Abnormal cerebellum and myelomeningocele on antenatal ultrasound. Patient is 22 weeks 4 days gestational age. Please note that imaging is tailored for assessment of the fetal brain and lower spine. The fetal abdomen is not specifically addressed on this study.Dedicated evaluation of the fetal brain demonstrates a small posterior fossa with a small and inferiorly displaced cerebellum which appears to partially enwrap the brainstem (best seen on image #12 of series #9). The appearance is similar to the ultrasound "banana" sign. The fourth ventricle in not clearly visualized which may indicate some compression, and there is a lack of CSF in the posterior fossa. The lateral ventricles are not dilated. However, the occipital horns of the lateral ventricles are angled inferiorly and medially, towards the posterior fossa, and it appears that the occipital poles are inferiorly positioned and possibly encroaching on the posterior fossa. The frontal region of the skull is narrowed due to bilateral frontal bone concavity, similar to the ultrasound "lemon" sign.The corpus callosum is too small to definitively assess at this point. A cavum septum pellucidum is present suggesting that the corpus callosum is present anteriorly. The posterior corpus callosum may be dysgenetic or agenetic. The gyral pattern is within range for reported gestational age. There is no parenchymal signal alteration or intracranial hemorrhage.Dedicated evaluation of the fetal spine demonstrates a dorsal vertebral defect involving the lower lumbar and upper sacral segments (starting at the L3 or L4 level and continuing into the sacrum, measuring 15 mm craniocaudally and 3 mm in transverse). There is suggestion of a neural placode extending across this lumbosacral defect with no clear soft tissue covering, compatible with an open myelomeningocele.
1. The constellation of small posterior fossa with inferior displacement/tethering of its contents, as well as the lumbosacral myelomeningocele, are compatible with Chiari II malformation.2. Abnormal morphology especially involving the posterior part of the lateral ventricles is probably related to medial and inferior displacement by the tethered posterior fossa structures. A partially dysgenetic or partially agenetic corpus callosum could also be contributing to this appearance. 3. Due to technical limitations, some of the more subtle features of Chiari II may not be detected on this exam. However, the ventricles are not dilated, and the brain otherwise shows a relatively normal volume and normal gyration.