The document discusses various subarachnoid cisterns in the brain. It describes the location and contents of key supratentorial cisterns like the carotid, chiasmatic, lamina terminalis, olfactory, and sylvian cisterns. It also describes infratentorial cisterns such as the interpeduncular, prepontine, ambient, and cisterna magna. The document traces the historical understanding of the subarachnoid space and cisterns and provides diagrams to illustrate locations. Knowledge of the cisterns' contents is important for neurosurgeons planning intracranial procedures.

1. Subarachnoid Cisterns
Dr Piyush Ramesh Thombare
Department of Neurosurgery
Topiwala National Medical College, and Nair Hospital, Mumbai

2. Introduction
• Knowledge of the neural and vascular contents of each of the basal cisterns is of particular
value to the neuro- surgeon in the planning and execution of intracranial procedures.
• The neurosurgeon may chart his intracranial approach like a road map in terms of the basal
cisterns.
• Many of the subarachnoid cisterns can be considered to be anatomically distinct
compartments, but others are not, being separated from each other by a porous trabeculated
wall with various sized openings.
• Under normal circumstances this permits a continuous exchange of CSF from one
compartment to another.
• These apertures can become plugged and partially or totally obliterated after subarachnoid
hemorrhage, infectious meningitis, chemical meningitis, spread of malignant cells in the
subarachnoid space and spread of proteinaceous exudate hindering the normal CSF circulation

3. HISTORY
• In 1555 Blaes (Blasius) is credited with naming of the arachnoid
• Vieussens (1690) noted that the pia and arachnoid existed as two separate membranes
• Ruysch (1697) showed that the arachnoid extended over the convexities of the brain.
• In 1802, Bichat proposed that the arachnoid formed a serous cavity similar to the peritoneal
cavity.
• Magendie (1822) gave the first modern description of the subarachnoid space as containing
cerebrospinal fluid
• He described the basal cisterns and the extensions of the arachnoid along cranial nerves II, V
VII and VII

4. original figure from the monograph (1875) by Key
and Retzius which shows the ventricular, arachnoid, and
cisternal spaces outlined by Berliner Blue.
• In 1875, Key and Retzius published a
monumental work in which they presented
drawings of the subarachnoid space that had
been injected with blue dye (Berliner-Blau) to
demonstrate the extensions and divisions of
the subarachnoid system
• They showed the relationship of the cerebral
vessels to the arachnoid and the numerous
trabeculae which suspend these vessels from
the walls of the cisterns.

5. • ln 1919 Dandy described injection of air into ' the lumbar subarachnoid space in order to
outline the cerebral ventricles.
• It was seen that the basal subarachnoid cisterns were also demonstrated in this manner
• Locke and Naffziger (1924) undertook a corrosion cast study of the subarachnoid cisterns in
dogs and humans, and demonstrated the shapes and intercommunications of the
subarachnoid space. They gave general names to the subarachnoid cisterns
• In 1937, Davidoff and Dyke published a textbook on the normal pneumoencephalogram in
which they discussed the shape and extension of the subarachnoid cisterns in some detail.
• Liliequist (1959) employed both techniques to provide a working normal anatomy of the
subarachnoid space

6. • The cisterns are divided into two major groups
• supratentorial
• infratentorial

8. Supratentorial Cisterns
• A) Anterior
• 1) Carotid cistern
• 2) Chiasmatic cistern
• 3) Lamina terminalis cistern
• 4) Olfactory cistern
• 5) Sylvian cistern
• B) Lateral
• 1) Crural cistern
• 2) Ambient cistern (anterior part)
• C) Posterior
• 1) Quadrigeminal cistern
• 2) Velum interpositum cistern
• D) Superior
• 1) Corpus callosum cistern

9. Infratentorial Cisterns
• A) Anterior
• 1) Interpeduncular cistern
• 2) Prepontine cistern
• 3) Premedullary cistern
• B) Lateral
• 1) Ambient cistern (posterior part)
• 2) (Superior) cerebellopontine
cistern
• 3) Inferior cerebellopontine or
lateral cerebellomedullary cistern
• C) Posterior
• 1) Cisterna magna
• 2) Superior cerebellar cistern
• D) Superior
• 1) Vermian cistern

