Definition: In 1914, Blackfan and Dandy described some alterations of the posterior fossa as cystic dilatation of the fourth ventricle, hypoplasia of the cerebellar vermis, separation of the cerebellar hemispheres, dilatation of the mesencephalic aqueduct and absence of the mediate and lateral apertures of the fourth ventricle. These abnormalities were referred to as Dandy-Walker malformation. In the 1970 and 80 different definitions were introduced for similar abnormalities of posterior fossa and then the classic definition of the Dandy-Walker malformation.
The term Dandy-Walker syndrome was introduced to define the association of
increase of the cisterna magna and
defect of the cerebellar vermis with a communication of the cyst with the fourth ventricle.
Now the complex term of Dandy-Walker is used to indicate a spectrum of anomalies of the posterior fossa. Using the computerized tomography the anomalies was classified in the following way:
Dandy-Walker malformation: increase of the posterior fossa, complete or partially agenesis of the cerebellar vermis, and a tentorium elevation
Variant of Dandy-Walker: hypoplasia of the cerebellar vermis in different degrees with or without increase of the posterior fossa.
Mega-cisterna magna: increase of the cisterna magna with integrity of the cerebellar vermis and fourth ventricle.
Incidence: It has an estimate prevalence of 0.3:10.000 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%, attributed to the associated malformations that are present in approximately 75% of the affected fetuses.
Recurrence risk: In the absence of recognized syndromes there is a recurrence risk from 1 to 5%.
Embryology and etiology: The formation of the cerebellar vermis begins in the ninth week of gestation by the fusion in the medium line of the cerebellar hemispheres beginning in the superior part and continuing inferior until the total formation of the vermis in the end of the fifteenth weeks. Original it was though that atresia of the foramen's of Luschka and Magendie would cause the dilatation of the ventricular system. Benda observed later that atresia of the foramen was not always present. Gardner proposes that the malformation would be due to an imbalance between the production of the fluid between the third, the lateral ventricle and the fourth ventricle. The exaggerating production of the fluid in the ventricles would result in a dilation and herniation of the rhombencephalus. The compression would take a secondary hypoplasia of the cerebellar vermis and the fourth ventricle would form a cyst. Genetic factors are the largest cause.
The Dandy-Walker malformation can occur as part of Mendelian disorders and chromosomal aberrations. Teratogens including infection viral, alcohol and diabetes are suggested as cause of the malformation.
Diagnosis: It has been diagnosed in the prenatal period in the first trimester during a routine ultrasound exam. The classic alteration is less common than the variant form. The classic malformation is characterized by the presence of increased 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 and frequently the hemispheres are small. As a consequence of the larger cistern magna the posterior fossa expands and elevates the tentorium. In the beginning of the second trimester, the inferior portion of the cerebellar vermis cannot be totally developed, so caution is warranted. The scan 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. Enlargement of the cistern magna is considered when greater than 10mm in the antero-posterior axis. The hydrocephaly was already considered as an element of the diagnosis, but recent studies suggest that it is not always present in the birth, but usually appear in the first months of life.
Differential diagnosis: Arachnoid cyst can be differentiated by the size and appearance of the cerebellum. Even in the second and third trimester the insonation angle can mistake the diagnosis. In the cases of doubt a sagittal plane helps visualization of the vermis. A transvaginal scan if possible will result in a better view too.
Associated anomalies: Postnatal studies indicate a frequency from 50 to 70% of associated anomalies. Other anomalies include ventriculomegaly, other midline anomalies, corpus callosum agenesis, holoprosencephaly, cephaloceles, polycystic kidneys, heart defects and cleft lip and palate. The chromosomal abnormalities include trisomies 18, 13 and Turner syndrome.
Prognostic: The classic malformation is usually clinically manifest in the first year of life with symptoms of hydrocephaly and/or neurological symptoms. The mortality is 24% but with current neurosurgical techniques mortality has been decreasing. The intellectual development in the survivors is controversial. Intellectual deficit may be in teh range of 40 to 70% of the cases. The isolated defect (no aneuploidies or other anomalies) seems to be associated with better prognostic.
Management: There is no modification of standard obstetric management. Cesarean delivery is indicated only if macrocrania is present.
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