Lets look at Chiari Malformation a bit.
First, it is believed to be congenital (there from birth), because the guy (Hans von Chiari) who first described it in 1891 was finding abnormalities in post-mortem exams (autopsies) on infants. Studies of family trees find that it seems to be a genetic disorder, with variable presentation (not everyone who has the gene has the malformation). unfortunatly, there is no conclusive evidence at this time.
here is a picture of an MRI with the malformation labeled.
A,B, C and E you can ignore; they are just anatomical structures only a geek like me are interested in, and don't help explain anything.
D shows the protrusion of the cerebellar tonsills into the spinal canal.
F is the cerebellar hemisphere, which lies just above the cerebellar tonsils.