12. The cisterns located in the posterior cranial fossa include paired and
unpaired cisterns.
• A. Unpaired cisterns
• 1. Interpeduncular cistern
• 2. Prepontine cistern
• 3. Premedulary cistern
• 4. Quadrigeminal cistern
• 5. Cisterna magna
• B. Paired cisterns
• 1. Cerebelopontine cistern
• 2. Lat Cerebelomedulary cistern

13. 1 Olfactory cistern
2a Callosal cistern
2b Lamina terminalis cistern
3 Chiasmatic cistern
4 Carotid cistern
5 Sylvian cistern
6 Crural cistern
7 Interpeduncular cistern .
8 Ambient cistern
9 Prepontine cistern
10 Superior cerebellar-pontine
cistern
11 Inferior cerebeliar-pontine
cistern (lateral cerebello-
medullary)
12 Anterior spinal cistern
13 Posterior spinal cistern

14. The relationship between
the basal cisterns and the
ventral cerebral arterial
system.

15. Carotid Cistern
• Bordered
• The lateral carotid membrane forms the lateral
wall
• The medial carotid membrane separates the
carotid and the chiasmatic cisterns.
• superiorly the dura over the anterior clinoid
process and the orbitofrontal lobe
• inferiorly by the cavernous sinus
• Medially - shares a wall with the chiasmatic
cistern
• laterally - bounded by the mesial temporal lobe
and the free margin of the tentorium.
• The inferior part of the carotid cistern and
superior part of the interpeduncular cistern are in
apposition - sometimes creating a single (Lilie-
quist's) membrane.

16. • The carotid cistern contains the
• supraclinoid portion of the internal
carotid artery,
• origin of ophthalmic artery
• Posterior communicating artery
• anterior choroidal arteries,
• small arteries to the optic nerves and
pituitary stalk,
• artery to the dura over the anterior
clinoid process

17. Chiasmatic Cistern (Cisterna
Chiasmatica)
• This cistern encloses the subarachnoid space,
around the optic nerves and chiasm.
• Superiorly - tightly adherent to the superior
surface of the optic nerves and chiasm and
contiguous with the inferior part of the lamina
terminalis cistern
• Inferiorly -shares a common wall with the
interpeduncular cistern
• Thick arachnoid membrane joining the chiasmatic
and interpeduncular cisterns being called
"Liliequist's membrane“

18. • Anteroinferiorly it extends to the infundibulum and
pituitary stalk and is bounded by the diaphragma
sellae
• Anteriorly - limited by the limbus sphenoidale
except at the optic foramina where short extensions
of the subarachnoid space follow the optic nerves
into the orbit.
• Laterally - shares a common wall with the carotid
cistern
• The chiasmatic cistern contains the optic nerves,
pituitary stalk, and numerous small internal carotid
branches to both structures. The ophthalmic artery
enters the chiasmatic cistern within the optic canal.

19. • Liliequist's membrane arises from the part of the arachnoid membrane
that rests against the dorsum sellae and splits into the diencephalic and
mesencephalic membranes.
• The diencephalic membrane is a complete membrane that separates the
chiasmatic and interpeduncular cisterns.
• The mesencephalic membrane, attaches along the junction of the
midbrain and pons, forms an incomplete wall between the
interpeduncular and prepontine cisterns with an opening through which
the basilar artery ascends.

