REVIEW PAPER

Anatomical variations and neurosurgical significanceof Liliequist’s membrane

Martin M. Mortazavi & Fareed Rizq & Olivia Harmon & Nimer Adeeb &

Mehrnoush Gorjian & Nicole Hose & Elham Modammadirad &

Pejman Taghavi & Brandon G. Rocque & R. Shane Tubbs

Received: 27 October 2014 /Accepted: 3 November 2014# Springer-Verlag Berlin Heidelberg 2014

AbstractIntroduction Liliequist’s membrane is an arachnoidmembranethat forms a barrier within the basilar cisternal complex. Thisstructure is an important landmark in approaches to the sellarand parasellar regions. The importance of this membrane waslargely recognized after the advance of neuroendoscopic tech-niques. Many studies were, thereafter, published reportingdifferent anatomic findings.Method A detailed search for studies reporting anatomic andsurgical findings of Liliequist’s membrane was performedusing “PubMed,” and included all the available literature.Manual search for manuscripts was also conducted on refer-ences of papers reporting reviews.Results Liliequist’s membrane has received more attentionrecently. The studies have reported widely variable results,which were systematically organized in this paper to addressthe controversy.Conclusion Regardless of its clinical and surgical significance,the anatomy of Liliequist’s membrane is still a matter of debate.

Keywords Liliequist’s membrane . Anatomy .

Neurosurgery . Endoscopic third ventriculostomy . Skull base

Introduction

Liliequist’s membrane (LM) (Fig. 1) was first described andillustrated by Key and Retzius [1] in 1875. In hispneumoencephalographic studies of the subarachnoid spaceand cisterns, Liliequist [2, 3], in 1956, provided the firstdetailed anatomic description of this membrane, which laterwas named after him. With the advance of neuroendoscopicand microscopic techniques, the interest of studying LM, aswell as other subarachnoid membranes and cisterns, wasrepopularized [4]. Since the earliest descriptions, many stud-ies have reported on the structure of LM. However, theirresults were contradictory, and controversy still exists(Table 1).

Within this study, the authors have provided a detailedreview of the various previously published descriptions ofLM, including variations.

Presence of LM

LM is found in most individuals, mainly separating theinterpeduncular, prepontine, and chiasmatic cisterns. It hasbeen reported to be absent in 15.4 [5] and 42.9 % [6] ofcadavers in different studies. In such cases, free communi-cation between the interpeduncular, prepontine, and chias-matic cistern is expected, forming one large cistern. Acarotid-interpeduncular wall separating the interpeduncularand carotid cisterns was introduced by Brasil and Schneider[7] in 1993, which could be absent unilaterally or bilaterally.However, this contradicts the description of Froelich [5],who stated that the interpeduncular cistern communicates

M. M. Mortazavi (*) :N. Adeeb :N. Hose : E. ModammadiradCalifornia Brain Institute, Los Robles hospital and Medical Center,215 W Janss Road, Thousand Oaks, CA 91360, USAe-mail: M_Mortazavi@hotmail.com

F. Rizq :O. Harmon : P. Taghavi :B. G. Rocque :R. S. TubbsPediatric Neurosurgery, Children’s Hospital of Alabama,Birmingham, AL, USA

M. GorjianInternational Neuroscience Institute, Hannover, Germany

R. S. TubbsDepartment of Anatomical Sciences, St. George’s University,True Blue, Grenada

R. S. TubbsCenter of Anatomy and Human Identification, University of Dundee,Dundee, UK

Childs Nerv SystDOI 10.1007/s00381-014-2590-5

anterolaterally with the carotid cistern along the posteriorcommunicating artery. Brasil and Schneider also stated thatin cases of single LM (see below), the prepontine cisternmay communicate with the carotid cistern anterolaterallyaround the posterolateral free border of LM, and with theambient cistern laterally along the course of the posteriorcerebral artery and across the posterolateral free border ofLM [5]. Moreover, Brasil and Schneider [7] described freecommunication between the carotid and crural cisternssuperolateral to the free margin of LM. Matsuno [8] alsoreported communication with the ambient and cruralcisterns.

Types of LM

Based on the various descriptions, LM has been divided intothree types. In type A, the LM is composed of two leaflets, thediencephalic leaf (DL) and mesencephalic leaf (ML) thatoriginate at the dorsum sellae (Fig. 2). In type B, the LMappears as one leaf anteriorly and two leaflets posteriorly, withthe LM arising as single membrane and then splits into DLand ML. In type C, the LM appears as a single (diencephalic)membrane [5].

Unless otherwise specified, this classification will be usedhereafter to describe the type of LM in different studies.

Single membrane (type C)

The classic description by Liliequist [2, 3] was of a singlemembrane with forward convexity, extending from the dor-sum sellae to the anterior edge of mammillary bodies (type C).Similar description was provided by Brasil and Schneider [7].Yaşargil [4, 9] added that this well-developed membranestretches like a curtain from one mesial temporal surface tothe other. Thus, a full description of the type C LM would beof a single, non-fenestrated membrane that arises inferiorlyfrom the basilar arachnoid membrane covering the dorsumsellae and the posterior clinoid processes, curves anteriorly,and attaches superiorly to the pia of the hypothalamus justanterior the mammillary bodies (premammillary), posterior tothe infundibulum. Laterally, it attaches to the pia of the mesialsurface of the temporal uncus [10]. This lateral extension (atthe carotid-interpeduncular wall) is perforated by the oculo-motor nerve (Figs. 1, 2, and 3) and the posterior communicat-ing artery [7].

Lateral extension

Other authors disagreed with the extent of the lateral exten-sion. Epstein [11] described the LM as a semilunar transversemembrane stretching obliquely between the oculomotornerves, which is similar to the description by Matsuno [8].Such a termination would render the LM as a continuous,imperforatedmembrane. Fox [12], on the other hand, acceptedboth types of lateral extension and added that it may also endjust medial to the oculomotor nerve, to which it will beconnected via arachnoid trabeculae. Similar results were re-ported by Anik [13] and Fushimi [14]. Froelich [5] describeddifferent relations with the oculomotor nerve. In 46.1 % ofspecimens, the nerve was surrounded by the lateral aspect ofthe ML, creating a separate oculomotor cistern. In 15.4 %, thenerve traveled between two lateral leaflets of the ML. Thesuperior leaf was attached to the pia of the mesial surface ofthe temporal lobe, and the inferior leaf was continuous withthe arachnoid above and below the level of the incisura. In23.1 %, the nerve was above the ML and connected to it by anarachnoidal ring [5]. Zhang [15] agreed that the ventral wall ofthe oculomotor cistern is partly formed by the lateral extensionof the LM (and partly by the basal arachnoid mater), but theauthor regarded the dorsal membrane (oculomotor membrane)as a different entity and differentiated it from the temporalmembrane (see below) described by him as the lateral exten-sion of the DL [15].

