Medical Definition of Chiari malformation
Note: Chiari malformation is usually classified into four types. Type 1 involves protrusion only of the lower part of the cerebellum into the spinal canal and is often asymptomatic. Type 2 is the Arnold-Chiari malformation. In type 3, the cerebellum is completely displaced into the spinal canal. In type 4, the cerebellum fails to develop completely.