21. Lamina Terminalis Cistern
• This cistern is defined primarily by the anterior
cerebral arteries
• Anteroinferiorly is the superior surface of the
optic chiasm where it is contiguous with the
chiasmatic cistern.
• Anterosuperiorly the rostrum of the corpus
callosum covers the cistern.
• The posterior boundary is the lamina terminalis
• Extensions laterally enclose each anterior cerebral
artery

22. • Thickened bands of arachnoid running from the olfactory area to the optic nerve
demarcate the most lateral limit of the cistern.
• These form a tunnel through which the anterior cerebral artery must pass on leaving the
carotid and entering the lamina terminalis cistern

23. • The lamina terminalis cistern contains
• the anterior cerebral arteries
• the most proximal A2 segments of the anterior
cerebral arteries,
• the anterior communicating artery complex
• medial striate branches (the recurrent artery of
Heubner)
• frontoorbital arteries,
• Anterior communicating and anterior cerebral
veins also lie within the cistern

24. Olfactory Cistern
• This cistern is formed by the arachnoid
over the olfactory tract between the
orbital gyri laterally and the gyrus rectus
medially.
• Inferiorly it is bounded rostrally by the
floor of the anterior fossa including the
cribriform plate of the ethmoid bone and
caudally by the chiasmatic cistern.
• Posteriorly it joins several other cisterns
above the internal carotid artery
bifurcation.

25. • Bilateral medial orbitofrontal arteries
(arrows) arising from the A2 segments and
disappearing into the olfactory sulci
beneath the olfactory tracts (OL.)
• The olfactory cistern contains the
• olfactory bulb and tract,
• parts of frontoorbital and olfactory
arteries their branches, and
• several frontobasal veins.

26. Sylvian Cistern
• Thickened bands of arachnoid completely enclose
the origin of the middle cerebral artery
• These form a tunnel through which the middle
cerebral artery passes before entering the Sylvian
fissure.
• The most medial and inferior extent is at the origin
of the middle cerebral artery from the internal
carotid
• The cistern narrows superiorly as the frontal and
temporal lobes approach each other
• The width of the cistern is usually about 0.5-1.0
cm on the surface

27. • In some cases the frontal and temporal lobes are closely
approximated thereby covering the substance of the cistern
• For this reason the sylvian cistern and its investing arachnoid can be
categorized as follows:
Category Cisternal Size Arachnoidal
Characteristics
1 Large Transparent + fragile
2 Small Transparent + fragile
3 Large Thickened + tough
4 Small Thickened + tough

28. • Microsurgical dissections of the Sylvian cistern during the pterional operative approach
are increasingly more difficult as the category of cistern increases according to the
above classification.
• Thus the exposure of a category 3 cistern is quite tedious, but easier than that of a
category 4, post-meningitic cistern, which is almost impossible
• The Sylvian cistern contains the middle cerebral artery and the origins of the
lenticulostriate, temporopolar and anterior temporal arteries
• The middle cerebral artery bifurcation
• The superficial and deep Sylvian veins

29. Crural Cistern
• Lies between the parahippocampal gyrus
and the cerebral peduncle.
• Extends to the carotid cistern anteriorly
• lies on top of the interpeduncular cistern
• the ambient cistern latero-posterior
• clearly demarcated from the carotid and
interpeduncular cisterns between the
anterior choroidal and posterior
communicating arteries.

30. • The importance of this cistern lies in the valuable surgical plane it
establishes between the anterior choroidal and posterior
communicating arteries.
• The crural cistern contains
• the anterior choroidal arteries
• medial posterior choroidal arteries
• the basal vein of Rosenthal.

31. Velum Interpositum Cistern
• Extends from the habenular
commissure to the foramen of Monro
• Located beneath the splenium of the
corpus callosum
• above the velum interpositum,
• with the roof of the third ventricle
below.
• Anteriorly it is beneath the fornix,
converging to a point at the foramen of
Monro.
• It lies between the pulvinar thalami
• Posteriorly there is no clear distinction
from the quadrigeminal cistern.

32. • The cistern contains
• the medial posterior choroidal
artery
• and the internal cerebral veins.

33. Corpus Callosum Cistern
• It extends from the falx cerebri medially
to the pia over the cingulate gyri laterally
• Anteriorly it follows the falx to the
crista galli and joins the lamina
terminalis cistern near the rostrum of
the corpus callosum.
• Arachnoid forming the roof of the
cistern is suspended from the inferior
margin of the falx.
• Inferiorly the cistern joins the
quadrigeminal and velum interpositum
cisterns at the end of the splenium.