Superior extension

Regarding its superior attachment, Vinas and Panigrahi [10]added that besides the premammillary attachment (see above),a retromammillary attachment may also be found. This is

Fig. 1 Schematic illustration of the basal surface of the brain and adja-cent arachnoid membranes. Note the abducens and oculomotor nervespiercing Liliequist’s membrane

Childs Nerv Syst

Tab

le1

Com

parisonbetweenvariousdescriptions

ofLiliequist’smem

brane

Study

Type

ofstudy

Num

berof

leaflets

Origin

Insertion

Lateralattachment

Freeborder

Relationto

the

PCoA

Relationto

theBA

[3]

Pneumoencephalo-

graph

1Arachnoid

overtheDS

andPCP

Prem

ammillary

Oculomotor

nerve

––

–

[11]

–1

Arachnoid

overtheDS

andPCP

Prem

ammillary

Oculomotor

nerve

Lateralto

theoculom

otor

nerve

––

[9]

–1

Arachnoid

overtheDS

andPCP

Prem

ammillary

Mesialsurface

––

–

[8]

–2(D

LandML)

Arachnoid

overtheDS

andPCP

DL:pre–or

postmam

millary.ML:

pontom

esencephalicjunctio

nOculomotor

nerve

–Lateralto

thelateral

border

ofthe

ML

Penetrates

theML

orissurrounded

byacuffform

edby

ML

[12]

–1

Arachnoid

overtheDS

andPCP

Prem

ammillary

At,medial,or

lateral

totheoculom

otor

nerve

Medialo

rlateraltothe

oculom

otor

nerve

––

[7]

Cadaveric

1Arachnoid

overtheDS

andPCP

Prem

ammillary

–Betweeninferolateral

border

oftheoptic

tractsandtheuncus

Penetrates

theLM

–

[6]

Cadaveric,and

sheet

plastin

ation

1Arachnoid

overtheDS

andclivus

Endsas

afree

border

Arachnoid

aboveand

belowtentorium

Inthesuperoposteriorpart

Lateralto

thelateral

border

oftheLM

–

[10]

Cadaveric

1Arachnoid

overtheDS

andPCP

Pre-

orpostmam

millary

Mesialsurface

–Penetrates

theLM

–

[18]

Cadaveric

3(D

L,M

L,

andDML)

DL:from

theMLover

theDSandPC

P.ML:

posteriorsurfaceof

infundibulum

andto

basalchiasmatic

mem

brane.DML:DL

anteriorly,and

MLand

MPM

Mor

LPM

Minferiorly

DL:ator

behind

(premam

millary)

theinfundibulum

.ML:junctio

nbetweensuperior

1/3andinferior

2/3or

midpointo

ftheBA.D

ML:

superiorly

tothelateralm

argin

ofMB

Mesialsurface

above

tentorium.A

rachnoid

covering

cerebellar

hemispheres

and

midbrainbelow

tentorium

DL:betweeninferolateral

border

oftheoptic

tracts

andtheuncus.MLb :in

fronto

ftheBA

bifurcation.DML:

posteriorlybetweenthe

attachmenttothelateral

marginof

theMBandthe

attachmenttotheML

Penetrates

theDL

MLparallelsthe

anterior

wall

oftheBA

[14]

3DCISSMRim

aginga

3(D

L,M

L,and

sellar)

Arachnoid

overtheDS

andPCP

DL:MB.M

L:−

Ato

rmedialtothe

oculom

otor

nerve

––

–

[5]

Cadaveric

Variable(1

or2,DLandML)

Arachnoid

overtheDS

andPCP

DL:betweentheMBand

infundibulum

.ML:ends

asafree

posteriorborder

DL:paramedian

perforatingsubstance.

ML:parahippocam

pal

gyrusandcontinues

with

arachnoidabove

andbelowtentorium

1an

dDL:posterolateral

betweenthecentral

attachmenttothe

diencephalon

and

tentorialedge.ML:in

fronto

ftheBAbifurcation

Variable

MLends

infront

ofBA

[17]

Cadaveric

2(D

LandML)

Arachnoid

overtheDS

andPCP

DL:prem

amillray,retrom

ammillary,

ormam

millaryapex.M

L:

pontom

esencephalicjunctio

n

Oculomotor

nerve

–Lateralto

thelateral

border

oftheDL

Penetrates

theML

orissurrounded

byacuffform

edby

ML

[19]

Cadaveric

3(D

L,M

L,

andHL)

DLan

dML:arachnoid

over

theDSand

PCP.HL:oculom

otor

nerveinferiorly

DL:prem

amillray,retrom

ammillary,

ormam

millaryapex,oranterior,

middle,or

posteriorto

thetuber

cinereum

.ML:ator

abovethe

pontom

esencephalicjunctio

nand

tothesuperior

segm

ento

ftheBA.

HL:superiorly

tothelateraledge

ofhypothalam

us

Oculomotor

nerve

DL:betweenthesuperior

attachmentand

the

occulomotor

nerve.

MLb :anterior

orlateral

border.H

Lb :anterior

orposteriorborder

Lateralto

thelateral

border

oftheDL

MLattaches

tothe

superior

segm

ent

oftheBA

Childs Nerv Syst

surgically important because in these patients, it is less impor-tant to fenestrate the LM during third ventriculostomy, be-cause the third ventricle will be opened into theinterpeduncular cistern following fenestration of the floor.While in cases of premammillary attachment, LM membra-notomy is important [10], and failure to open the LM duringthird ventriculostomy might result in failure of the procedure[16]. Similar findings were also described by Matsuno [8],regarding the attachment of the DL. Inoue [17] reported bothtypes of superior attachments of the DL. The DLwas attachedto the posterior edge of the mammillary bodies(retromammillary) in 47 % of the brains examined, the apex

Tab

le1

(contin

ued)

Study

Type

ofstudy

Num

berof

leaflets

Origin

Insertion

Lateralattachment

Freeborder

Relationto

the

PCoA

Relationto

theBA

[13]

Cadaveric

Variable(1

or2,

or3,DML)

Arachnoid

overtheDS

andPCP

1or

DL:prem

amillray,

retrom

ammillary,or

mam

millary

apex.M

L:BAbifurcationor

vertebrobasilarjunctio

nand

continuesas

MMPM

Ato

rmedialtothe

oculom

otor

nerve

Variable

–MLattaches

tothe

BAbifurcationor

vertebra-basilar

junctio

n

[15]

Cadaveric,and

histologic

Variable(1

or2,

DLandML)

DLan

dMML:Arachnoid

over

theDSandPC

P.LML:DLandMML

medially,and

the

arachnoidover

the

posteriorborder

ofthe

oculom

otor

trigone

anteriorly

DL:ator

prem

ammillary.ML:above,

ator

belowtheBAbifurcation.

