tonsils did not fall far enough below this line, the MRI was called normal.  As you probably know, if we did MRIs on everyone, alot of normal folks would have an Arnold-Chiari I malformation.  I think you need to just follow your son's health clinically and try not to worry about the small depression of the cerebellar tonsils (I know it is easy for me to say).  I hope your son does well.
Sincerely,
CCF Neuro[P] MD