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The posterior fossa findings of Cleland-Chiari malformation probably result from a normal size cerebellum developing in a small posterior fossa with low tentorial attachment. The consequence of this discrepancy is that the cerebellum encroaches on other posterior fossa structures and is “squeezed” out of the posterior fossa as it grows. The cerebellum protrudes through the tentorial incisure and the foramen magnum as it wraps around the brainstem. The pons, medulla, and cervical spine are stretched inferiorly. The inferior displacement of the cervical spine is limited by the dentate ligaments. The characteristic cerebromedullary kink occurs as the medulla is stretched down farther than the dentate ligaments will allow the cervical spine to move inferiorly. The tectum is stuck below the tentorium and takes the shape of a beak. These posterior fossa abnormalities influence supratentorial development. The splenium of the corpus callosum is thin or absent and neuronal migrational errors are often present. Hydrocephalus occurs because of a combination of diminished fourth ventricular foramina outflow and aqueductal stenosis. Magnetic resonance imaging of the brain is diagnostic (Figure 122.1). Treatment consists of closure of the myelomeningocele. Ventriculoperitoneal shunt is usually required.

Figure 122.1.Cleland-Chiari malformation. T1-weighted sagittal image demonstrates large massa intermedia, beaked tectum, towering cerebellum, elongated fourth ventricle, and inferior extension of the cerebellum.

An entity related to Cleland-Chiari malformation is Chiari III. Chiari type III malformation is characterized by the displacement of the posterior fossa content (brainstem and cerebellum) through a C1-C2 spina bifida (Figure 122.2).

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Figure 122.2.Chiari type III malformation. [A] Lateral view; [B] posterior view; [C] MRI showing displacement of the posterior fossa content into the cervical area.

 

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cerebellum elongated fourth ventricle beaked tectum large massa intermedia McLone, 1989 Two options: (1) click on figure for animated labels; or (2) pause pointer on figure (arrows) for labels. Figure must be centered.