Discussion of Dandy-Walker malformation
The original definition of the Dandy-Walker malformation in 1914 was formulated by Blackfan and Dandy. They described this as a constellation of alterations of the posterior fossa consisting of fourth ventricular cystic dilatation, cerebellar vermian hypoplasia, separation of the cerebellar hemispheres, dilatation of the aqueduct and absence of the Foramen of Magendie and Luschka. Later, different definitions were introduced for similar abnormalities of posterior fossa and to distinguish them from the original description of the Dandy-Walker malformation.
The all-encompassing term of "Dandy-Walker syndrome" was introduced to include the association of ventriculomegaly, large cisterna magna and defect of the cerebellar vermis with a communication of the posterior fossa cyst with the fourth ventricle. Therefore, at the present time, the term is used to indicate a spectrum of anomalies of the posterior fossa. The three main types that are included within this term are:
- Dandy-Walker malformation: Large posterior fossa, complete or partially agenesis of the cerebellar vermis, and elavation of the tentorium cerebelli.
- Dandy-Walker variant: hypoplastic cerebellar vermis with or without a large posterior fossa.
- Mega cisterna magna: Large cisterna magna with integrity of the cerebellar vermis and fourth ventricle.
Estimated prevalence of the Dandy walker syndrome is in the order of 0.3 per 10.000 births and it is found in 4 to 12% of the cases of infantile hydrocephalus. The incidence of the different forms of the complex is uncertain. The Dandy-Walker malformation is associated with a high mortality that is around 70%, resulting from associated malformations that are present in approximately 75% of the affected fetuses (see below).
Etiology: It is proposed in one of the prevalent theories that the malformation results from an imbalance between production of fluid between the third, the lateral ventricle and the fourth ventricle. The excessive production of fluid in the ventricles results in a dilation and herniation of the rhombencephalon. This results in secondary hypoplasia of the cerebellar vermis due to compressive effects and the fourth ventricle becomes enlarged, with cyst formation. The Dandy-Walker malformation can occur as part of genetic disorders and chromosomal aberrations. Teratogens including infection, viral, alcohol and diabetes have been quoted as possible causes of the malformation. There is a high frequency of associated anomalies (up to 70%). Other anomalies include ventriculomegaly, other midline anomalies, corpus callosum hypoplasia or agenesis, holoprosencephaly, encephaloceles, polycystic kidneys, cardiac anomalies and cleft lip and palate. The chromosomal abnormalities include trisomies 18, 13 and Turner syndrome.
Prognosis: The classic malformation is usually clinically manifest in the first year of life with symptoms of hydrocephalus and associated neurological symptoms. With current neurosurgical techniques, mortality has been decreasing, although it is still quoted as up to 25%. Isolated malformation, without other systemic involvement or genetic alterations is associated with better prognosis.
 Imaging Findings for Dandy-Walker malformation
Sonographic diagnosis in the first trimester on routine antenatal study is the most common presentation in healthcare systems where screening antenatal studies are carried out regularly. MRI is used in almost all cases in larger institutions in the late second trimester (after 18 weeks) to look for associated anomalies and plan further management at an early stage. The classic malformation is characterized by the presence of large cistern magna, communicating with the fourth ventricle through a defect in the cerebellar vermis going from the hypoplasia of the inferior part to the complete agenesis. The cerebellum is small. As a consequence of the larger cistern magna, the posterior fossa expands and the tentorium is lifted up. In the beginning of the second trimester, the inferior portion of the cerebellar vermis cannot be totally developed, and false positive diagnosis is a possibility at this stage. A combination of MRI and ultrasound at 18 weeks or later is recommended, if there is the suspicion of agenesis of the vermis, especially if the inferior portion is not seen. Hydrocephalus was originally considered as an element of the diagnosis, but recent studies suggest that it is not always present in fetal life, but usually appear in the first months of post-natal period.
The differential diagnosis includes arachnoid cysts and large intraventricular bleeds, which can be usually differentiated by the size and appearance of the cerebellum. Fetal MRI scans are now used more frequently to confirm and clarify sonographic findings.
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 References for Dandy-Walker malformation
- 1. Altman NR, Naidich TP, Braffman BH: Posterior fossa malformations. AJNR Am J Neuroradiol 1992 Mar-Apr; 13(2): 691-724
- 2. Barkovich AJ, Kjos BO, Norman D, Edwards MS: Revised classification of posterior fossa cysts and cystlike malformations based on the results of multiplanar MR imaging. AJR Am J Roentgenol 1989 Dec; 153(6): 1289-300.
- 3. Bordarier C, Aicardi J: Dandy-Walker syndrome and agenesis of the cerebellar vermis: diagnostic problems and genetic counselling. Dev Med Child Neurol 1990 Apr; 32(4): 285-94.
- 4. Cavalcanti DP, Salomao MA: Dandy-Walker malformation with postaxial polydactyly: further evidence for autosomal recessive inheritance. Am J Med Genet 1999 Jul 16; 85(2): 183-4.
- 5. Raybaud C: Cystic malformations of the posterior fossa. Abnormalities associated with the development of the roof of the fourth ventricle and adjacent meningeal structures. J Neuroradiol 1982; 9(2): 103-33.
- 6. Strand RD, Barnes PD, Poussaint TY, et al: Cystic retrocerebellar malformations: unification of the Dandy-Walker complex and the Blake's pouch cyst. Pediatr Radiol 1993; 23(4): 258-60.