There many theories related to the causes of Arnold Chiari type ll including pulling of the brainstem and lower cerebellum into the foramen magnum by anchoring of the spinal cord by the meningomyelocoele.
None really explains all the malformations.
Chiari l may be due to a small posterior fossa. Arnold Chiari type ll is almost always associated with a meningo- myelocoele. Consists of malformation of the brainstem and cerebellum leading to extension of the medulla and cerebellum through the foramen magnum where the cervical spinal cord should rest. Usually associated with hydrocephalus. Arnold Chiari type l is not related to spina bifida, is often seen in adults and consists of herniation of the cerebellar tonsils through the foramen magnum around the cervical spinal cord.