rhoton - perimesencephalic cistern and pca

7/23/2019 Rhoton - Perimesencephalic Cistern and PCA

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SURGICAL ANATOMY AND TECHNIQUE

MICROSURGICAL APPROACHES TO THEPERIMESENCEPHALIC CISTERNS AND RELATED SEGMENTSOF THE POSTERIORCEREBRAL ARTERY: COMPARISONUSING A NOVEL APPLICATION OF IMAGE GUIDANCE

Arthur J. Ulm, M.D.

Department of Neurosurgery,University of Florida College of

Medicine, Gainesville, Florida

Necmettin Tanriover, M.D.

Department of Neurosurgery,University of Florida College of

Medicine, Gainesville, Florida

Masatou Kawashima, M.D.

Department of Neurosurgery,University of Florida College ofMedicine, Gainesville, Florida

Alvaro Campero, M.D.

Department of Neurosurgery,

University of Florida College ofMedicine, Gainesville, Florida

Frank J. Bova, Ph.D.



Albert L. Rhoton, Jr., M.D.



Reprint requests:

Arthur John Ulm, M.D.,Department of Neurosurgery,University of Florida College of

Medicine, P.O. Box 100265,Gainesville FL 32610.

Email: [email protected]

Received,December 18, 2003.

Accepted,February 12, 2004.

OBJECTIVE: To describe the exposure obtained through six approaches to the per-imesencephalic cisterns with an emphasis on exposure of the posterior cerebral arteryand its branches.

METHODS: Dissections in 12 hemispheres exposed the crural, ambient, and quadri-geminal cisterns and related segments of the posterior cerebral artery. A Stealth ImageGuidance workstation (Medtronic Surgical Navigation Technologies, Louisville, CO)was used to compare the approaches.

RESULTS: The transsylvian approach exposed the interpeduncular and crural cisterns.The subtemporal approach exposed the interpeduncular and crural cisterns as well asthe lower half of the ambient cistern. Temporal lobe retraction and the position of thevein of Labb limited exposure of the quadrigeminal cistern. Occipital transtentorialand infratentorial supracerebellar approaches exposed the quadrigeminal and lowertwo-thirds of the ambient cistern. Transchoroidal approaches exposed the posteriorthird of the crural cistern, the upper two-thirds of the ambient cistern, and the proximalquadrigeminal cistern. Transchoroidal approaches exposed the posterior portion of theP2 segment (P2p) in 9 of 10 hemispheres and were the only approaches that exposedthe lateral posterior choroidal arteries and the plexal segment of the anterior choroidalartery. Occipital transtentorial and infratentorial supracerebellar approaches providedaccess to the P3 segment in all cases and exposed the P2p segment in 4 of 10hemispheres. The subtemporal approach provided access to the cisternal and cruralsegments of the anterior choroidal and medial posterior choroidal arteries and exposedthe P2p segment in 3 of 10 hemispheres.

CONCLUSION:Surgical approaches to lesions of the perimesencephalic cisterns mustbe tailored to the site of the pathological findings. The most challenging area to exposeis the upper half of the ambient cistern, particularly the P2p segment of the posteriorcerebral artery.

KEY WORDS:Basal cisterns, Lateral ventricle, Posterior cerebral artery, Surgical approach, Temporal lobe,Transchoroidal

Neurosurgery 54:1313-1328, 2004 DOI: 10.1227/01.NEU.0000126129.68707.E7 www.neurosurgery-online.com

Approaching lesions within the perimes-encephalic cistern presents uniquechallenges to the neurosurgeon. The

anatomy of this region is complex, with nu-merous arteries, perforating vessels, deep ve-nous drainage pathways, and cranial nervescoursing through its confines. The pathologi-cal findings of the region are diverse and en-compass the entire spectrum of neurosurgery.

The perimesencephalic cisterns lie deepwithin the brain, and portions are shielded byoverlying structures, which include the tem-poral lobe, parahippocampal gyrus, limen in-sula, tentorial incisura, and vein of Labb. Nosingle surgical approach can provide access tothe entire perimesencephalic cistern, but ap-proaches must be tailored to the site and typeof the patients pathological findings with a

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thorough understanding of the anatomy of this region.The ambient portion of the perimesencephalic cistern and

accompanying posterior portion of the P2 segment (P2p) of theposterior cerebral artery (PCA) is a particularly challengingarea to expose. Selecting the appropriate surgical approach tothis portion of the PCA remains controversial (14, 16, 25, 35,

44, 51). The goals of the present study were to define theanatomic segments of the perimesencephalic cisterns, describethe microanatomy of six approaches to the region, comparethe approaches using a novel application of image guidance,and outline a strategy for approaching lesions in this area.

