1. 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

2. 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

3. 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

4. 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

5. Typical appearances
Same fetus at 28 weeks
Septa less visible

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

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

8. 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

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

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

11. 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

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

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

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

15. 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

16. 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 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 Apr;4(2): 155-9
the straight sinus
Mahony BS, Callen PW, Filly RA, et al. The fetal cisterna magna. Radiology Dec;153(3): 773-6

17. 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 Sep;180(3):
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:

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

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

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

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

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

23. 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

24. 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 weeks, resolving by weeks gestation
Bronshtein M, Zimmer EZ, Blazer S. Isolated large fourth ventricle in early pregnancy--a possible benign transient phenomenon. Prenat Diagn Oct;18(10):
Knutzon RK, McGahan JP, Salamat MS, et al. Fetal cisterna magna septa: a normal anatomic finding. Radiology Sep;180(3):

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

26. 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

27. 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

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

29. 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

30. Thankyou for your attention
Please also see our poster OP 03.17