34. • The cistern contains
• the pericallosal arteries
• the origins of the frontopolar and callosomarginal arteries.
• Small anterior cerebral veins may be present sometimes making connections
with the inferior sagittal sinus.
• Also contains the posterior pericallosal veins.

36. Interpeduncular Cistern
• situated between the cerebral peduncles and the
leaves of Liliequist's membrane
• roof of the cistern is formed by the inferior surface of the
mesencephalon and the lower diencephalon, the posterior
perforated substance, and the mammillary bodies
• posterior wall - formed by the posterior perforated
substance.
• upper border is situated at the posterior edge of the
mamillary bodies.
• lower border is situated at the junction of midbrain
and pons.
• anteroinferior boundary is the clivus, and laterally the
cistern joins the ambient cistern
• superiorly is limited by the carotid and crural cisterns
and the mesial temporal lobes

37. • bordered rostrally and caudally by Liliequist's
membrane.
• the diencephalic membrane separates the
chiasmatic and interpeduncular cisterns.
• the mesencephalic membrane separates the
interpeduncular and prepontine cisterns.

38. • The oculomotor nerves course in the lateral wal and form
the pillars to which the leaves of Liliequist's membrane
attach. Also the site of attachment of other arachnoid
membranes
• The membranes that converge on and form a sleeve
around the nerve are
• the mesencephalic membrane, which separates the
interpeduncular and prepontine cisterns;
• the diencephalic membrane, which separates the
interpeduncular and chiasmatic cisterns
• the anterior pontine membrane, which separates the
cerebelopontine and prepontine cisterns;
• the lateral pontomesencephalic membrane, which
separates the ambient and cerebelopontine cisterns;
• the medial carotid membrane, which separates the
chiasmatic and carotid cisterns;
• the lateral carotid membrane, which forms the lateral
wal of the carotid cistern.

39. • interpeduncular cistern contains the
• the bifurcation of the basilar artery and
the origins of the posterior cerebral and
superior cerebellar arteries
• medial posterior choroidal and
thalamogeniculate arteries, and their branches
• the basal veins of Rosenthal, median anterior
pontomesencephalic veins, and the vein of
the pontomesencephalic sulcus
• and the oculomotor nerves.

41. Ambient Cistern
• This cistern covers the lateral aspect of the
mesencephalon
• both supra- and infratentorial justifying its
inclusion in both categories
• Its medial boundary is the cerebral
peduncle and the interpeduncular cistern
• Its lateral boundaries are
• supratentorially the mesial temporal lobe
and
• infratentorially the lobu-lus quadrangularis
of the cerebellum.
• Inferiorly it shares an arachnoid wall with
the cerebellopontine cistern.
• Anteriorly the cistern is related to the crural
cistern.

42. • The ambient cistern contains
• segments of the posterior cerebral artery,
• numerous arteries to the midbrain from both
PCA's,
• the basal vein of Rosenthal.
• The superior cerebellar artery and the
trochlear nerve have their own arachnoid
sleeves around the peduncle.

44. Quadrigeminal Cistern
• The anterior limits -are the dorsal
mesencephalon, the quadrigeminal plate,
and the pineal gland.
• Posteriorly arachnoid is attached to the
tentorium and extends from the
splenium of the corpus callosum
inferiorly to the lingula of the cerebellar
vermis, above the anterior medullary
velum of the fourth ventricle.
• The cistern is contiguous superiorly with
the velum interpositum cistern and
laterally with the ambient cisterns.

45. • anterior wall is formed by the pulvinar,
superior, and inferior colliculi and the
superior cerebellar peduncles.
• lateral wall has an anterior and a posterior
part.
• anterior part is formed by the segment of
the crus of the fornix that wraps around the
pulvinar.
• posterior part is formed by the part of the
occipital cortex located below the splenium.
• roof is formed by the lower surface of the
splenium and the broad membranous
envelope that surrounds the great vein and
its tributaries.
• The cistern extends downward between the
cerebellum and midbrain into the
cerebellomesencephalic fissure.