LML:laterally

tothearachnoid

covering

thefree

edge

ofanterior

tentorium

cerebella.P

osteriorly

tothelateralp

ontomesencephalic

junctio

n

Mesialsurface

and

continueswith

arachnoidaboveand

belowtentorium

DL:betweeninferolateral

border

oftheoptic

tracts

andtheoculom

otor

sheath

Lateralto

thelateral

border

oftheDL

Above,atorbelow

theBAbifurcation

aThree-dim

ensionalconstructiv

einterference

insteady

stateMRim

aging

bUncom

mon

LM

Liliequist’smem

brane,MLmesencephalicleaf,D

Ldiencephalicleaf,D

MLdiencephalic-m

esencephalicleaf,H

Lhypothalam

icleaf,M

MLmedialm

esencephalicleaf,L

MLlateralm

esencephlaicleaf,

MPMM

medialpontom

esencephalic

mem

brane,

LPMM

lateralpontom

esencephalic

mem

brane,MBmam

millarybodies,DSdorsum

sellae,PCPposteriorclinoidprocesses,BAbasilarartery,PCoA

posteriorcommunicatingartery

Fig. 2 Schematic illustration of LM illustrating its component parts (afterFroelich et al. [5])

Fig. 3 Endoscopic view of LM following traversing the floor of the thirdventricle. Note the following structures deep to the intact LM: B basilarartery,DS dorsum sella, VI abducens nerve. The oculomotor nerve is seenin the upper pole of the photograph

Childs Nerv Syst

of the mammillary bodies in 33 %, and the anterior edge ofmammillary bodies (premammillary) in 20 %. Anik [13], onthe other hand, reported the direct attachment to the mammil-lary bodies to be the most common type, found in 70.8 % ofspecimens. Lu and Zhu [18] described a more anterosuperiorattachment of the DL on the posterior surface of the infundib-ulum, found in 50 % of cadavers. In the other 50 %, the DLhad a superior attachment that is 1.96 to 4.92 mm posterior tothe infundibulum (premammillary) [18].

Free border

Brasil and Schneider [7] have divided the single LM based onits cisternal relations into three parts (or walls): two carotid-interpeduncular walls laterally and a chiasmatic-interpeduncular wall medially. Each one of these walls couldbe selectively absent. They also added that besides its classicsuperior attachment, the LM also attaches to the pia of theinferior surface of the optic tracts. A free margin between theinferolateral border of the optic tracts and the uncus is ob-served. Superolateral to this free margin, free communicationbetween the carotid and crural cisterns is present [7]. The samefree margin was also reported by Lu and Zhu [18] in the DL ofLM. On the other hand, Zhang [15] observed this free lateralborder of the DL between the inferolateral border of the optictracts and the oculomotor sheath, found in only 16.7 % ofspecimens [15].

Based on their description on the lateral extension of LM(see above), Epstein [11] and Fox [12] reported the presenceof free border lateral or at (or medial to) the oculomotor nerve,respectively. Froelich [5] described the free border to belocated posterolaterally between the central attachment to thediencephalon and tentorial edge in type C and in DL of typesA and B and in the posterior end of ML in front of the basilarbifurcation in types A and B. Zhang and An [6] found that thefree border of LM is located on its superoposterior part. All ofthese authors agree that the free border will be attached to thesurrounding structures via arachnoid trabeculae. These trabec-ulae are not considered as part of the LM, with some excep-tions [15] (see below), and they also render it meaningless toidentify the border of an irregular arachnoid trabecular net-work, because they vary greatly, and may be the cause ofvariations in gross anatomic findings. This was also supportedby the findings of Anik [13], who proclaimed that various freeedges were found in the ML in 29.2 % of specimens, and thatthe LM was totally closed in 50 % of specimens.

The carotid-chiasmatic walls

Brasil and Schneider [7] contradicted the previous descriptionof Epstein [11], Fox [12], and Yaşargil [9] that the LM maysurround the infundibulum to create a “hypophyseal cistern.”They suggested that these surrounding membranes represent

the carotid-chiasmatic walls (or chiasmatic membrane), andadded that the LM is always located posterior to the infundib-ulum [7]. The same description was made by Vinas andPanigrahi [10], who also added that although the chiasmaticmembranes arise from LM, but they represent a differententity.

These controversies were further clarified by Zhang andAn [6]. Using modified E12 sheet plastination method, ca-daveric dissections and electron microscopy, the authors con-cluded that the arachnoid membrane-like LM has an architec-ture that is significantly different from that of the arachnoidtrabeculated carotid-chiasmatic walls. The LM appearsthicker, imperforated and with a more clean surface, and iscomposed of two layers of arachnoid mater (presumed to bethe DL and ML described in other studies) that are bettervisualized in its middle portion. However, in 42.9 % of ca-davers, no double layers could be found. The carotid-chiasmatic walls, on the other hand, are composed of accu-mulated, irregularly oriented arachnoid trabeculae that extendfrom the LM to the pia mater, covering the surface of the opticchiasm. Unlike LM, these walls have openings of varioussizes, and are punctured by perforating branches from theposterior communicating and internal carotid arteries, whichgive these walls a vascularized appearance. This is surgicallyimportant, as it suggests that the most suitable site for openingLM and approaching the interpeduncular cistern is the part ofLM between the carotid-chiasmatic wall and the oculomotornerve [6], with careful attention to the posterior communicat-ing artery and its branches [18].

Regarding the lateral extension and free border, Zhang andAn [6] proclaimed that the free border of LM is located on itssuperoposterior part and continues along the entire length andcan be grossly visualized. At this free border, LM is connectedvia arachnoid trabeculae to the surrounding structures, includ-ing the infundibulum and mammillary bodies. Laterally, theLM neither attaches to the uncus nor to the oculomotor nerve.But, instead, it fans out near the free edge of the tentorium andcontinues with the arachnoid mater covering the tentoriumbelow and above the tentorial notch. Zhang and An [6] did notcomment on the anterior attachment of LM but regarded themembrane as continuous anteroinferiorly with the arachnoidmater covering the tentorium, the dorsum sellae, and theclivus [6].

