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).
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