Arnold-Chiari Malformation
Basics
Description
- 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:
Epidemiology
Prevalence
- 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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Citation
Domino, Frank J., et al., editors. "Arnold-Chiari Malformation." 5-Minute Clinical Consult, 27th ed., Wolters Kluwer, 2020. Medicine Central, im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/816125/all/Arnold_Chiari_Malformation.
Arnold-Chiari Malformation. In: Domino FJF, Baldor RAR, Golding JJ, et al, eds. 5-Minute Clinical Consult. Wolters Kluwer; 2020. https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/816125/all/Arnold_Chiari_Malformation. Accessed August 31, 2024.
Arnold-Chiari Malformation. (2020). In Domino, F. J., Baldor, R. A., Golding, J., & Stephens, M. B. (Eds.), 5-Minute Clinical Consult (27th ed.). Wolters Kluwer. https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/816125/all/Arnold_Chiari_Malformation
Arnold-Chiari Malformation [Internet]. In: Domino FJF, Baldor RAR, Golding JJ, Stephens MBM, editors. 5-Minute Clinical Consult. Wolters Kluwer; 2020. [cited 2024 August 31]. Available from: https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/816125/all/Arnold_Chiari_Malformation.
* Article titles in AMA citation format should be in sentence-case
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T1 - Arnold-Chiari Malformation
ID - 816125
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ED - Baldor,Robert A,
ED - Golding,Jeremy,
ED - Stephens,Mark B,
BT - 5-Minute Clinical Consult, Updating
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PB - Wolters Kluwer
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DB - Medicine Central
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