MATERIALS AND METHODS

A total of 12 hemispheres from six cadaveric heads infusedwith colored silicone were examined in the study. Magnifica-tion 3 to 40 and microsurgical techniques were used tocompare six surgical approaches to the perimesencephalic

cisterns. The approaches studied were the transsylvian pre-temporal, subtemporal, occipital transtentorial, infratentorialsupracerebellar, transtemporal transchoroidal, and transinsu-lar transchoroidal. The segments and major branches of thePCA exposed through each approach were identified in 10hemispheres. Special attention was paid to the anatomic struc-tures limiting visualization in each approach.

Three cadaveric heads were studied with magnetic reso-nance imaging and registered on a Stealth Image Guidanceworkstation (Medtronic Surgical Navigation Technologies,Louisville, CO) using standard protocols. Three-Tesla mag-netic resonance imaging scans using a Siemens MagnetomAllegra scanner (Siemens Medical Systems, Inc., Erlangen,

Germany) were obtained and transferred to a Stealth work-station. Before scanning, bilateral large orbitozygomatic fron-totemporal, bilateral occipital, and suboccipital craniotomieswere performed. In essence, three bridges of bone were leftintact over the convexity, to which fiducials were attached.The bony bridges were over the superior sagittal sinus andparieto-occipital cortex. Care was taken to maintain an intactdura. Ten fiducial landmarks to be used for image registrationwere placed in a uniform manner over the bony bridges. Theremaining calvaria provided a stable platform for registrationand maintenance of relative anatomic positioning during mi-crodissection. A three-dimensional model was built, and adynamic reference array was attached to the cadaver. Thecadaver was registered to the three-dimensional model andimage, and an estimated accuracy of better than 2 mm at 10 cmwas established. An active probe was then used to indicate thesurgical exposure on the three-dimensional model and threeorthogonal magnetic resonance imaging views. These imageswere recorded through screen captures. The limits of exposurethrough each of the approaches to the perimesencephalic cis-terns were determined by image guidance under direct micro-scopic visualization. Measurements included the superior, in-ferior, anterior, and posterior limits of surgical exposure aswell as the trajectory and working distance for each approach.

Basic Anatomy

The perimesencephalic cisterns consist of cerebrospinalfluid-filled spaces that surround the midbrain. We divide theregion into four segments. The first segment is the interpe-duncular cistern, which lies between the cerebral peduncles

and communicates anteriorly with the suprasellar cistern andsuperolaterally with the sylvian fissure (3, 24). The secondsegment is the crural cistern, which lies within a spacebounded by the cerebral peduncle medially, the posteriorsegment of the uncus laterally, and the optic tract superiorly(3, 24). The crural cistern communicates superiorly with thesylvian fissure, medially with the interpeduncular cistern, andposteriorly with the ambient cistern. The third segment is theambient cistern, which extends from the posterior margin ofthe crural cistern to the lateral edge of the midbrain colliculi(24). The walls of the ambient cistern are as follows: anteriorly,the posterior surface of the cerebral peduncle; medially, thelateral surface of the midbrain; laterally, the tentorial edge,

parahippocampal gyrus, fimbria of the fornix, and choroidalfissure; and superiorly, the pulvinar of the thalamus, lateralgeniculate body, and optic tract (Fig. 1). When viewed in acoronal plane, the ambient cistern is shaped like a Caroundthe parahippocampal gyrus (Fig. 1D). The final segment is thequadrigeminal cistern, which lies posterior to the colliculi,below the splenium, anterior to the apex of the cerebellum,and superior to the cerebellomesencephalic fissure (Fig. 1, AandD) (3, 22, 24).