Double membranes (types A and B)

The first description of the LM as double membranes wasmade by Matsuno [8]. Similar description was also providedby other authors including Inoue [17]. The classic descriptionis of an arachnoid membrane arising from the arachnoidcovering of the dorsum sellae and the posterior clinoid

Childs Nerv Syst

processes (some also add the arachnoid covering of the pos-terior petroclinoid and adjacent tentorial edge [5]), extendingbetween the oculomotor nerve and then splits into two leaflets,DL and ML (type B). According to Matsuno [8], the previousdescription of a single membrane made by Liliequist [2, 3]was of the DL, which can be visualized by the pneumatographdue to its imperforated nature. In contrast, the perforated MLwas only visualized in later cadaveric studies [8].

The DL

The DL is a thicker and mostly imperforated membrane thatattaches to the posterior or anterior edge, or to the tip of themammillary bodies, separating the interpeduncular and chias-matic cisterns. At its superior end, the DL sends many arach-noid trabeculae to attach to the surrounding structures, includ-ing the infundibulum and mammillary bodies [5, 8, 17]. TheDL is thick in most cases (54 % [13], 75 % [18]) but may alsobe thin (25 % [18], 46 % [13]). It is also mostly semitranspar-ent (83.3% [13], 87.5% [18]) but may also be opaque (12.5%[18], 16.7 % [13]). The DL may also be a largely poroustrabeculated membrane [15, 19], and in these cases, it mayhave only a small crescent-shaped, dense, non-porous part inits anteroinferior attachments [15]. The DL may also be ab-sent, as reported in 12.5 % [13], 25% [15], and 26.7 % [19] ofcadavers in different studies. In these cases, the chiasmatic andinterpeduncular cistern will merge into one cistern [19].

The lateral attachment of the DL was related to the arach-noid sheath surrounding the oculomotor nerve, with numeroustrabeculae extended from the oculomotor nerve to the uncusand tentorium. This goes along the classic description as acontinuous and imperforated membrane [8, 17]. Lu and Zhu[18] have reported a case in which a unilateral window wasfound in the DL, connecting the posterior communicatingcistern with the interpeduncular cistern [18].

The length of DL from the inferior to the superior edgeranges between 5.0 and 13.0 mm (mean, 10.8 mm). The widthat the inferior edge ranges between 10.0 and 28 mm (mean,19.9 mm), and the width at the superior edge ranges between5.0 and 14.0 mm (mean, 9.5 mm) [19].

The ML

ML is a thinner and mostly perforated (by the basilar artery)membrane that extends backward and attaches to thepontomesencephalic junction, separating the interpeduncularand prepontine cisterns [5, 8, 17]. Lu and Zhu [18] described adifferent course and termination of the ML (see below). Otherauthors reported a free posterior border for the ML that isconnected to the basilar bifurcation and surrounding structuresby arachnoid trabeculae [5]. Occasionally, the ML may bethick (or equal in thickness to the DL) and have small perfo-rations. It may also form a tight cuff around the basilar artery,

but it more commonly has a large opening through which thebasilar artery ascends [8, 17].

The length of the ML from the anterior to the posterior endranges between 2.5 and 10.8 mm (mean, 4.8 mm). The widthat its posterior attachment ranges between 2.5 and 14 mm(mean, 8.3 mm). In cases of type A LM, the width of theanterior edge of theML is similar to that of the inferior edge ofthe DL. In cases of type B LM, the width of the anterior edgeof the ML ranges between 3.0 and 22.0 mm (mean, 12.6 mm)[19].

Findings of Froelich

In the cadaveric study of Froelich [5], LM was identified in84.6 % of cadavers. When present, the LM consisted of eitherone (type C, most common) or two leaflets (types A and B).Type A was found in 15.4 % of specimens, and besides theclassic description (see above), the author reported a lateralattachment of the DL to the paramedian perforating substanceand of the ML to the pia of the parahippocampal gyri (mesialsurface of the temporal lobe). Type B was also found in15.4 % of specimens, where the LM arises as a single mem-brane and then splits into two leaflets, DL and ML, withsimilar connections as type A. In these two types, the authordescribed the free border to be located in the posterior end ofthe ML, in front of the basilar bifurcation but may occasion-ally cover the basilar tip partially. Type C was found in 53.8 %of specimens, with a posterolaterally located free border be-tween the central diencephalic attachment and the tentorialedge. This free border allows communication between theprepontine and carotid cisterns [5].

The author added that the LM is continuous with thearachnoid covering the temporal fossa floor and the superioraspect of the tentorium, above the tentorial edge. Below thetentorial edge, the membrane was continuous with the arach-noid, covering the inferior aspect of the tentorium and poste-rior fossa dura [5]. Froelich [5] also agreed with other authorsthat the LM is continuous laterally with the lateralpontomesencephalic membrane [8] that separates the ambientcistern from the cerebellopontine cistern, and with the caudaloculomotor membrane [20–22], as they share a commonembryological origin [5].

Relation with the posterior communicating artery

Froelich [5] also described the different relationship of LMwith the posterior communicating artery. In specimens withsingle membrane (type C) and posterolateral free border, theposterior communicating artery coursed above the membraneand crossed the free border to join the posterior cerebral artery.In specimens with DL and ML, the artery coursed betweenthose leaflets [5]. Zhang [15] stated that in all cases, theposterior communicating artery courses lateral to the lateral

Childs Nerv Syst

border of the DL [15]. Lu and Zhu [18], on the other hand,stated that the artery penetrates the DL to enter theinterpeduncular cistern. They also reported one specimen inwhich a unilateral posterior communicating artery penetratedthe DL at its inferior border, coursed within the leaf, and left atthe superior border to enter the deep part of theinterpeduncular cistern [18].

Histological layers

Similar to the description of Zhang and An [6], Zhang [15]found the LM to be composed of two layers of arachnoidmater, named the basal and attaching layers. The basal layerarises from the basal arachnoid mater covering the dorsumsellae and posterior clinoid processes. The basal arachnoidmater is multiplied into several cellular layers at the basalattachments of the LM, and then extends superoposteriorlyand folds upon itself to form the uninterrupted basal layer ofLM. This basal layer extends as the inferior part of the DL (nocomments were made on the ML) and extends laterally be-yond the oculomotor nerve to continue with the arachnoidmater covering the tentorium below and above the tentorialnotch. In the midsagittal plane, the basal part comprised morethan half of the entire length of LM in 50 % of specimens andless than 20 % of the length in 25 % of specimens. Theattaching layer spreads from the posterior border of the basallayer to attach on the surrounding structures. The diencephalicpart of this layer (attaching the DL to the mammillary bodies)is composed of accumulated arachnoid trabeculae. Thus, incontrast to previous studies (see above), Zhang [15] haveconsidered these arachnoid trabeculae as a part of the LM.

