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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 mental impairment.

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Arnold-Chiari syndrome
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Caudal displacement of the cerebellar tonsils (between arrows) into the opened spinal canal (dorsal laminae removed) in a patient undergoing surgery for Arnold-Chiari syndrome.

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Arnold-Chiari syndrome
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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).

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Cerebromedullary Malformation Syndrome.

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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 noncommunicating hydrocephalus.

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  • 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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