Cindy-
I asked a physician friend:
The malformation is more properly called Arnold Chiari.
There is a type 1 and a type 2.  Type 1 is generally less severe.
In type 1 there is an elongation of the cerebellar tonsils.  The
cerebellum is herniated into the 4th ventricle of the brainstem.  The
medulla and pons are decreased in size.  It is usually associated with
hydrocephallus.  The cerebro-spinal fluid cannot drain properly.  Most of
the consequences of the Arnold Chiari type 1 malformation is because of
the hydrocephallus.

Sarah Friend Barnett   LLLL, IBCLC
Bronx (New York City), NY  -  [log in to unmask]
" You are not obliged to finish the task,
 neither are you free to neglect it."       R. Tarfon