Temporal membrane

Regarding the gross lateral extension, Zhang [15] described atriangular membrane, found in 58.3 % of specimens, whichspreads from the lateral free border of the DL to the mesialtemporal uncus. The author named it the temporal membraneand considered it as part of the DL. This membrane separatesthe carotid from the ambient cistern, and sends variable arach-noid trabeculae to attach to the arachnoid sheath surroundingthe oculomotor nerve. In contrast to the DL, the temporalmembrane is usually porous and trabeculated and is occasion-ally a sheet-like membrane with small openings. When thetemporal membrane is present, the posterior communicatingartery always courses above the temporal membrane [15].

Medial and lateral ML

The ML was divided by Zhang [15] into three parts, onemedial ML and two lateral ML. The medial ML extendsbetween the oculomotor nerves, and is referred to as the MLin previous studies, separating the interpeduncular and

prepontine cisterns [15]. Similar division was also appliedby Qi [23] who used the name anterior perimesencephalicmembrane to describe the medial ML [23].

The relationship between the DL and medial ML is of thesame types described earlier as type A, B, or C (the classicalclassification of LM). In the study of Zhang [15], however,type B was the most common, followed by type A and then C.When the DL is absent, the medial ML is defined as thecombination of an anteriorly located crescent-shaped densenon-porous part, which attaches to the basal arachnoid mem-brane (exactly the same as that of DL) and courses betweenthe oculomotor nerves (to which it laterally attaches) and aposteriorly located porous trabeculated part, which extends tothe caudal end of the medial ML [15].

According to Zhang [15], regardless of the type of medialML present, its caudal end may be above, at, or below thelevel of the terminal basilar bifurcation. The lateral ML isequal to the lateral pontomesencephalic membrane (see be-low). It separates the ambient and cerebellopontine cisterns,and it communicates medially with the DL and medial MLbelow the oculomotor nerve (the part of the LM that formspart of the ventral sheath of the oculomotor), and attaches tothe anterior tentorial edge laterally. Anteriorly, it attaches tothe basal arachnoid mater covering the posterior border of theoculomotor trigone. Posteriorly, it sends out variable arach-no id t r abecu l ae to a t t ach on the an te ro la t e ra lpontomesencephalic junction. The size of the non-porous,sheet-like portion of the lateral ML varies and is inverselyproportional to the non-porous sheet-like portion of the DL or,when the DL is absent, the ML. Therefore, when the non-porous, sheet-like part of the DL or ML is prominent, thecounterpart of the lateral ML is smaller, and vice versa [15].

Based on these findings, Zhang [15] have proposed anotherclassification of the LM according to presence (type I) orabsence (type II) of the DL. In type I, most common, theLM has similar appearance to the one previously describedbeing divided into types A, B, and C (classical classification).In type II, only the medial and lateral ML are present, withsimilar course as described above, and with no separationbetween the interpeduncular cistern and the chiasmatic cistern[15] (Table 2).

Oculomotor membrane

Regarding the oculomotor membrane, Zhang [15], who dis-tinguished it from the temporal membrane, have describedthree coronal configurations based on its relation to the carotidmembrane: inverted Y-shaped (most common), inverted V-shaped, and inverted U-shaped (least common). In theinverted Y-shaped configuration, lateral carotid membraneconstitutes the upper arm of the inverted Y and attachessuperiorly to the mesial temporal uncus near the attachmentof the temporal membrane. In the inverted V-shaped

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configuration, the lateral carotid membrane is absent, and theapex of inverted V directly attaches on the mesial temporaluncus near the attachment of the temporal membrane. In theinverted U-shaped configuration, the dome of the oculomotormembrane is adhered to the dorsal surface of the oculomotornerve and may be connected to the mesial temporal uncus byscattered arachnoid trabeculae, which represents a less well-developed lateral carotid membrane. Therefore, according tothe author, LM can attach to the mesial temporal uncus eitherdirectly by its temporal membrane or indirectly by the oculo-motor membrane [15].

Triple membranes

Diencephalic-mesencephalic leaf

Variable descriptions of the ML

The first description of the LM as composed of three leafletswas made by Lu and Zhu [18]. These were the DL, ML, andthe diencephalic-mesencephalic leaf (DML). In contrast to theDL, their description of the ML was different from that byother authors. In their study on eight cadavers, Lu and Zhu[18] proclaimed that the ML represents an intact, thick, dense,and imperforated membrane that forms the anteroinferior wallof the interpeduncular cistern. According to them, the MLintersects with the DL at the dorsum sellae and posteriorclinoid processes, rather than originating from this point.Rostrally, it spreads along the surface of the diaphragma sellaeand attaches to the posterior surface of the infundibulum, 1.48to 3.98 mm above its bottom, and fuses with the basal chias-matic membrane (the arachnoid covering the floor of thechiasmatic cistern). Inmost cases (87.5%), theMLwas totallyabove the diaphragma sellae and did not enter the intrasellarregion. In the remaining 12.5 %, the ML turned downwardaround the infundibulum through the opening of thediaphragma sellae along with the chiasmatic membrane toencircle the pituitary gland. Caudally, the ML ends at thejunction of the superior one-third and the inferior two-thirdsof the basilar artery or the midpoint of the basilar artery andthe ventral surface of the pons at this level. In contrast to the

classic description, ML does not attach to the pons directly orform a cuff around the basilar artery, but only parallels theanterior wall of the basilar artery and the ventral surface of thepons. The authors also added that it is not the ML but themedial pontomesencephalic membrane that separates theinterpeduncular and prepontine cisterns and is penetrated bythe basilar artery. This medial pontomesencephalic membranewas also found to join the ML to the pons in most cases(87.5 %). The ML was found to have a free posterior borderin the remaining cases (12.5 %), similar to that described byFroelich [5]. Laterally, the ML attaches to the mesial surfaceof the temporal lobe, but also encircles the oculomotor nervefor variable distance (2.72 to 4.98 mm) through the cavernoussinus [18].

Anik [13], who regarded the LM as being single, double, ortriple membrane, made a similar description. However, theauthor regarded that the posterior end of the ML at the basilarbifurcation may also course towards the vertebrobasilar junc-tion through the prepontine cistern. Anik [13] also reportedone specimen in which two ML were identified, one splitsfrom the anterior portion and attaches to the basilar artery atthe level of the vertebral artery bifurcation, and the other splitsfrom the posterior portion of the LM and attaches to the basilarartery bifurcation. The author, however, did not comment onthe anterior extension of ML, described by Lu and Zhu [18],but regarded the ML to be in direct continuation anteriorlywith the basal arachnoid membrane [13]. Fushimi [14] on theother hand, recognized the anterior extension as a distinct partof the LM, named the sellar part [14]. Although this part couldbe identified in most cases, it may also not be visualized.

