Presentation
24 weeks pregnant, prenatal ultrasound revealed a posterior fossa cyst
Patient Data
- Enlarged posterior fossa. A tiny amount of cerebellar tissue is seen in the expected position of the superior vermis in the midline with absent vermis foliation. Tegmentovermian angle measures 950 . Cerebellar hemispheres are seen abnormally small. Retrocerebellar CSF space is seen communicating with the 4th ventricle.
- The lateral ventricle atrium width is about 11 mm with dilated occipital horns and parallel appearance of the lateral ventricles (colpocephaly).
- The subarachnoid space is compatible with the gestational age.
- The degree of gyration and sulcation is as expected for the stated gestational age.
- No structural abnormalities are noted in the cerebral hemispheres.
- The cavum septum pellucidum is normally formed.
- Corpus callosum is thin with indistinct posterior body , splenium and rostrum (dysgenesis).
- The craniocervical junction is unremarkable with no sign of Chiari or other malformation.
Case Discussion
Findings are in keeping with Dandy walker malformation with dysgenesis of corpus callosum.
Dandy-Walker malformation is the most common posterior fossa malformation, characterized by the triad of: enlarged posterior fossa , hypoplasia of the vermis and cephalad rotation of the vermian remnant and cystic dilatation of the 4th ventricle posteriorly.
Dandy-Walker continuum, also referred to as Dandy-Walker spectrum corresponds to a group of congenital anomalies believed to represent a continuum spectrum of posterior fossa malformations, characterized by inferior vermian hypoplasia and incomplete formation of the fourth ventricle which communicates with a prominent posterior fossa CSF space.
The tegmentovermian angle is an important measurable parameter in the assessment of posterior fossa abnormalities in the fetus. The angle is constructed on midsagittal images of the fetal brain, ideally on MRI. In case of Dandy- Walker malformation it is > 450 .