We have previously described the anatomy of the PCA anddivided it into relevant segments (52). Briefly, the P1 segmentbegins at the basilar apex and ends at the insertion with theposterior communicating artery. The P2 segment is dividedinto the anterior portion (P2a) and the P2p. The P2a extendsfrom the junction of the posterior communicating artery withthe PCA and ends at the posterior margin of the crural cisternat the back edge of the cerebral peduncle. The P2p begins atthe posterior border of the crural cistern and ends at the lateraledge of the midbrain colliculi. The P2p often courses superi-orly and laterally within the ambient cistern to lie on thesuperior surface of the parahippocampal gyrus. The P3 seg-ment traverses the quadrigeminal cistern, and the P4 segmentis formed by the cortical branches arising from the PCA alongits course (Fig. 1).

The choroidal arteries are intimately associated with theperimesencephalic cisterns. The anterior choroidal arteryarises several millimeters distal to the takeoff of the posteriorcommunicating artery from the posterior surface of the inter-nal carotid artery and courses along the anterior and proximalposterior uncal segment before entering the lateral ventriclethrough the inferior choroidal point (3, 8, 9, 45). The medialposterior choroidal artery most commonly arises from the P1;however, a significant number arise more distally from the P2.The medial posterior choroidal artery travels inferior andmedial to the PCA through the crural and ambient cisternsand turns medially to enter the quadrigeminal cistern (9, 23,30, 45). The artery then turns forward to enter the velum

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interpositum and supplies the choroid plexus in the roof of thethird ventricle (9, 30, 45). The lateral posterior choroidal arter-ies arise most commonly from the P2p as one or severalbranches, course laterally along the upper edge of the para-hippocampal gyrus within the ambient cistern, and passthrough the choroidal fissure to enter the posterior part of thetemporal horn and atrium (Fig. 1, Aand C) (8, 9, 24, 30).

The PCA gives rise to cortical branches along its coursethrough the crural, ambient, and quadrigeminal cisterns. Fromproximal to distal, these branches include the inferior tempo-ral, parieto-occipital, calcarine, and splenial arteries (52). Theinferior temporal arteries comprise a group of vessels thatinclude the hippocampal, anterior temporal, middle temporal,and posterior temporal branches (Fig. 1, AC) (52).

FIGURE 1. A, specimen showing supe-rior view of the perimesencephalic cis-terns. The perimesencephalic cisterns in-clude the interpeduncular, crural,ambient, and quadrigeminal cisterns sur-rounding the midbrain. The interpedun-

cular cistern lies between the cerebral pe-duncles. The crural cistern lies betweenthe posterior segment of the uncus and thecerebral peduncle. The ambient cistern ex-tends from the posterior edge of the cruralcistern to the lateral edge of the midbraincolliculi. The head, body, and tail of thehippocampus and the choroid plexus areexposed in the floor of the temporal horn.Note how the right P2p courses above themedial edge of the parahippocampal gy-rus, whereas the left P2p courses medial tothe gyrus. B, specimen showing inferiorview of the basal cisterns. The right P2aand P2p have been removed to expose the

roof of the crural and ambient cisterns.The lower part of the temporal lobe hasbeen removed to expose the temporal hornand basal cisterns. The anterior choroidalartery enters the choroid plexus in thetemporal horn. The lateral geniculate bodyand optic tract sit in the roof of the cis-terns. The P1, P2, and P3 segments; in-

ferior temporal branches; and medial pos-terior and lateral posterior choroidalarteries are exposed. C, specimen showinganterior inferior temporal lobe below thechoroidal fissure removed to expose therelationship between the P2p and choroi-dal fissure. The choroidal fissure begins atthe posterior edge of the posterior uncalsegment at the site where the anteriorchoroidal artery enters the temporal hornto become the plexal portion. Note howtheright P2 ascends and courses laterally toreach the upper surface of the medial edgeof the parahippocampal gyrus in close

proximity to the choroidal fissure. D, axialmagnetic resonance imaging scan of a ca-daver using the Stealth workstation dem-onstrating the segments of the perimesen-cephalic cisterns. E, coronal magneticresonance imaging scan demonstrates the C shape of the ambient cistern. A., artery;AChA, anterior choroidal artery;Ant. Seg., anterior segment of uncus;BasalV., basal vein; Body, body of hippocampus; Chor. Plex., choroid plexus; Cist., cistern; CN III, oculomotor nerve; Head, head of hippocampus; Inf. Temp. A., inferior

temporal artery; Lat. Gen. Body, lateral geniculate body; LPChA, lateral posterior choroidal artery; MPChA, medial posterior choroidal artery; Optic Tr., optic tract;Parahippo. Gyrus, parahippocampal gyrus;PCoA, posterior communicating artery; Post. Seg., posterior segment of uncus; SCA, superior cerebellar artery; Tail,tail of hippocampus.