In the study of Anik [13], the ML continues caudally as themedial and lateral pontomesencephalic membrane. The medi-al pontomesencephalic membrane (MPMM) courses antero-laterally, medial to the trigeminal and abducens nerves. Thelateral pontomesencephalic membrane (LPMM) coursesposterolaterally, lateral to the trigeminal and abducens nerves.Both membranes are connected between these two nerves.The LPMM joins the formation of a ring that covers the thirdnerve, attaching to the DL on the distal posterior surface of thenerve and attaching to the mesial temporal surface coursinglateral to the uncus. Both MPMM and LMPMM continueinferiorly as the medial and lateral pontomedullary mem-branes, respectively. These pontomedullary membranes

Table 2 Classifications of Liliequist’s membrane

Classic classification as described by Froelich (based on the number of leaflets andtheir origin)

Classification of Zhang (based on the presence or absence of DL)

Type A LM is composed of DL and ML that originate from the DS. Type I DL is present, and the LM might be of type A, B, or C.

Type B LM arises as single membrane and then splits into DL and ML. Type II The DL is absent, and only ML is present.

Type C LM is composed of only 1 leaf (described as DL). – –

LM Liliequist’s membrane, DL diencephalic leaf, ML mesencephalic leaf, DS dorsum sellae

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continue as a single membrane through the prepontine andpremedullary cisterns, as the prepontine and premedullarymembranes, respectively. They attach to the dura, coveringthe superior aspects of the cranial nerves laterally [13]. Ac-cording to Matsuno [8] and Lu and Zhu [24], the medialpontomedullary membrane is located at the medial part of thepontomedullary sulcus and separates the prepontine andpremedullary cisterns, and the lateral pontomedullary mem-brane is located at the lateral pontomedullary sulcus and sep-arates the cerebellopontine and cerebellomedullary cisterns.

The diencephalic-mesencephalic leaf

The DMLs are pairs of two parallel leaflets that connect theDL to the ML. Each leaf represents a triangular or quadrilat-eral membrane that is dense and intact in most cases (87.5 %)but may also be composed of sparse networks [18]. In thestudy of Anik [13], the DMLs were not consistently presentbut were seen in 70.8 % of specimens [13]. They are locatedventral to the cerebral peduncles and medial to the oculomotornerves, separating the interpeduncular and oculomotor cis-terns, and, as well as the other LM leaflets, taking part in theformation of the oculomotor sheath. The DML attaches to theDL anterosuperiorly, the lateral margin of the mammillarybodies superiorly, the ML anteroinferiorly, and the medial orlateral pontomesencephalic membranes inferiorly (which rep-resents the direct continuation of the ML) and has a freeposterior borders (between the attachment to the lateral marginof the mammillary body and the attachment to the ML). Thespace between the posterior margin of the DML and thecerebral peduncles allows the communication between theinterpeduncular cistern with the crural and ambient cisterns.Lu and Zhu [18] stated that absence of the DMLs in previousstudies is probably due to these leaflets being fragile, thin, andeasily destroyed. Therefore, they are often overlooked ordescribed as arachnoid trabeculae joining the LM to the ocu-lomotor nerves [18].

Hypothalamic membrane

Variable descriptions of the DL and ML

In their study on 15 cadavers, Wang [19] divided LM intothree leaflets: the DL, ML, and hypothalamic leaf (HL).Besides the previously described superior attachment of theDL, Wang [19] added that the DL may be attached to theanterior, middle, or posterior edge of the tuber cinereum.Laterally, the DL attaches to the oculomotor nerve and iscontinued with the inferior oculomotor sheath. Between thesuperior attachment and the oculomotor nerve, a free border isfound to form a cuff around the posterior cerebral artery. Theauthor also found that the posterior communicating artery onlyrarely penetrates the DL, as a small branch of the basilar artery

may also do. Many arachnoid trabeculae were also foundextending from the DL to the posterior communicating andposterior cerebral arteries and from the superior end of the DLto the surrounding structures [19].

Similar to the findings of Matsuno [8], the ML in the studyof Wang [19] extended to the pontomesencephalic junctionbut could also reach up to 5.0 mm (mean, 2.5 mm) above thejunction. Beside the classical types A and B of ML origin, theauthor reported the ML arising from the HL in 20 % ofspecimens and having a free anterior edge in 6.7 % of spec-imens. The lateral attachment of the ML was mostly to theoculomotor sheath but was also occasionally seen to be at-tached to the posterior cerebral or the superior cerebellararteries. Free lateral borders were also seen in some speci-mens. Many arachnoid trabeculae were also found extendingfrom the ML to the surrounding neurovascular structures [19].

Between the sellar segment of the DL, the anterior part ofthe ML, and the posterior part of the superior clivus, a spacecould be identified anteroinferior to the interpeduncular cisternand was called post-dorsum cistern by Wang [19]. There wasno arachnoidmater separating the post-dorsum cistern from theprepontine cistern, and when the DL was absent, all the spacebeneath the ML became the prepontine cistern, and the MLconstituted the superior wall of the prepontine cistern [19].

The hypothalamic leaf

The HLs described by Wang [19] were pairs of triangular orquadrilateral membranes that extended upwards from the oc-ulomotor nerve and/or the lateral part of the DL (or ML) toattach to the lateral edge of the hypothalamus. In most cases,the HL was thin and trabeculated, with many scattered holes.Only rarely, it may develop into complete net-like shapes. Intwo specimens with absent DL, the HLwas developed so wellthat it seemed to replace the DL. Inferiorly, HL was mostlyattached to the superomedial or inferomedial edge of theoculomotor nerve. It may also be attached to superior wallof the oculomotor nerve sheath, the lateral part of the DL, orthe lateral part of the ML (when the DL is absent). The HLmay extend laterally above the oculomotor nerve attaching tothe temporal lobe, with an attaching line ranging from 1.5 to8 mm. Inferoanteriorly, the HL is attached to the arachnoidmembrane covering the diaphragma sellae, the dorsum sellae,or the superior wall of the cavernous sinus by some fibroustrabeculae. Superiorly, the HL attached mostly to theinferolateral surface or the lateral edge of the tuber cinereum.It may also attach to the lateral edge of the median eminencearea or the side of the infundibulum, the paramedian areabelow the optic chiasm/tract, or extend to the inferolateralsurface of the mammillary bodies. The length of the superioredge ranged between 6 and 20 mm (mean, 10 mm), and thedistance between the upper attaching edges of HLs rangesbetween 2 and 8 mm. The anterior edge of the HL was mostly

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attached to the dorsum sellae or the posterior clinoid process.It may also be continuous with the carotid–chiasmatic walls(which indicate that they represent distinct structures), con-nected with the fibrous trabecular network in the chiasmaticcistern, attached to the anterior edge of the DL or ML, or maybe completely free. The posterior edge was mostly adhered inmultiple places to the posterior communicating artery or itsbranches, which coursed through the HL to supply the tubercinereum and themammillary bodies. It may also be complete-ly free, surrounding the posterior cerebral artery in a cuff-likeshape and connect via arachnoid trabeculae to the tubercinereum, the mammillary bodies, or the cerebral peduncle.Similar to the DL and ML, the HL may also send manyarachnoid trabeculae to the surrounding structures, includingthe posterior cerebral artery or its branches. The HL was alsofound to separate the superolaterally located posterior commu-nicating cistern from the interpeduncular cistern [19].

