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Enlarged posterior fossa on prenatal imaging : Differential diagnosis , associated anomalies and post-natal outcome Running headline : Enlarged posterior fossa : Ultrasound and MRI

Introduction: The primary aim of this study was to ascertain the prevalence the individual conditions and of associated anomalies in fetuses with the prenatal diagnosis of enlarged posterior fossa and to explore the diagnostic accuracy of ultrasound in these anomalies. The secondary aim was to evaluate the post-natal outcome of children affected by posterior fossa anomalies. Material and methods: All fetuses with enlarged posterior fossa detected by prenatal sonography at a referral center from 2001-2015 were analyzed retrospectively. Some were also studied by fetal magnetic resonance imaging (MRI) or volume ultrasound examinations. Fetal sonographic and MRI were compared using following classification: Dandy-Walker malformation; Megacisterna magna (MCM); Blake’s pouch cyst; isolated vermian hypoplasia; vermian agenesis; posterior fossa arachnoid cyst; and cerebellar hypoplasia. Results: Among the 69 fetuses the ultrasound diagnoses were: MCM (n=29; of these isolated n=15), Dandy-Walker malformation (n=28, isolated n=4), vermian hypoplasia (n=5, isolated n=4), Blake’s pouch cyst (n=4, isolated n=1), cerebellar hypoplasia (n=2; none isolated) and arachnoid cyst in the posterior fossa (n=1, isolated). 13 of the 41 karyotyped fetuses were aneuploid, including seven with Dandy-Walker malformation. Associated malformations were found in 37/69 cases. There were 39 live births, including 11 with confirmed Dandy-Walker malformation six of whom show a normal development. 12 infants with truly isolated MCM show normal development. There were eight false-positive prenatal diagnoses (or resolution until birth) of “enlarged posterior fossa”: Three with Blake’s pouch cyst, two MCM and one with vermian hypoplasia. Conclusions: An enlarged posterior fossa A cc ep te d A rt ic le This article is protected by copyright. All rights reserved. requires specific diagnoses for best possible counseling. The term “Dandy-Walker variant” should not be used any more. Isolated MCM and Blake’s pouch cyst can either resolve or be normal variants, but may also indicate the presence of a more severe anomaly or associated malformations.