SURGICALAPPROACHES TO THE PERIMESENCEPHALICCISTERNS



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RESULTS

Transsylvian Pretemporal Approach

The heads were placed in a standard surgical position forapproaching the basilar artery through the transsylvian expo-

sure (4, 15, 34, 35, 49). The sylvian fissure was widely dissectedand opened along its entire length. The cisterns surroundingthe optic nerve and internal carotid artery were likewise dis-sected. After exposure of the areas between the optic andoculomotor nerves and carotidarteries, attention was directedto the exposure of the perimes-encephalic cisterns. The tempo-ral lobe was retracted postero-laterally away from the frontaloperculum to expose the basilartip and origin of the PCA. Atthis point, head positioning andscope angulation were opti-mized to expose the course ofthe PCA around the cerebral pe-duncle. The posterior limit oftemporal lobe retraction was atthe posterior edge of Meckelscave (Fig. 2, AD).

In the six hemispheres thatwere studied using the StealthImage Guidance workstation,several measurements and local-izations were made. In particular,the anterior, posterior, superior,and inferior limits of exposure

within the perimesencephalic cis-tern were determined. The pre-

temporal approach exposed the interpeduncular and crural cis-terns but not the ambient and quadrigeminal portions. The major

anatomic limitations to exposure included the limen insula, the

medially projecting uncal apex, and the excessive retraction ofthe posterior temporal lobe required to reach some areas (Fig. 2,EG).

The approach provided excellent exposure anterior to the mid-

brain and included the internal carotid, anterior cerebral, and

middle cerebral artery complexes. The PCA origin and the P1

FIGURE 2. Pretemporal transsylvianapproach.A, specimen showing right or-bitozygomatic craniotomy exposing thesylvian fissure and frontal and temporallobes. Theinsetshows the site of the two-

piece orbitozygomatic craniotomy. B,specimen showing widely opened proxi-mal portion of the sylvian fissure, expos-ing the M2 branches and insular surface.C, specimen showing opened opticoca-rotid triangle, located between the optic nerve, carotid artery, and A1 segment of the anterior cerebralartery, and opened carotid-oculomotor space, located between the oculomotor nerve and carotid artery.Note the limen insula obstructing the view into the crural cistern. D, specimen showing temporal loberetracted posteriorly and proximal M1 elevated to expose the P2ain the anterior part of thecrural cistern.The P1s, P2a, anterior choroidal artery, and PCA come into view. EG, Stealth magnetic resonanceimaging scans revealing the limits (arrows) of the exposure obtained with the pretemporal approach.

Anterior exposure through the approach is excellent and provides access to the interpeduncular cistern.The posterior and superior exposures are limited by the limen insula. The posterior limit of exposure isat the posterior edge of the cerebral peduncle, and the upper limit is on the upper third of the uncus. A1,

A1 segment of the anterior communicating artery; AChA, anterior choroidal artery; CN II, optic nerve;CN III, oculomotor nerve;ICA, internal carotid artery;M1, M1 segment of middle cerebral artery;M2,

M2 segment of the middle cerebral artery;PCoA, posterior communicating artery.

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and P2a were exposed in all cases. In addition, we were able toexpose the origin and most of the cisternal segment of the ante-rior choroidal artery. The medial posterior choroidal artery originwas identified in 9 of 10 hemispheres arising from the P1 or P2a.In one hemisphere, the medial posterior choroidal artery arosefrom the P2p within the ambient cistern and could not be ex-

posed. The lateral posterior choroidal vessels, which arise mostcommonly from the P2p, could not be exposed through thetranssylvian approach. In addition, the P2p and P3 were inacces-sible through this approach (Table 1).

