Objectives: To compare magnetic resonance imaging (MRI) and fetopathological findings in the evaluation of non-cystic fetal posterior fossa anomalies and to describe associated abnormalities.
Methods: This was a prospective study from 2000 to 2005 of fetuses identified on ultrasound as having sonographic suspicion of posterior fossa malformation. All underwent a thorough MRI examination of the fetal brain, after which we classified each fetus as presenting one of the following pathologies: vermian hypoplasia or agenesis, cerebellar and/or brain stem hypoplasia, destructive or dysplastic lesions. All of the pregnancies were then terminated, after which the whole fetus underwent fetopathological examination. We compared the findings from MRI and fetopathological examinations and recorded the associated cerebral and extracerebral abnormalities.
Results: Twenty-five fetuses were included. MRI was performed at a mean gestational age of 31 weeks, and fetopathological examination at 33 weeks. In 12 cases we observed vermian hypoplasia, six had partial vermian agenesis, 11 had cerebellar hemisphere hypoplasia, seven had brain stem hypoplasia, four had destructive lesions and six had dysplastic lesions. The two techniques were similar in their performance with respect to the detection of vermian agenesis, brain stem hypoplasia and destructive lesions. There were four false-positive results of MRI for vermian hypoplasia and a poor agreement regarding cerebellar hemisphere hypoplasia. No dysplastic lesions were diagnosed by MRI. None of the posterior fossa malformations was isolated and many cerebral and extracerebral abnormalities were found.
Conclusion: A systematic analysis of the posterior fossa in fetal MRI makes it possible to diagnose accurately most posterior fossa malformations. These malformations never occurred in isolation in our study.