Chiari malformation, also known as an Arnold-Chiari malformation, is a congenital (present at birth) defect occurring in the back of the head where the brain and spinal cord connect.There are four types of Chiari malformations:1  Type 1 – Occurring when the base of the skull and upper spinal area do not form properly, a type 1 Chiari malformation commonly goes unnoticed until problems arise in the adolescent or adult years of life.here are four types of Chiari malformations: 1  Type 1 – Occurring when the base of the skull and upper spinal area do not form properly, a type 1 Chiari malformation commonly goes unnoticed until problems arise in the adolescent or adult years of life.