46. • The quadrigeminal cistern contains the
• medial posterior choroidal arteries
• the great vein of Galen and its tributaries
• The trochlear nerves arise in the
quadrigeminal cistern just below the inferior
colliculi and course forward around the
midbrain to enter the ambient cisterns.
• The PCAs commonly bifurcate into their
calcarine and parieto-occipital branches within
the cistern.
• The SCAs course through the part of the
cistern that extends into the
cerebelomesencephalic fissure.

47. • The internal cerebral veins exit the velum
interpositum and the basal veins exit the
ambient cisterns to reach the quadrigeminal
cistern, where they join the vein of Galen.
• The latter vein passes below the splenium to
enter the straight sinus at the tentorial apex.
• Other veins that converge on the cistern include
the posterior pericallosal veins , the atrial veins,
and the internal occipital veins

49. Prepontine Cistern
• lies between the anterior surface of
the pons and the clivus surrounding
the basilar artery
• separated from the interpeduncular
cistern by the mesencephalic leaf of
Liliequist's membrane.
• The lateral edges of the prepontine
cistern are separated from the
cerebelopontine cisterns by the
paired anterior pontine membranes.

50. • The lower boundary of the cistern is situated at
the level of the pontomedulary sulcus, the site
of a less wel-defined membrane called the
medial pontomedulary membrane.
• The inferior arachnoidal wall of the cistern is
thickened as the vertebral arteries join to form
the basilar artery beneath the pontomedullary
sulcus
• The cistern contains the
• basilar artery,
• the origin of the anterior inferior cerebellar
artery,
• the entire free course of the abducens nerve
from the pons to Dorello's canal

53. CEREBELLOPONTINE CISTERN

54. CEREBELLOPONTINE
CISTERN
• The cerebelopontine cistern lies between
the anterolateral surface of the pons and
cerebellum
• Superiorly this cistern is separated from the
ambient cistern by the lateral
pontomesencephalic membrane.
• This membrane is attached to the
brainstem at the junction of the midbrain
and pons and to the outer arachnoidal
membrane
• Anteriorly, it intersects the oculomotor
nerve.
• This membrane spans the interval between
the PCA and SCA.

55. • Inferiorly, the cerebelopontine cistern is
separated from the cerebelomedulary
cistern by the lateral pontomedulary
membrane
• This membrane stretches from the
junction of the pons and medula to the
outer arachnoidal membrane.
• It crosses the subarachnoid space
between the vestibulocochlear and
glossopharyngeal nerves.
• Lateraly, the cerebelopontine cistern
extends to the edge of the cerebelar
surface that wraps around the pons to
form the cerebelopontine fissure.

56. • Laterally the cistern extends along the posterior petrous portion of the
temporal bone entering the internal auditory meatus and extending outwards
into Meckel's cave.
• Trigeminal nerve has its own cisternal sleeve which forms a recess into the
cerebellopontine cistern
• The cerebellopontine cistern contains the anterior inferior cerebellar artery
cranial nerves V, VII and VIII, and the lateral pontomesencephalic vein.

57. • The trigeminal nerve arises from the midpons and courses
through the superolateral portion of the cistern.
• The abducens nerve arises at the level of the
pontomedulary sulcus and ascends just lateral to the
anterior pontine membrane.
• The facial and vestibulocochlear nerves arise in the
inferior part of the cerebelopontine cistern just above the
lateral pontomedulary membrane.
• The outer arachnoid membrane extends into the internal
auditory canal and surrounds the intracanalicular segment of
the facial and vestibulocochlear nerves.
• The flocculus projects into the cerebe lopontine cistern
behind the facial and vestibulocochlear nerves.

