Assessment of Fetal Posterior Fossa Anomalies at 11-13+6 Gestational Weeks in the Midsagittal Cranial Plane by Three-Dimensional Multiplanar Sonography: A Prospective Study.

Objective: The aim of this study was to describe the sonographic appearance of posterior fossa anomalies in fetuses at 11-13+6 weeks' gestation.

Methods: This prospective study included 60 healthy fetuses and 15 fetuses with an abnormal posterior brain at 11-13+6 weeks' gestation. All three-dimensional images were processed using multiplanar image correlation to view the posterior fontanelle in the midsagittal views. The final diagnosis of all fetuses was confirmed using second-trimester ultrasonography, fetal magnetic resonance imaging, and/or genetic testing.

Results: The brainstem morphology, fourth ventricle, choroid plexus of the fourth ventricle, vermis, and physiologic Blake pouch were clearly visualized at 11-13+6 weeks' gestation through the posterior fontanelle from the midsagittal view. Among the 15 fetuses analyzed, two had abnormal brainstem morphology, which was subsequently diagnosed as Walker-Warburg syndrome. The remaining 13 fetuses were diagnosed with posterior fossa cystic malformations (Dandy-Walker malformation, 2 fetuses; Blake's pouch cyst, 2 fetuses; Noonan syndrome, 1 fetus; trisomy 21, 2 fetuses; trisomy 18, 1 fetus; and transient dilatation of the fourth ventricle, 5 fetuses). The extended anterior membranous area and dysplastic vermis were strong markers of Dandy-Walker malformation. In fetuses with Blake pouch cysts, the vermis was visible, with the choroid plexus of the fourth ventricle located backward.

Conclusions: Sonography enables clear visualization of morphological changes in posterior fossa anomalies at 11-13+6 gestational weeks. An extended anterior membranous area, dysplastic vermis, and abnormal brainstem morphology are direct signs of early recognition of severe posterior fossa anomalies.