Acquired Chiari Malformations
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)
There is still no consensus about whether these types of cerebellar ectopia should be classified as a type of acquired Chiari or not. The lack of consensus of the definition, in general, makes this harder. There is also no consensus on how to treat. The general thought is to address the underlying cause of the cerebellar tonsils being herniated first. Then one may consider if a chiari decompression is still required. One needs to make sure they speak to a doctor who is very well educated in this area and discuss the risks and benefits of how they will be approaching this.