Arnold-Chiari Malformation



  • Chiari disorders involve the cerebellum and craniocervical junction and include alteration of cerebrospinal fluid (CSF) hemodynamics and herniation of the cerebellar tonsils into the spinal canal.
  • Classification of Chiari malformation (1):
    • Chiari 0 malformations:
      • Cerebellar tonsillar extension <3 mm with syringomyelia and/or obstruction
    • Chiari I malformations:
      • Herniation of the cerebellar tonsils >5 mm below the foramen magnum (ages 15+ years) or >6 mm (age <15 years)
      • Associated with syringomyelia 60–70% (2)
      • Hydrocephalus is uncommon and constrictive.
    • Chiari 1.5 malformations:
      • Caudal displacement of cerebral tonsils with herniation
      • Displacement of brainstem and 4th ventricle
    • Chiari II malformations (“classic”):
      • Variable descent of cerebellar tonsils and brainstem into spinal canal
      • Aqueduct and 4th ventricle elongation associated with hydrocephalus, spina bifida, myelomeningocele, and dysgenesis of the corpus callosum (3)
    • Chiari III malformations (rare):
      • Small posterior fossa with an associated encephalocele containing the cerebellar structures
      • Hydrocephalus in 50% of cases due to obstruction
    • Chiari IV (very rare)
      • Primary cerebellar hypoplasia or agenesis



  • True incidence is estimated at 1 to 5 in 1,000.
  • Female to male 3:2 (2)
  • Chiari I malformations may present at any age and normally do not become clinically apparent until adolescence or adulthood.
  • MRI has led to an increase in the diagnosis of asymptomatic Chiari I malformations.
  • Chiari I and II malformations are typically discovered during prenatal testing due to the associated neural tube defects (NTDs) (3).

Etiology and Pathophysiology

  • Several models describe Chiari malformations currently. Evidence does not support one over another.
  • The crowding theory implicates restricted volume in the posterior cranial fossa forcing the cerebellum through the foramen magnum.
  • The oligo-CSF theory suggests that insufficient CSF leads to a smaller posterior fossa due to reduced hydrostatic forces.
  • Although there is no clear inheritance yet identified, a simple genetic malformation may also cause aberrant cerebellar growth.

Risk Factors

Maternal folate deficiency is a proposed (but unproven) risk factor.

General Prevention

Folic acid supplementation in women of childbearing years may prevent Arnold-Chiari malformations.

Commonly Associated Conditions

5.4–8.6% of patients with Chiari I malformations have NF1 (3).

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