The cisterna magna septa: a vestigial remnant of the roof of the rhombencephalic vesicle Ashley Robinson1,2, Ruth Goldstein1 1University of California San Francisco 2Children’s & Women’s Hospital of British Columbia

The cisterna magna septa Introduction The phrase: “Posterior fossa cyst communicating with fourth ventricle” Dandy-Walker malformation however it is actually a description of normal developmental anatomy

The cisterna magna septa Introduction are seen in up to 92% of fetuses in 2nd and 3rd trimesters usually straight, arising at the cerebellovermian junction course directly to the occipital bone axial

Septa become more difficult to discern later in gestation Typical appearances 22 weeks Septa become more difficult to discern later in gestation even in the same fetus but the posterior fossa structures develop normally

Typical appearances Same fetus at 28 weeks Septa less visible

Typical appearances Same fetus at 35 weeks Septa not visible Structures appear to have developed normally

Typical appearances 28 weeks - neonatal Above vallecula 4th ventricle walls continuous with septa via vallecula 4th ventricle walls separate from septa

Typical appearances 28 weeks At vallecula 4th ventricle walls continuous with septa via vallecula CSF space enclosed between septa is in direct continuity with CSF space of the fourth ventricle

Variable appearances 28 weeks - neonatal axial The falx cerebelli is visible as a midline septum superiorly in the posterior fossa

Variable appearances 27 weeks - neonatal Sometimes the septa and falx cerebelli can be seen as three septa on the same scan plane

Typical appearances 20 weeks coronal space between the septa is always completely anechoic space outside of the septa is usually slightly echogenic especially at earlier gestations

Embryology of the dura epidural conjunctive tissue containing rich vascular plexi meninx primitiva developing cerebrum

Embryology of the dura as hemispheres overgrow midline structures dura become apposed the vascular plexi coalesce and meninx primitiva cavitates

Embryology of the dura where dural leaves don’t fuse sinuses form superior sagittal inferior sagittal

Colour Doppler 21 weeks If the cisterna magna septa were dural leaves there would vascular spaces between them flow in occipital sinus no flow between septa

The cisterna magna septa Discussion We therefore don’t agree that the septa represent dural folds - inferior attachments of falx cerebelli Pretorius DH, Kallman CE, Grafe MR et al. Linear echoes in the fetal cisterna magna. J Ultrasound Med. 1992 Apr;11(4): 125-8 the Torcula Herophili Pilu G, Romero R, De Palma L, et al. Ultrasound investigation of the posterior fossa in the fetus. Am J Perinatol. 1987 Apr;4(2): 155-9 the straight sinus Mahony BS, Callen PW, Filly RA, et al. The fetal cisterna magna. Radiology. 1984 Dec;153(3): 773-6

The cisterna magna septa Discussion We also don’t agree that the septa represent bridging arachnoid septations Knutzon RK, McGahan JP, Salamat MS, et al. Fetal cisterna magna septa: a normal anatomic finding. Radiology. 1991 Sep;180(3): 799-801 they are thicker than other pia-arachnoid septations seen in the subarachnoid space We believe that the cisterna magna septa represent the walls of Blake’s pouch Blake JA. The roof and lateral recesses of the fourth ventricle considered morphologically and embyrologically. J Comp Neurol 1900;10: 79-108

Normal embryology rhombencephalon sagittal focal dilatation of the central canal of the neural tube rhombencephalic vesicle

Normal embryology rhombencephalon sagittal Dorsal pontine flexure Transverse crease Anterior membranous area (AMA) Posterior membranous area (PMA)

Normal embryology rhombencephalon sagittal AMA develops into vermis Choroid plexus forms in crease Cavitation starts in meninx primitiva

Normal embryology rhombencephalon sagittal PMA evaginates = ependyma-lined diverticulum into the meninx primitiva Blake’s pouch Further cavitation in meninx primitiva

Normal embryology rhombencephalon sagittal Multiple pia-arachnoid trabeculations Blake’s pouch fenestrates variably down to obex neck of Blake’s pouch = foramen of Magendie

Normal embryology rhombencephalon fluid more echogenic trabeculated by pia-arachnoid septations axial As Blake’s pouch expands walls are visible as cisterna magna septa fluid anechoic intra-axial

Lateral displacement of the septa 24 weeks neonatal The septa are sometimes deviated laterally giving impression of a cyst Fenestration of Blakes’ pouch variable in timing Prior to this, increased intra-axial CSF pressure may lead to the transient enlargement of 4th ventricle that has been demonstrated at 14-16 weeks, resolving by 22-23 weeks gestation Bronshtein M, Zimmer EZ, Blazer S. Isolated large fourth ventricle in early pregnancy--a possible benign transient phenomenon. Prenat Diagn. 1998 Oct;18(10): 997-1000 Knutzon RK, McGahan JP, Salamat MS, et al. Fetal cisterna magna septa: a normal anatomic finding. Radiology. 1991 Sep;180(3): 799-801

Lateral displacement of the septa 24 weeks neonatal outward bowing of the cisterna magna septa may be due to this delay in fenestration

Conclusions Our findings support current theories that Blake’s pouch cyst Dandy-Walker continuum Mega cisterna magna are a single spectrum of developmental abnormalities of roof of the rhombencephalic vesicle

Conclusions differences depend on: degree of dilatation of Blake’s pouch degree & timing of fenestration of the 4th ventricular outlet foramina degree of vermian hypoplasia

Elevation of the vermis probably simply due to an expanded Blake’s pouch may lead to false-positive diagnosis of vermian hypoplasia

The cisterna magna septa Conclusions The cisterna magna septa represent the walls of Blake’s pouch, a normal dorsal expansion of the roof of the rhombencephalic vesicle into the cisterna magna may be potential new marker for normal development of roof of the rhombencephalon

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