Chiari malformation

2005-10-24-11 Heron Werner, MD*, Pedro Daltro, MD*, Dorothy Bulas, MD # * Clínica de Diagnóstico por Imagem (CDPI) & Instituto Fernandes Figueira (IFF) – FIOCRUZ Rio de Janeiro – Brazil
# 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
Findings 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
Ventricles 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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