Chiari malformations (CMs), are structural defects in the cerebellum that may develop when the space in the lower rear of the skull is smaller than normal, causing the cerebellum and brain stem to be pushed downward into the foramen magnum and into the upper spinal canal. The resulting pressure on the cerebellum and brain stem may affect functions controlled by these areas and block the flow of cerebrospinal fluid (CSF)—the clear liquid that surrounds and cushions the brain and spinal cord—to and from the brain. Read more here.