OP14.02: Cystic abnormalities of fetal posterior fossa at 11–13+6 weeks scan

Abstract

To study the diagnostic value of cystic abnormalities of the posterior fossa in the first trimester as a marker for chromosomal diseases and to assess how often it is associated with other anatomical defects. This was a retrospective analysis of data from 15778 pregnant women sent to two referral fetal medicine centres for detailed ultrasound examination of fetal anatomy at 11 to 13+6 gestational weeks. All scans included extended systematical transabdominal and/or transvaginal examination of the fetal brain. In case of suspicion for brain anomaly 3D volumes were routinely acquired and analysed offline by 3D multiplanar reconstruction tool. Overall we diagnosed 50 fetuses with cystic abnormalities of the posterior fossa. In 43 cases (86%) it was associated with defects in other anatomical structures, in 7 fetuses (14%) posterior fossa abnormalities were isolated. 27 out of 50 pregnant women with fetal posterior fossa abnormalities opted for fetal karyotyping. In 20 of them (40%) various chromosomal abnormalities were found and the pregnancy was terminated. 23 women with multiple fetal abnormalities decided to terminate the pregnancy without further prenatal invasive tests. In 7 cases with isolated fetal anomaly the karyotype was normal. All 7 women in this group opted to continue with the pregnancy. At 20 weeks ultrasound scan 3 fetuses were diagnosed with Dandy Walker anomaly, 1 had an arachnoid cyst of the posterior fossa, 2 had Blake's pouch cyst and in 1 case the posterior fossa had normal appearance. Two women with Dandy Walker anomaly decided to terminate the pregnancy at 20 weeks, the others continued and the sonographic diagnosis was confirmed in all of them after birth. Cystic abnormalities of the posterior fossa in the first trimester of pregnancy is a strong marker for fetal chromosomal abnormalities. In most cases cystic abnormalities of the posterior fossa are associated with multiple malformations of other fetal structures. The prognosis of posterior fossa cystic abnormalities diagnosed in the first trimester of pregnancy is generally poor.