Heron Werner, MD & Pedro Daltro, MD
ClÃnica 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
The cystic malformation of the posterior fossa has been classically divided into Dandy-Walker deformity, Dandy-Walker variant and mega cisterna magna. The Dandy-Walker deformity is characterized by cystic dilatation of the third ventricle and enlargement of the posterior fossa together with posterior displacement of the tentorium and torcular Herophili (confluence of the venous sinuses), presenting a variable degree of aplasia or vermian hypoplasia (Table 1). The Dandy-Walker variant is characterized by a mild hypoplasia of the inferior cerebellar vermis with the connection of the posteroinferior portion of the fourth ventricle with the cistern magna. Characteristically, the posterior fossa is of normal dimension and the brain stem is preserved. In the mega cisterna magna the vermis, the cerebellar hemispheres and the fourth ventricle are normal.
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Table 1:Â Dandy-Walker malformation
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Cranium and duraenlargement of posterior fossa;high insertion of tentorium;high transverse sinuses
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Cerebrospinal fluid space and ventricular systempresence of floor of fourth ventricle, opening up posteriorly into a big cyst of the posterior fossa;hydrocephalus: 80% of cases (generally after birth)
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Cerebellum, vermis and brain stemcerebellar and vermin hemispheric hypoplasia;anterosuperior remnant of cerebellar vermis;brain stem can be hypoplastic and anterior displaced;heterotopia and cerebellar dysplasia
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Associated anomalies of the central nervous system – 70% of casesagenesis of corpus callosum: 20% to 25% of cases;heterotopia, gyral anomaly, schizencephaly;occipital cephalocele.
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Other associated anomaliespolydactylycardiac
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The study of the posterior fossa by MRI must be done within the third semester. Garel et al. (1998) describe their experience of seven cases of Dandy-Walker deformity which were evaluated by ultrasound and MRI. In all cases, ultrasound had no difficulty in making the diagnosis of the cystic dilatation of the fourth ventricle as well as the agenesis or hypoplasia of the cerebellar vermis. However, the evaluation of such fetuses by MRI may provide some additional information, such as the identification of agenesis of the corpus callosum.