An Arnold Chiari malformation is an abnormality of the brain. In this disorder, the skull surrounding the back of the brain (cerebellum) is small and this pushes a portion of the brain (cerebellar tonsils) down through the hole between the brain and the spinal column (foramen magnum). The fluid around the brain (CSF) normally flows through this hole and down to the spinal column. An Arnold Chiari malformation can result in an obstruction of this flow called hydrocephalus. Arnold Chiari malformations are thought to be present at birth although they may not cause symptoms until later in life. The cause of the malformations is unknown.