P02.10: The cisterna magna septa—a probable vestigial remnant of the roof of the rhombencephalic vesicle

Linear echoes are often seen in the fetal and neonatal cisterna magna. These lines have been variously described as the straight sinus, the Torcula Herophili, dural folds of the inferior attachments of the falx cerebelli, or bridging arachnoid septations. Through improved knowledge of posterior fossa embryology, anatomy and malformations and our own observations in fetuses and neonates, we believe that these septa represent a remnant of Blake's pouch, a normal dorsal extension of the developing 4th ventricle. We also demonstrate the probable relationship between these septa, the Dandy-Walker continuum, mega cisterna magna and Blake's pouch cyst. The parallel septa consistently observed in the cisterna magna are seen infero-posterior to the cerebellar vermis. The falx cerebelli is visible separately. There is continuity of the parallel septa with the inferior aspect of the fourth ventricle at the caudal aspect of the cerebellar vermis. The CSF space between the cisterna magna septa is in direct continuity with the fourth ventricle. The CSF between the lines is completely anechoic because it is intra-axial. The CSF outside the lines is usually slightly echogenic because it is in the subarachnoid space which is trabeculated by pial extensions. On sagittal views the superior margin of Blake's pouch can be seen separately from the dural fold that forms the inferior margin of the falx cerebelli. Sometimes the lines are bowed outwards giving the impression of a cyst. In light of these observations we propose that it is likely that the cisterna magna septa represent the walls of Blake's pouch, a phylogenetically vestigial structure observed during ontogeny. Additionally our observations support current opinion that Blake's pouch cyst and mega-cisterna magna may represent (less severe) abnormalities within the Dandy-Walker continuum.