SYRINGOMYELIA (SM)
Primer

Above is a normal MRI image of a Dalmation

Red circle is normal sized ventricle in brain--no hydrocephalus
Yellow is normal sized cerebellum--no squashing or pinching
Green is beginning of spinal cord
Extension to left of spinal cord is the brain stem/medulla

 

 

Above is a Cavalier with SM

Red circle is showing enlarged ventricle in brain--hydrocephalus
Yellow is showing severely squashed/pinched cerebellum
Green is showing multiple syrinxes inside spinal cord, one quite large (the white is fluid)
Blue is showing medulla--main part of the brain stem

 

Spinal cord
Faint white line through center of spinal cord is central spinal canal

 

Here is a comparison of squashing/pinching of cerebellum--from very full to very squashed.
As you look at these you can easily see the cerebellum is more and more squashed the further down
you go.  There also appears to be a noticeable shortening of the area behind the main brain in these particular dogs.  If you look at the many MRIs on this site, you will note that the VAST
majority of Cavaliers are within range of the 2nd and 3rd ones.
The one at the top is extremely rare in this breed.  It appears almost normal.

 

 

They have done measurements on human chiari sufferers and a normal control group as well.
The following is a link to a website with this article.  It appears there are significant differences
in the hind skull development between the two groups.

Conquer Chiari Site