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Articles » Magnetic Resonance Imaging » MRI, Central nervous system
2005-10-24-11 MRI, Chiari malformation © Werner

Chiari malformation

Heron Werner, MD*, Pedro Daltro, MD*, Dorothy Bulas, MD #

* Heron Werner, MD & Pedro Daltro, MD*
Clínica de Diagnóstico por Imagem (CDPI) & Instituto Fernandes Figueira (IFF) – FIOCRUZ
Rio de Janeiro – Brazil

# Dorothy I. Bulas M.D.
Professor of Radiology and Pediatrics
Children"s National Medical Center
George Washington University Medical Center
111 Michigan Ave, NW,   Washington D.C. 20010

Chiari malformation is one of the main anomalies of the posterior fossa. The most frequent forms are the types I and II. Chiari I consists in the displacement of the cerebellar tonsils towards the superior cervical canal, whereas Chiari II consists in the herniation of the inferior part of vermis cerebellar and of the fourth ventricle. Chiari II is found in 65 percent to 100 percent of grave cases of bifida spine (Gillet, 1990). The posterior fossa is small, the cerebellar hemispheres are hypoplastic, and a ventricular dilatation is almost always present. There is still another grave form of Chiari malformation, the so-called type III (Table 1). This is a rare form of anomaly of the posterior fossa and consists in encephalocele with herniation of the content of the posterior fossa and, occasionally, of the occipital lobe (cephalocele). The herniated tissue is always abnormal, presenting areas of necrosis, gliosis and fibrosis.


Table 1: Intrauterine visualization of alterations of Chiari Malformation


Chiari I 

Chiari II 

Chiari III 

 Posterior fossa

Displacement of cerebellar tonsils towards the superior cervical  canal

Small posterior fossa with inferior displacement of vermis and superoanterior of cerebellum
Pointed quadrigeminal plate
High cervical encephalocele with herniation of content of posterior fossa and / or occipital lobe
 Brain Usually normal Dysgenesis of corpus callosum, heterotopia and polymicrogyria There might be correspondence with Chiari II findings

Hydrocephalus (mild to  moderate in 20% of cases)

Fourth ventricle is small, elongated and with inferior displacement

Hydrocephalus (90% of cases) There might be correspondence with Chiari II findings
 Marrow Hydrosyringomyelia (in 30% to 60% of cases) Myelomeningocele (approximately 100% of cases)
Hydrosyringomyelia (in 50% to 60% of cases)
There might be correspondence with Chiari II findings
 Associative malformations

Difficult prenatal evaluation

Bone anomalies of base of skull

Difficult prenatal evaluation Posterior bifida spine in topography on C1 and C2



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