58. • The SCA and AICA course through the cerebelopontine cistern
• The SCA enters the cerebelopontine cistern by passing through the junction of the
anterior pontine membrane and the oculomotor nerve. It courses below the
trochlear nerve and the lateral pontomesencephalic membrane, and above the
trigeminal nerve in its passage through this cistern.
• The AICA enters the lower part of the cerebelopontine cisterns by passing through
or below the anterior pontine membrane. It commonly bifurcates into its rostral
and caudal trunks within this cistern.
• The veins in this cistern converge and unite to form the superior petrosal veins,
which empty into the superior petrosal sinus

59. CP angle cistern with arachnoid cyst

60. Lateral Cerebellomedullary Cistern (or
Inferior Cerebellopontine Cistern)
• Cerebellomedullary cistern lies anterior and
lateral to the medulla
• Its anterosuperior border is the sulcus between
the medulla and the pons.
• Arachnoid over cranial nerves IX, X, and the
cranial portion of XI separate this cistern from
the cisterna magna dorsally and from the
cerebellopontine cistern superiorly.
• Ventrally a less clear arachnoid sheet separates
the cistern from the premedullary cistern.
• Its inferior border is located at the level of
the foramen magnum.

61. • extends from the pontomedullary sulcus
superiorly to the foramen magnum inferiorly
• reaches laterally along with short sleeves into
the jugular and hypoglossal foramina
accompanying the respective nerves.
• separated from the cerebelopontine cistern by
the lateral pontomedulary membrane
• The cistern contains the vertebral artery, the
origin of the posterior inferior cerebellar artery,
the retroolivary and lateral medullary veins,
• cranial nerves IX, X, XI and XII.

62. • The glossopharyngeal and vagus nerves and the
medulary portion of the accessory nerve arise within
and course through this cistern to reach the jugular
foramen.
• The spinal portion of the accessory nerve ascends
from the posterior spinal cistern to reach the
cerebellomedulary cistern.
• The lateral recess of the fourth ventricle communicates
with this cistern through the foramen of Luschka.
• The vertebral artery enters at the lower border of this
cistern and immediately leaves it to enter the
premedullary cistern.
• The (PICA) enters this cistern by reaching the anterior
surface of the rootlets of the glossopharyngeal, vagus,
and accessory nerves
• From here, the artery passes dorsaly between the
rootlets of these nerves and pursues a posterior course
around the medula to enter the cisterna magna.

63. The lateral cerebello-medullary or inferior
cerebello- pontine cistern containing cranial nerves
IX, X, and XI (arrows).
The inferior cerebello-pontine cistern has been
opened revealing cranial nerves IX, X, and XI,
while nerves VIII and VII remain hidden within the
upper cerebello-pontine cistern (arrow).

64. The inferior cerebello-pontine cistern has been opened
revealing cranial nerves IX, X, and XI, while nerves VIII and
VII remain hidden within the upper cerebello-pontine cistern
(arrow).
Both upper and lower cerebellopontine cisterns have now
been opened revealing nerves 7,8,9,10,11 and loop of aica(
arrow)

65. PREMEDULLARY CISTERN
• The premedulary cistern lies between the
anterior surface of the medula and the
arachnoid membrane covering the lower
part of the clivus
• Its upper border is located at the
junction of the pons and medula.
• It is separated from the prepontine
cistern by the medial pontomedulary
membrane.
• Inferiorly, the premedullary cistern is
continuous with the anterior spinal
cistern.

66. • The rootlets forming the hypoglossal
nerves arise in the posterior wall of this
cistern between the medulary pyramids
and the inferior olives.
• The vertebral arteries enter this cistern
by ascending through the foramen
magnum.
• They ascend obliquely through this
cistern and join at the junction of the
premedulary and prepontine cisterns.
• The paired anterior spinal arteries arise
from the vertebral arteries and join to
form a single trunk that courses in the
midline on the anterior surface of spinal
cord.

