Heron Werner, MD & Pedro Daltro, MD
Clínica de Diagnóstico por Imagem (CDPI) & Instituto Fernandes Figueira (IFF) – FIOCRUZRio 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