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Chiari Malformation

Patologie Malformative o della Dinamica Liquorale

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Definition and Etiology

Chiari Malformation (CM) is a pathology of the posterior cranial fossa, in which the cerebellum and the brainstem are normally contained. CM is characterised by an “encumbrance” of the neural structures in the area of the passage between cranium and spine and by a reduced size of the posterior cranial fossa. The cerebellar tonsils and the brainstem are therefore displaced through a small opening, called the foramen magnum, in the spinal cavity. Chiari malformation may also be associated with many other diseases, including myelomeningocele, syringomyelia, spina bifida and hydrocephalus.


Two main types of CM have been identified, with different etiology, age of onset, and severity: Type I and Type II.



Type of Malformation

Chiari Type I

Chiari Type II

displaced neural structures

cerebellar tonsils

Cerebellar vermis, lower portion of the cerebellar hemispheres, medulla oblungata and forth ventricle

typical age of onset

young adults


associated diseases

Hydrocephalus, abnormalities of the skull base, syringomyelia

Spina bifida, hydrocephalus, syringomyelia


It is believed that the malformation is related to an insufficient development of the posterior cranial fossa, causing the cerebellum to be pushed downwards, through the foramen magnum.

Onset of Type I usually occurs in young adults. Morphologically it is characterised by the displacement of the cerebellar tonsils in the upper portion of the cervical canal. 30-85% of all cases are associated with syringomyelia and 1/4 of all cases are affected by bone abnormalities in the skull base.

Onset of Type II usually occurs at birth, and is typically associated with spina bifida. Morphologically, also the cerebellar vermis, the lower portion of the cerebellar hemispheres, the medulla oblungata and the forth ventricle are displaced into the cervical canal.

Type II is more severe than Type I, and its treatment is more complex. 

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