68. Cisterna Magna
• As the dorsal spinal subarachnoid space opens
into the intracranial cavity through the foramen
magnum, it widens into a large cistern, the cisterna
magna.
• limited anteriorly by the dorsal surface of the
upper spinal cord and lower medulla, and extends
to the posterior medullary velum.
• Its posterior wall is formed by the arachnoid
membrane that conforms to the inner surface of
the occipital bone above the foramen magnum.
• Superiorly in the midline it communicates with
the fourth ventricle at the foramen of Magendie,
forming a cephalad extension of the cistern
called the vallecula
• It usually ends near the lobulus pyramidalis of the
vermis but may extend all the way up the
tentorium.

69. • Superiorly, the cisterna magna projects both
anterior and posterior to the cerebelar vermis.
• The cisterna magna also opens behind the
vermis into the posterior cerebelar incisura.
• Anteriorly, it opens into the cerebe lomedulary
fissure.
• This cistern contains the inferior vermian
branches of the posterior inferior cerebellar
arteries and the median tonsillar veins.
• The PICAs pass posteriorly around the medula.
They enter the cisterna magna commonly divide
into a
• lateral trunk, which supplies the hemisphere
and tonsil,
• and a medial trunk, which supplies the vermis

71. Cisternal tap
• Suboccipital access to the cisterna magna.
• Usually done with patient sitting, with neck slightly
flexed
• A 22 gauge spinal needle is inserted exactly in the
midline between the inion and the C2 spinous
process, directed superiorly towards the glabella until
the needle strikes the occiput or enters the cisterna
magna.
• If the occiput is encountered, the needle is
withdrawn slightly and reinserted directed slightly
inferiorly, the process is repeated until the cisterna
magna is entered (a “pop” will be felt).
• The distance from the skin surface to the cisterna
magna is 4–6cm

72. SUPERIOR CEREBELLAR CISTERN
• This cistern is situated between the superior part
of the vermis and the arachnoid membrane that
rests against the lower border of the straight
sinus.
• Anteriorly, it opens into the quadrigeminal cistern
• Posteriorly, it communicates below the torcular
with the cisterna magna.
• Lateraly, it blends into the subarachnoid space
over the cerebelar hemispheres.
• The cistern contains the median and paramedian
branches of the SCAs and the superior vermian
vein.

74. • Opening a cisternal wall, with the resultant escape of cerebrospinal fluid, facilitates
the approach to lesions in front of the brainstem and cerebellum.
• Allowing cerebrospinal fluid to escape from the cisterna magna during posterior fossa
operations facilitates the exposure of lesions in the cerebellopontine,
cerebellomedullary, prepontine, and premedullary cisterns.
• Pathological processes in the subarachnoid space may conform to cisternal boundaries.
• The arachnoid septa and trabeculae separating the cisterns may prevent the spread of
blood to adjacent cisterns after aneurysm rupture.
• The resulting location of the blood, as seen on C T scans and MRI, often provides
information pinpointing the site of a ruptured aneurysm.

75. • Yasargil notes that aneurysms may become invested with the arachnoidal walls of the
cisterns and tension on the arachnoid membranes may be transmitted to the fundus of
the aneurysm
• Even when dissection is being carried out some distance away In dissecting an aneurysm, it
is helpful to know which membranes may be attached to the aneurysm.
• Aneurysms arising at the basilar apex and at the origin of the SCA may project into the
leaves of Liliequist's membrane;
• aneurysms arising at the origin of the AICA may have the anterior pontine membrane
stretched around their surface
• aneurysms arising at the origin of the PICA from the vertebral artery may project upward
into the lateral pontomedulary membrane
• aneurysms arising at the junction of the vertebral with the basilar arteries may be
enmeshed in the thick trabeculae that form the medial pontomedulary membrane

76. • An understanding of the arachnoidal membranes is especialy important in dealing with
aneurysms pointing in the direction of the oculomotor nerves.
• Traction on any of the membranes converging on the oculomotor nerve may rupture these
aneurysms.
• The outer surface of the arachnoidal membranes that are adherent to an aneurysm may
provide a plane of dissection that allows easier separation of the aneurysm from adjacent
structures.
• It may be necessary to leave some of the arachnoid membrane attached to the fundus
and wall of the aneurysm to prevent rupture of the aneurysm before a clip is applied.

77. Thank you