Surgical considerations

The arachnoid membranes, including the LM, are of para-mount surgical importance. Although they can complicate theexposure and dissection of brain lesions, but they help todemarcate the contour of these lesions, hence protecting near-by brain structures. Therefore, neurosurgeons should alwayslook for the arachnoidal interface between the lesion andadjacent brain structures as far as possible, in order to preventinjury to the surrounding neurovascular structures. Moreover,the arachnoid membranes can limit the spread of blood (e.g.,from a ruptured aneurysm) from spreading to other cisterns,which would allow localization of the injury by observing theblood-filled cistern [25].

The DL is of special interest in surgical approaches to thesellar and parasellar region. It represents an accurate anatomiclandmark that divides the cisterns of the skull base into pre- andpost-liliequist groups. The ML is not less surgically importantthan the DL. It forms an important barrier between thesupratentorial and infratentorial cisterns. In perimesencephaliclesions (e.g., diaphragm sellae meningiomas and trigeminalneurinomas), the ML can be displaced upward, yet preserved.Hence, if clearly followed, it may provide a safe and clearsurgical plane for operating on these tumors [5]. Understandingthe endoscopic of LM and its relation to the surroundings,determined on preoperative imaging, helps surgeons plan theroute of access, improve exposure, and minimize injuries.

Pterional approach

In 1979, Avman and Kanpolat [26] reported a technique ofmicroscopic third ventriculostomy using an approach through

the pterion. During the procedure, opening LM offeredinferolateral access to the floor of the third ventricle, andprovided an additional pathway for the flow of cerebrospinalfluid (CSF) [16].

According to Wang [19], if the operation is performedthrough the space between the optic nerve and the internalcarotid artery, the first arachnoid mater encountered will be thecarotid-chiasmatic wall, and the second is most likely the DLof LM. When approaching the upper clivus through the spacebetween the optic nerve and the internal carotid artery and thelateral space of the internal carotid artery, the LM should becut [27–29]. After incising LM, the operation should then beperformed with great care in order to avoid injury to theposterior communicating artery. Moreover, because of therotation and inclination of the head during pterional approach,the medial carotid membrane might be mistaken as the DL ofLM, and if it is bluntly or sharply dissected; this may injure theperforating arteries [10]. Therefore, few, if any, vessels may befound passing through the central part of the DL and ML;correct identification and careful incision of these leaflets areimportant for avoiding unnecessary dissections or injuries tosurrounding structures.

Sometimes after opening the DL, the basilar artery bifur-cation membrane is encountered, and it must be opened inorder to expose the interpeduncular cistern completely [18].

Supratentorial approach

When operating on the upper basilar complex bysupratentorial approaches, the HL and DL should be dissectedalong the medial surface of the oculomotor nerve, pushing theposterior communicating artery superiorly. LM should bedissected sharply without pulling or stripped gently withcareful attention to the delicate structures attached to it [19].Blunt pulling of the LM was reported as one of the causes ofthe post-operative diabetes insipidus, as it may damage thehypothalamus and its blood supply [25].

Moreover, when supratentorial approaches are used to gainaccess to the posterior ambient cistern, it is also important topreserve the perimesencephalic membrane (which was de-scribed by Qi [23] as part of the ML) in order to protect thesuperior cerebellar artery and the trochlear nerve below it fromsurgical injury [23].

Retrosigmoidal approach

During the retrosigmoidal approach to the tentorial notch, theoculomotor nerve can be clearly seen. Thereafter, it might besafe to carefully incise the LM through the central spacebetween the bilateral oculomotor nerves. Again, becausebranches of the posterior communicating artery might be

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attached to the superomedial area of the oculomotor nerve,care should be taken. In addition, care should be given to thesuperior cerebellar and the posterior cerebral arteries. Botharteries intersect at 90° against the posterior segment of theoculomotor nerve, with the initial segment of the superiorcerebellar artery running parallel, close to, or adherent, to theoculomotor nerve [19].

Endoscopic third ventriculostomy (ETV)

Understanding the anatomic appearance of LM and its relationto the floor of third ventricle is important for neurosurgeonswhen performing endoscopic third ventriculostomy (ETV)(Fig. 3). In some cases, the LM has a retromammillary, ratherthan premammillary, attachment superiorly (see above). Inthese cases, opening the LM is less important [10]. Otherwise,it has been recommended that any arachnoid membrane orgliotic tissue encountered following the floor fenestrationshould be opened, and it is only then the procedure will beconsidered complete. Both third ventricular floor and LMlayers should be punctured and expanded by a balloon or adissector, one at a time. Otherwise, the CSF will not flowsmoothly from the third ventricle to the interpeduncular cis-tern, which leads to ETV failure.

In most cases, the LM might not be seen during ETV, asboth LM and the third ventricular floor are opened simulta-neously, so the surgeons consider both as a single membrane.Moreover, in hydrocephalus, the third ventricular floor ispushed downward against the membrane, making it hard toidentify LM as a separate membrane [5].

Zhang [15] has described the findings during ETV basedon his and classic classifications. In LM type I, the DL is oftenopened simultaneously with the floor of third ventricle. Themedial ML can then be seen right under the surgical stoma(type B) or found stretching between the posterior edge of thestoma and the terminal basilar quadrification (type A). Some-times, however, the DL and even the temporal membrane canbe encountered after incising the floor of third ventricle.Whilein LM type II, only the medial ML can be seen under thestoma due to the absence of the DL. In these cases, just aspreviously mentioned, these membranes have to be opened toensure free communication between supratentorial andinfratentorial cisterns [15] .

Many authors only pay attention to the DL, which isusually the first layer to be seen after opening the thirdventricular floor. However, in some cases, where theinterpeduncular cistern represents a closed space, openingthe DL may not be sufficient. In these cases, the ML becomesa significant structure to be incised, allowing free communi-cation out of the cistern, mainly to the prepontine cistern [13].

