Chiari Malformations. A Chiari malformation is a brain defect involving posterior fossa contents; the major types are described in Table 11-3. The type II Chiari malformation, seen almost exclusively with myelomeningocele, is characterized by herniation of a small cerebellum, medulla, pons, and fourth ventricle into the cervical spinal canal through an enlarged foramen magnum. The resulting obstruction of CSF flow causes the hydrocephalus.