Subtemporal Approach

Three variations of the subtemporal surgical approach werestudied: the anterior, middle, and posterior extensions (5, 7,10, 11, 20, 27, 35, 39, 43). The approach involves placing thesagittal suture parallel to the floor with the vertex angledinferiorly to allow for maximal visualization along the tento-rial surface to the perimesencephalic cisterns. The zygomaticarch and squamosal and petrous portions of the temporalbones were drilled to form a flat trajectory along the floor ofthe middle fossa. The arachnoidal trabeculae connecting themesial temporal lobe to the tentorial edge were sharply dis-sected and removed to expose the underlying structures (Fig.3, AC).

In all specimens, the interpeduncular, crural, and lower halfof the ambient cistern could be exposed subtemporally (Fig. 3,DG). In 6 of 10 specimens, the posterior subtemporal routecould access the quadrigeminal cistern (Fig. 3E). In 4 speci-mens, the vein of Labbblocked exposure of the posterior partof the ambient cistern (Fig. 3F). Another anatomic obstacle toexposure through this approach is the parahippocampal gy-rus. The upper one-third to one-half of the ambient cistern,

which extends above the rounded medial edge of the para-hippocampal gyrus, was inaccessible from the subtemporalapproach (Fig. 3G).

The basilar tip, P1, P2a, and medial posterior choroidal arterywere exposed in all 10 specimens. The origin and most of thecisternal segment of the anterior choroidal artery could also beexposed through the subtemporal approach. The proximal andinferior surface of the P2p was accessible in three hemispheres. Inseven hemispheres, the P2p coursed superiorly at the posterior

edge of the cerebral peduncle and became hidden from viewabove the medial edge of the parahippocampal gyrus (Fig. 3, Aand B). The lateral posterior choroidal arteries and P3 wereinaccessible in all hemispheres (Table 1).

Occipital Transtentorial and InfratentorialSupracerebellar Approaches

Two posterior approaches, one directed above and onebelow the tentorium, were examined. The approaches exam-ined were the occipital transtentorial and supracerebellar tran-stentorial approaches (1, 2, 18, 19, 28, 29, 31, 32, 35, 37, 40, 46,47, 51, 53). Both were found to provide nearly identical expo-sure of the perimesencephalic cisterns and structures within.Each involves a paramedian incision through the tentorium.The obvious difference involves retraction of the cerebellumversus retraction of the occipital pole (Figs. 4and 5).

Both the occipital transtentorial approach and the supracer-ebellar transtentorial approach allow for excellent exposure ofthe quadrigeminal and ambient cisterns as well as access tolesions extending below the tentorial edge (Figs. 4and5). Theposterior portion of the crural cistern can be reached throughboth approaches, with a slight increase in exposure gainedthrough the supracerebellar route (Figs. 4C and 5E). Comparedwith the more anterior approaches, the working distance issignificantly greater when accessing the anterior ambient cis-

tern and posterior crural cistern (Figs. 4Fand5I). The averageworking distance from the surface of the cortex to the anteriorambient cistern for the posterior approaches was approxi-

TABLE 1. Exposure of posterior choroidal artery segments and choroidal arteriesa

P1 P2a P2p P3 AChr MpChr LpChr

Subtemporal 10/10 10/10 3/10 inferior

surface

0/10 10/10 origin,

cisternal

10/10 origin,

crural, ambient

0/10

Pretemporal 10/10 10/10 0/10 0/10 10/10, origin,cisternal

9/10, crural 0/10

Occipital

interhemispheric

0/10 8/10 posterior

third

4/10, 2/10

inferior trunk

10/10 0/10 10/10, ambient,

quadrigeminal

0/10

Supracerebellartranstentorial

0/10 8/10 posteriorthird

4/10, 2/10inferior trunk

10/10 0/10 10/10, ambient,quadrigeminal

0/10

Transtemporal

transchoroidal

0/10 6/10 posterior

third

9/10 4/10 proximal

fourth

10/10 plexal 0/10 10/10

Transinsulartranschoroidal

0/10 6/10 posteriorthird

9/10 4/10 proximalfourth

10/10 plexal 0/10 10/10

a AChr, anterior choroidal artery; MpChr, medial posterior choroidal artery; LpChr, lateral posterior choroidal artery.




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mately 8 cm, whereas the working distance for the anteriorapproaches ranged between 4 and 5 cm.

