Antenatal Posterior Fossa Cystic Malformations

Ashish Jain, Kaushaki Shankar

Abstract

Antenatally detected posterior fossa cystic malformations encompass lesions ranging from normal variants to lethal anomalies. Accurate diagnosis of these malformations is the cornerstone for optimally counseling parents regarding further follow-up and management. The aim of this article is to discuss the diagnosis of these cystic lesions in a simplified way using various antenatal ultrasonography (USG) planes and to guide in the diagnosis, antenatal counseling, and management. The axial transcerebellar plane, coronal transcerebellar plane, and median sagittal plane are the most important for antenatal diagnosis of posterior fossa cystic malformations on USG. To start with, the cisterna magna is assessed in the axial view, and it is determined if it is normal (< 10 mm) or enlarged (> 10 mm). If enlarged, then the communication with the fourth ventricle is determined. A lack of communication confirms the diagnosis of mega cisterna magna. In case a communication is identified with the fourth ventricle, then the vermis, fastigium, and tentorium must be viewed in the sagittal view of the brain. If all these structures are normal in the sagittal view, then a Blake pouch cyst is the most likely diagnosis. If the vermis and fastigium are abnormal, then the tentorium is analyzed for the final diagnosis. The tentorium is elevated in Dandy–Walker malformation, and it is normal in vermian hypoplasia.