The Chiari II malformation: Part IV. The hindbrain deformity.

Computed tomography successfully delineates the multiple components of the Chiari II malformation at the craniocervical junction, the hindbrain, and the cervical spinal cord. These include wide foramen magnum and upper cervical spinal canal; incomplete fusions of the posterior arches of C1 and lower cervical vertebrae; cascading protrusions of vermis, fourth ventricle, medulla, and cervical cord into the spinal canal; cervicomedullary "kinking"; anterior displacement and sequential sagittal compression of each protrusion by the protrusions posterior to it; compression of all protrusions by the posterior lip of foramen magnum and the posterior arch of C1; and associated cervical hydromyelia, cervical diastematomyelia, and cervical arachnoid cysts.