The posterior fossa, the compartment of the cranial cavity that houses the brainstem and the cerebellum (the “hindbrain”), is abnormally small in patients with myelomeningocele. It cannot accommodate the brain structures that belong in it. As the brain grows, the bottom of the brain stem and an adjacent portion of the cerebellum are pushed out of the head into the cervical spine. This displacement of the cerebellum (and the brainstem) is called the Chiari malformation. The Chiari malformation type 2 is the pattern of deformity that occurs with myleomeningocele.
