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- 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
- Chiari 0 malformations:
- 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.
Maternal folate deficiency is a proposed (but unproven) risk factor.
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).