These posterior approaches allow for excellent exposure ofthe P3. The P1 is inaccessible through these approaches, andonly the posterior third of the P2a can be exposed. In fourhemispheres, the inferior surface of the P2p could be exposed.In two hemispheres, the P2p existed as two trunks, a superiortrunk and an inferior trunk. In these hemispheres, the inferiortrunk could be accessed, whereas the superior trunk was

hidden from view above the medial edge of the parahip-pocampal gyrus. In the remaining four hemispheres, the P2pwas inaccessible above the medial edge of the parahippocam-pal gyrus (Figs. 4Band 5D; Table 1).

Both approaches allowed similar access to the choroidal ves-sels. The anterior choroidal artery could not be accessed in any ofthe hemispheres because it entered the temporal horn anterior tothe exposure in the posterior crural cistern. The ambient andquadrigeminal portions of the medial posterior choroidal artery

FIGURE 3. AC, left subtemporal ap-proach to the perimesencephalic cisterns.A,specimen showing anterior subtemporal ap-

proach exposing the interpeduncular andlower half of the crural and ambient cisterns.Structures exposed include the internal ca-

rotid, anterior choroidal, posterior communi-cating, basilar, superior cerebellar and medialposterior choroidal arteries, P1 and P2a, andanterior and posterior segments and apex ofthe uncus. B, specimen showing middle sub-temporal approach exposing the P2a withinthe crural cistern, P2p within the ambientcistern, medial posterior choroidal and supe-rior cerebellar arteries, trochlear nerve, andtentorial edge. C, specimenshowingposteriorsubtemporal exposure with view into ante-rior quadrigeminal cistern exposing the col-licular plate, trochlear nerve, and basal vein.DG, Stealth magnetic resonance imagingscans demonstrating the limits (arrows) of

the approach. D, left anterior subtemporalexposure provides access to the interpedun-cular cistern and across to the medial aspectof the contralateral cerebral peduncle. E, pos-terior limit of exposure in hemispheres notlimited by an anterior position of the vein ofLabbwas the lateral part of the quadrigem-inal cistern on the side ipsilateral to the ap-

proach.F, posterior limit of exposure ipsilat-eral to the approach in hemispheres in whichananterior positionof the vein of Labblimitstemporal lobe retraction.G, superior limit ofexposurewithinthe ambient cistern was lim-ited to the lower one-half to two-thirds of thecistern because of the prominence of the me-dial edge of the parahippocampal gyrus.Reaching the upper part of the cistern wouldrequire excessive temporal lobe retraction.AChA, anterior choroidal artery;Ambient,ambient cistern; Ant. Seg., anterior segmentof uncus; Apex, apex of uncus; Bas. Tr.,basilar trunk; Basal V., basal vein ofRosenthal; CN III, oculomotor nerve; CNIV, trochlear nerve; Crural, crural cistern;ICA, internal carotid artery;SCA, superiorcerebellar artery;Tent edge, tentorial edge;MPChA, medial posterior choroidal artery;PCoA, posterior communicating artery;Post. Seg., posterior segment of the uncus.

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could be accessed but not the origin from the P1. The lateralposterior choroidal arteries were inaccessible secondary to theirorigin from the superior or lateral surface of the P2p segment andsubsequent lateral course toward the temporal horn above theparahippocampal gyrus (Table 1).

Transtemporal Transchoroidal and TransinsularTranschoroidal Approaches

The perimesencephalic cisterns can be accessed through thechoroidal fissure of the tempo-ral horn (16, 24, 25, 26, 30, 45).The choroidal fissure is the cleftbetween the pulvinar of thethalamus and the fimbria of thefornix along which the choroidplexus in the temporal horn isattached. The fissure begins atthe inferior choroidal point,where the anterior choroidal ar-tery enters the temporal horn atthe posterior aspect of the cruralcistern and posterior edge of theposterior uncal segment, mark-ing the beginning of the plexalportion of the artery. In additionto the anterior choroidal artery,the lateral posterior choroidalartery branches enter the ven-tricular system through the fis-

sure within the posterior part of the temporal horn and atrium(24, 30, 45). Two approaches through the choroidal fissure

were examined, one through the inferior temporal lobe and

the other through the inferior limiting sulcus of the insula(Figs. 69) (12, 16, 25, 26, 36, 42, 45, 50).

