Case History: A 26-year-old pregnant patient came for fetal ultrasound.
Figure1. No hydrocephalus seen
Figure 2: Ultrasound image of the fetal brain: an axial section showing the slightly enlarged cisterna magna and absence of the inferior cerebellar vermis. The cerebellar hemispheres are seen.
Diagnosis: A case of a Dandy-Walker variant on fetal ultrasound
Discussion: Dandy-Walker malformation is a rare abnormality of the central nervous system (CNS) with a reported incidence of 1 in 25,000 to 35,000 live births and a slight female predominance. It accounts for 1 percent to 4 percent of cases of antenatally detected hydrocephalus. Dandy-Walker variant forms part of the spectrum of Dandy-Walker malformation. It is characterized by partial agenesis of the vermis, resulting in communication between the fourth ventricle and the cisterna magna.
Dandy-Walker malformation demonstrates an enlarged posterior fossa, with high insertion of the torcula herophili, as well as other features. In the diagnostic workup, ultrasound is reliable and accurate for making the diagnosis. In utero, fetal MRI can be used to confirm the diagnosis and also to identify any associated abnormalities.
The formation of the vermis begins at the ninth week of gestation by fusion of the cerebellar hemispheres and is completely formed by the end of the nineteenth week. Because of the craniocaudal development of the vermis in embryogenesis, partial agenesis of the vermis is always inferior.
Ultrasound at 18 weeks of gestation would normally detect vermian agenesis . There are several abnormalities associated with Dandy-Walker malformation. The most common are ventriculomegaly (70 percent to 90 percent) and cardiac defects. Midline defects such as agenesis of the corpus callosum, holoprosencephaly, encephalocele, polycystic kidney disease and facial dysmorphism etc have been variously reported to be associated with this condition.
The incidence rate of karyotypic abnormalities, mainly trisomy 18 and 13 and triploidy, is up to 40 percent. The Dandy-Walker variation is also part of more than 50 genetic syndromes. The outcome following prenatal diagnosis varies widely.
A large proportion of pregnancies will be terminated owing to the identification of associated abnormalities and the karyotypic defects. Overall mortality rates of 12 percent to 50 percent have been reported; associated congenital abnormalities contribute to 83 percent of postnatal deaths and subnormal intelligence is reported in 40 percent to 70 percent of cases. The isolated Dandy-Walker variant abnormality has the highest incidence of survival, and there are reported cases of people who have had Dandy-Walker variant their entire lives without any symptoms.
The accurate diagnosis of antenatal conditions is essential not only for the management of the current pregnancy but also for future pregnancies to aid in the identification of inheritable diseases and provide information for accurate pre-natal counselling
The term Dandy-Walker syndrome was introduced to define the association of ventriculomegaly, 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 estimated prevalence of 0.3:10.000 births and it is found in 4 percent to 12 percent 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 percent, attributed to the associated malformations that are present in approximately 75 percent of the affected fetuses.
Recurrence risk: In the absence of recognized syndromes there is a recurrence risk from 1 to 5 percent. 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.
Harpreet Singh, MDJP Scan Private Diagnostic Center
Khanna, Punjab, India