Can you explain arnold chiari?

Chiari Malformation: Chiari malformations are when the base opening of the skull is too big allowing part of the cerebellum to come through the opening and compress the brainstem or spinal cord. They can be mild or very severe with associated abnormalities like a syrinx(hole in the spinal cord), and other problems.

Arnold chiari: Arnold chiari is a congenital malformation of the cerebellar tonsils and medulla, they displace downward through the foramen magnum causing hydrocephalus. Suggest you look at a book of anatomy to understand this complicated syndrome.