Chiari malformation Chiari type I malformation Arnold-Chiari type I malformation refers to a relatively mild degree of herniation of the posteroinferior region of the cerebellum (the cerebellar tonsils) into the cervical canal with little or no displacement of the fourth ventricle. It is characterized by one or both pointed (not rounded) cerebellar tonsils that project 5 mm below the foramen magnum, measured by a line drawn from the basion to the opisthion (McRae Line) Chiari I malformation Arnold-Chiari type I malformation UMLS:C0750929 Arnold Chiari type I malformation SNOMEDCT_US:253185002 HP:0007099 HP:0002440