The cisterna magna septa:a vestigial remnant of the roof of the rhombencephalic vesicle

The cisterna magna septa:a vestigial remnant of the roof of the rhombencephalic vesicle

Ashley Robinson1,2, Ruth Goldstein1

1University of California San Francisco2Children’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

axial•The cisterna magna septa:–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

Typical appearances

•Septa become more difficult to discern later in gestation–even in the same fetus

•but the posterior fossa structures develop normally

22 weeks

Typical appearances

Septa less visibleSame fetus at 28 weeks

Typical appearances

Septa not visible

Structures appear to have developed normally

Same fetus at 35 weeks

Typical appearances28 weeks - neonatal

Above vallecula

4th ventricle wallsseparate from septa

4th ventricle walls continuous with septa via vallecula

Typical appearances28 weeks

At vallecula 4th ventricle wallscontinuous with septa via vallecula

CSF space enclosed between septa

is in direct continuity with CSF space of the fourth ventricle

Variable appearances28 weeks - neonatal

axial •The falx cerebelli is visible as a midline septum superiorly in the posterior fossa

Variable appearances27 weeks - neonatal

•Sometimes the septaand falx cerebellican be seen as three septa on the same scan plane

•space between the septa is always completely anechoic

•space outside of the septa is usually slightly echogenic–especially at earlier gestations

Typical appearances20 weeks

coronal

Embryology of the dura

epidural conjunctive tissue containing

rich vascular plexi

meninx primitiva

developing cerebrum

Embryology of the dura

the vascular plexi coalesce

as hemispheres overgrow midline structures

dura become apposed

and meninx primitiva cavitates

Embryology of the dura

where dural leavesdon’t fusesinuses form

superior sagittal

inferior sagittal

Colour Doppler 21 weeks

flow in occipital sinus

no flow between septa

If the cisterna magna septa were dural leaves there would vascular spaces between them

The cisterna magna septaDiscussion

•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 septaDiscussion

•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 embryologyrhombencephalon

focal dilatation of the central canal of the neural tuberhombencephalic vesicle

sagittal

Normal embryologyrhombencephalon

sagittal

Dorsal pontine flexure

Transverse crease

Anterior membranous area (AMA)

Posterior membranous area (PMA)

Normal embryologyrhombencephalon

sagittal

Choroid plexus forms in crease

AMAdevelops into vermis

Cavitation starts in meninx primitiva

Normal embryologyrhombencephalon

sagittal

PMA evaginates= ependyma-lined

diverticulum into the meninx primitiva

Blake’s pouch

Further cavitation in meninx primitiva

Normal embryologyrhombencephalon

sagittal

Blake’s pouch fenestrates variably down to obex

neck of Blake’s pouch= foramen of Magendie

Multiple pia-arachnoid trabeculations

Normal embryologyrhombencephalon

As Blake’s pouch expands

walls are visibleas cisterna

magna septa

fluid more echogenictrabeculatedby pia-arachnoid septations

fluid anechoicintra-axial

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

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

Thankyou for your attention

Please also see our poster OP 03.17