Arnold-Chiari malformation classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Classification

The Austrian pathologist Hans Chiari in the late 19th century described seemingly related anomalies of the hindbrain, the so called Chiari malformations I, II and III. Later, other investigators added a fourth (Chiari IV) malformation. The scale of severity is rated I - IV, with IV being the most severe. Types III and IV are very rare.

Type	Presentation	Other notes
I	A congenital malformation. Is generally asymptomatic during childhood, but often manifests with headaches and cerebellar symptoms. Herniation of cerebellar tonsils.	The most common form.
Syndrome of occipitoatlantoaxial hypermobility	An acquired Chiari I Malformation in patients with hereditary disorders of connective tissue. Patients who exhibit extreme joint hypermobility and connective tissue weakness as a result of Ehlers-Danlos syndrome or Marfan Syndrome are susceptible to instabilities of the craniocervical junction and thus acquiring a Chiari Malformation. This type is difficult to diagnose and treat.
II	Usually accompanied by a lumbar myelomeningocele leading to partial or complete paralysis below the spinal defect. As opposed to the less pronounced tonsillar herniation seen with Chiari I, there is a larger cerebellar vermian displacement. Low lying torcular herophili, tectal beaking, and hydrocephalus with consequent clival hypoplasia are classic anatomic associations. The position of the torcular herophili is important for distinction from Dandy-Walker syndrome in which it is classically upturned. This is important because the hypoplastic cerebellum of Dandy-Walker may be difficult to distinguish from a Chiari malformation that has herniated or is ectopic on imaging. Colpocephaly may be seen due to the associated neural tube defect.
III	Causes severe neurological defects. It is associated with an occipital encephalocele.
IV	Characterized by a lack of cerebellar development.

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Syringomiyelia associated with Chiari malformation

Other conditions sometimes associated with Chiari Malformation include hydrocephalus, syringomyelia, spinal curvature, tethered spinal cord syndrome, and connective tissue disorders such as Ehlers-Danlos syndrome and Marfan Syndrome.