Upload
bernadette-thompson
View
215
Download
1
Tags:
Embed Size (px)
Citation preview
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