Both Chiari type l and ll may be due to a malformed posterior fossa. 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.

Prognosis:
Chiari ll is almost always seen with meningomyelocoele and hydrocephalus with all their clinical problems. It may cause brainstem or cervical cord compression.
Chiari l is sporadic or occasionally inherited. It can cause headaches, brainstem compression and rarely sudden death. Usually becomes manifest during middle life.