It is well known that the raised intracranial pressure maylead to abnormal structural changes in the ventricular system

[30]. Findings of Oi [31] also showed that the duration ofhydrocephalus might cause various intraventricular anomalies[31]. Etus [32] also found associated histological changes inthe structure of LM. The author showed that in recent-onsethydrocephalus, LM assumes its usual membrane-like,arachnoidal structure. In contrast, LM appears thick andtough, with marked increase in type I collagen in long-standing hydrocephalus, suggesting a fibrotic process. Thesechanges in the structure of LM can complicate ETVand makefenestration of LM more technically demanding, where it isnot possible to use balloon dilatation to keep the LM patentafter its perforation. Therefore, the author suggested a helpfulmaneuver, whereby a small piece of the membrane can begently excised using endoscopic scissors in order to maintainadequate flow through the LM.

Etus [32] also advised against using lasers and hot coagu-lating probes during the fenestration, regardless of the maneu-ver used, due to possible accidental injury to the basilar arteryor the oculomotor nerve.

Even in patients without hydrocephalus, the LM, and es-pecially its DL, is still surgically important because it may beattached to the bottom of the third ventricle. In hydrocephalus,however, this attachment will be closer [19].

In some cases of suprasellar arachnoid cysts (SSAC; seebelow), they may herniate into the third ventricle leading toobstructive hydrocephalus [26]. The treatment of choice forthese lesions is the fenestration of the cyst into the ventricularsystem together with an ETV, or cistoventriculocisternostomy.Nevertheless, there still is no general agreement among authorson whether it is enough to fenestrate the cystic lesion into theventricular system or an additional cystocisternostomy withopening of LM has to be performed [33].

Approaches to skull base aneurysms

As Rhoton [34] pointed out, aneurysms arising from the partof circle ofWillis located anterior to LM can be approached bya frontotemporal (pterional) craniotomy, while aneurysmslocated behind LM can be exposed through either afrontotemporal or subtemporal craniotomy. Lesions caudalto the floor of the sella, however, need to be reached througha petrosal approach. Tumors located anterior to LM, betweenthe optic chiasm and the sellar floor, are commonly operatedon by the trans-sphenoidal or subfrontal route.

When dissecting an aneurysm it is helpful to know thearachnoid membranes attached to it [8]. The force of retractionor blunt pulling on these membranes may be transmitted to thewall or dome of the aneurysm, causing rupture. Therefore,sharp dissection of the arachnoid membrane and surroundingtrabeculae helps to avoid rupture of aneurysms before clipapplication [35]. Aneurysms arising from the basilar apex andat the origin of superior cerebellar artery may project into the

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leaflets of LM [8]. In cases where the basilar artery is to beexposed, surgeons need to open LM, bearing in mind itspossible anatomic variations. Moreover, when the secondsegment of the oculomotor nerve is surrounded by LM, mo-bilizing it to widen the carotid-oculomotor window requiresfull dissection of the arachnoid sleeve surrounding the nerve[5].

Extended endoscopic trans-sphenoidal approach

Extended endoscopic trans-sphenoidal approach (EETS) wasrecently developed to provide an alternative route to accessmidline skull base lesions [36–40]. One of the drawbacks ofthis approach is the limited visibility of the posterior aspect ofthe lesions.

According to Anik [13], EETS is preferred over traditionalapproaches when operating on lesions (e.g., tuberculum sellaemeningiomas, craniopharyngiomas, and pituitary adenomas)located in the infra-chiasmatic region and extending to thethird ventricle, mammillary bodies, and prepontine area. Dur-ing EETS, the basal arachnoid membrane (BAM) and LM inthe infra-chiasmatic area provide more secure entrances tothese lesions. In tuberculum sellae meningiomas, the BAMis displaced inferiorly, while in pituitary adenomas,craniopharyngiomas and dermoid tumors, it is displaced su-periorly. In pituitary adenomas extending to this region, theML is a leading and protecting layer. The significance of theBAM (which continues posteriorly as the ML with no distinctborder) relies on its protection of the posterior-superior surfaceof these lesions, and forms an essential border that preventsdamage to the structures located posterior and superior to thelesions during surgeries. Moreover, its flexibility keeps it frombeing lacerated during tumor growth [13].

Perimesencephalic hemorrhage

Perimesencephalic hemorrhage may complicate aneurysmalor non-aneurysmal subarachnoid hemorrhage. The latter isless common but carries better prognosis [41–44].

Later supported by the findings of Froelich [5], Schwartzand Solomon [44] had suggested the importance of LM in thedistribution of blood in perimesencephalic hemorrhage. Whenpresent, LM can obstruct flow of the blood, and hence divertsthe hemorrhage into the interpeduncular cistern. However,due to the anatomic variations of LM, communication be-tween the anterior basal cisterns is extremely variable, whichproduces multiple radiological patterns of perimesencephalichemorrhage. When LM is absent, there is no obstruction toflow [44].

Suprasellar arachnoid cysts

The role of LM, as any other arachnoid membrane, in theformation of SSAC has been described in literature [5, 45–48].

Based on their locations and relation to the leaflets of LM,Miyajima [47] proposed a new classification for these cysts. Intype I, the intra-arachnoid cyst of the DL, the cysts are non-communicating and the basilar bifurcation is pushed posteri-orly against the brainstem. In type II, the cystic dilation of theinterpeduncular cistern, the cysts occur between the two leaf-lets of LM and the basilar bifurcation lies inside the cyst [47].

There is controversy about the indications for surgery inSSACs. Asymptomatic SSACs that are not evolving usuallydo not require surgery regardless of their current size. Con-servative treatment with closed clinical and radiological mon-itoring is preferred in these cases [49, 50]. Cases of SSACsthat spontaneously disappeared during follow-up have beenreported [51, 52]. Moreover, most of the cysts forming in theprenatal period either completely disappear or become stabi-lized without any deficits on follow-up [49, 53].

The enlargement of arachnoid cysts is explained by severalmechanisms. One theory is the one-way valve mechanism andpumping of CSF through the valve into the cyst by arterialpulsation. Another possible mechanism is the accumulation offluid in arachnoid cysts driven by the osmotic pressure gradi-ent [5, 54]. Other authors [27] had reported that formation ofarachnoid cyst within the cisterns, including interpeduncularcistern, is often related to structural abnormalities and error inthe splitting of the arachnoid membranes, in this case, the LM[54].

Pathogenesis of hydrocephalus

As been described above, the LM (especially its thick, non-fenestrated DL) usually forms a complete division in the basalsubarachnoid space. The only route of communication of CSFis through the fenestrated arachnoid trabeculae, spreadingfrom the free border of LM to the surrounding structures andalong the space around the posterior communicating artery asit penetrates the leaf. These spaces are very narrow and easilyobstructed in cases of subarachnoid hemorrhage or meningealinfections [18], contributing to the pathogenesis of obstructivehydrocephalus.

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