by Dr Noreen Norfaraheen Lee Abdullah MD (UKM), MMed Radiology (UKM), AM(Mal)

Answer to Case Of the Month 
Dandy Walker Malformation

Discussion of Findings
There is a large posterior fossa cyst, which is in communication with the 4th ventricle. The tentorium cerebelli is elevated as best seen in the sagittal view. The cerebellum is small and hypoplastic. The lateral and 3rd ventricles are dilated. The corpus callosum is not seen on the sagittal view and its absence is confirmed on the axial view. The interhemispheric fissure dips down till the third ventricle. Features favour Dandy Walker malformation.

This disorder arises between the 7th till the 10th week of fetal gestation. It is due to the defective development of the roof of the fourth ventricle and adjacent meninges. These infants usually present at birth or early infancy with an enlarged head with a localized bulging of the occiput. Some present with raised intracranial pressure secondary to hydrocephalus.

A triad of maldevelopment characterizes Dandy Walker malformation ie.
Absence of the Foramen of Magendie
Massive dilatation of the fourth ventricle
Absent or small inferior vermis

The massive dilatation of the fourth ventricle presents as a posterior fossa cyst compressing and displacing the cerebellar hemispheres anterolaterally and the superior vermis upward. The cerebellum is usually hypoplastic due to external compression by the large posterior fossa cyst. In extreme cases, only a small nubbin of cerebellar tissue is identified. Other associated anomalies of the central nervous system include agenesis of the corpus callosum, holoproscencephaly and migrational arrest. Sagittal magnetic resonance imaging (MRI) best demonstrates this disorder. The massive posterior fossa cyst, cystic dilatation of the fourth ventricle, small/hypoplastic cerebellum, torcular lambdoidal inversion and dysgenesis of the corpus callosum are visualized clearly. A prominent signal void on T2 W images within the aqueduct and proximal fourth ventricle rules out the diagnosis of aqueductal stenosis.

Differential diagnoses of Dandy Walker Malformation
Several conditions do give rise to albeit similar features and may be confused with Dandy Walker malformation. The Dandy Walker variant is one such condition. It is characterized by the absence of the inferior vermis, a less dilated and better formed fourth ventricle which communicates with a retrocerebellar cyst. In Dandy Walker variant, other congenital malformations such as dysgenesis of the corpus callosum, holoprosencephaly, migrational disorders and posterior cephaloceles are associated.

Another condition is the primary vermian cerebellar hypoplasia. It usually presents at birth and consists of varying degrees of absence or hypoplasia of the cerebellar vermis and hemispheres. This clinical entity develops secondary to an atrophic process incurred postnatally.  Pontomedullary hypoplasia is frequently associated. The differentiating factor is there is no posterior fossa enlargement. There is another condition that may look like Dandy Walker malformation is megacisterna magna or commonly known as giant cisterna magna. The posterior fossa is enlarged with a large cisterna magna, intact vermis and normal 4th ventricle.

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Last Updated:
Tuesday, 04 January 2005