Depending on the type, the disorder can be associated
with hydrocephalus, raised intracranial pressure, and respiratory and
cardiac center dysfunction. Infants may exhibit vomiting, weakness, and
Caudal displacement of the cerebellar tonsils (between arrows) into the opened
spinal canal (dorsal laminae removed) in a patient undergoing surgery for
The CT scan of a patient with Arnold-Chiari syndrome shows herniation of the
cerebellar tonsils in the upper cervical spinal canal and the presence of a significant
syringomyelia (different patient).
Cerebromedullary Malformation Syndrome.
In 1883, John Cleland, a British poet and anatomist,
first described the abnormality of the brainstem and cerebellum. The two
German pathologists Hans Chiari in 1891 and Julius Arnold in 1894 further
detailed this anomaly in children with hydrocephalus. The term
Arnold-Chiari syndrome (ACS) was coined in 1907 by the students of Arnold (not honoring Cleland's
work). It comprises a variety of anatomical abnormalities resulting from a
bony defect in the posterior fossa and upper cervical spine. This process
results in herniation of the cerebellar vermis and choroid plexus through
the foramen magnum, with elongation of the medulla and fourth ventricle, and
Type I: Displacement of the often elongated cerebellar tonsils through
the foramen magnum into the cervical spinal canal. The medulla oblongata may
be slightly displaced, kinked, or compressed. The vermis cerebelli and the
fourth ventricle most often are not or are only mildly involved. The degree
of hydrocephalus, hydromelia (distension of the central canal), and
syringomyelia (distension of the paracentral cavities) is variable.
Myelomeningocele is not a feature of this type of ACS; however, other
cranial and vertebral abnormalities occur in approximately one fourth of patients and may
include atlantooccipital fusion, abnormal positioning of the odontoid
process, Klippel-Feil Syndrome, cervical ribs, fused thoracic ribs, and
kyphoscoliosis. This type of ACS is also called the adult type because many of the
patients were not (and often still are not) diagnosed before the second or
third decade of life in the era before CT and MRI.
Type II: Usually manifested by severe hydrocephalus and myelomeningocele in
infancy. Radiologically, type II is characterized by a relatively small
posterior fossa, which results in elongation of the pons and the fourth
ventricle into the spinal canal, displacement of the medulla oblongata, and
crowding of the cervical cord roots into an upward course (upward
herniation). Kinking of the medulla at the cervicomedullary junction occurs
in more than 50% of cases. This form of ACS is frequently associated with
other anomalies of the central nervous system, such as aqueductal stenosis
with occlusive hydrocephalus (often presenting after repair of
myelomeningocele, dysgenesis of the corpus callosum, ...
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