Chiari Malformations are generally considered to be a congenital condition, but there are cases where the cerebellar tonsils can herniate later in life. Due to a lack of consensus over the definition of Chiari Malformation some people call these herniations "Acquired Chiari" or call it cerebellar ectopia. The argument is that some believe the diagnosis for "Chiari Malformation" is not related to the cerebellar tonsils being herniated, but the small posterior fossa causing the herniation. 

 

In "classic" Chiari it is felt that the small posterior fossa limits space for the cerebellum and so the brain is shoved until the tonsils slip out of the skull. So the posterior fossa decompression surgery addresses this concern of limited space for the brain. When the cerebellar tonsils are herniated because they are pushed down by too much CSF pressure (or something else occupying the space in the skull) or pulled down (through tethered cord, CSF leak or LP shunts) creating space does not address the cause of the herniation and creates a bigger hole for more brain tissue to slip down (cerebellar ptosis)

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