In the 10 hemispheres studied, the temporal horn was foundto be located 2.5 to 3.5 cm posterior to the anterior tip of the

temporal lobe and approximately 2 to 2.5 cm deep to the

surface of the middle temporal gyrus (Fig. 7B). The transtem-

FIGURE 4. Right occipital transtentorial approach.A, specimen showing posterior view, in which the

insetdemonstrates the route of the exposure. The ten-torium was divided adjacent to the straight sinus.The exposure included the P2a, P2p, P3, pineal

gland, colliculi, superior cerebellar artery, basal veinand vein of Galen, trochlear nerve, and inferior tem-

poral branches of the PCA. B, specimen showingenlarged view above the cerebellum into the ambientcistern. The approach is directed lateral to the apex ofthe cerebellar vermis. The P2a has ascended to be hid-den above the medial edge of the parahippocampal

gyrus in the upper part of the ambient cistern. TheP3 re-emerges in the quadrigeminal cistern. CG,Stealth magnetic resonance imaging scans demon-strating the limits (arrows) of exposure. C, anteriorlimit of exposure is the back edge of the crural cis-tern. D and E, approach provides excellent exposureof the quadrigeminal cistern. F, working distance tothe deepest point of exposure in the six hemispheresaveraged 8 cm. G, trajectory of approach to the ante-rior limit of exposure. H, superior limit of exposurein ambient cistern. Basal V., basal vein; CN IV,trochlear nerve; Inf. Temp. A., inferior temporalartery; Parahippo. Gyrus, parahippocampal gyrus;Pineal, pineal gland; SCA, superior cerebellar artery;Sup. Col., superior colliculus; Tent., tentorium; V.of Galen, vein of Galen.




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poral transchoroidal approach involves making a 2-cm corti-cectomy in the inferior temporal gyrus and opening the lateralwall of the temporal horn (Fig. 6, Aand B). After entering thetemporal horn, the choroidal fissure is opened between thefimbria and choroid plexus to avoid the thalamic drainingveins and perforating arteries traversing the thalamic side ofthe fissure (Fig. 6C). At this point, the upper portion of theposterior crural cistern and the ambient cistern are exposed. Inmost hemispheres, the lateral geniculate body in the roof ofthe cistern and the basal vein are the first structures encoun-

tered after the initial opening of the fissure (Fig. 6C). Withminimal retraction of the hippocampus inferiorly, the upperhalf of the ambient cistern and related segments of the PCAcan be exposed (Fig. 6D). More extensive retraction and dis-section provide more inferior exposure of the ambient cisternas well as exposure of the posterior crural and anterior quad-rigeminal cisterns (Fig. 6, EG). The major drawback to thetranstemporal transchoroidal approach is the need to performa corticectomy in the temporal lobe. Several authors havereported success in using the approach for lesions in the

FIGURE 5. Infratentorial supracerebel-lar transtentorial approach. A, specimenshowing posterior view of the cerebellum.Note the bridging veins blocking access tothe quadrigeminal cistern. B, specimenshowing infratentorial midline view of the

quadrigeminal cistern. Exposed struc-tures include the pineal gland, complex ofveins emptying into the vein of Galen, P3,superior cerebellar artery, and medial pos-terior choroidal artery. C, specimen show-ing left paramedian approach with thetentorium intact exposing the collicular

plate, fourth nerve, and superior cerebellarartery. The P3 is exposed above and me-dial to the tentorial edge. D, specimenshowing view with the tentorium dividedexposing the P2p in the ambient cisternand theP2a in the posterior crural cistern.Thedistal P2p ascends in theanterior partof the ambient cistern to become hidden

above the medial edge of the parahip-pocampal gyrus. EJ, Stealth magneticresonance imaging scans demonstratingthe limits (arrows) of exposure. E, ante-rior limit of exposure is at the back edge ofthe crural cistern.FandG, approach pro-vides excellent exposure of the quadrigem-inal cistern. H, trajectory of approach tothe anterior middle portion of the ambientcistern.I, working distance from the pos-terior cortical surface to the most anterior

point of exposure averaged 7 to 8 cm.J,superior limit of the exposure extendedinto the upper one-third of the ambientcistern, similar to that obtained throughthe occipital transtentorial approach.Basal V., basal vein of Rosenthal; Cist.,cistern; CN IV, trochlear nerve; Col-liculi, superior and inferior colliculi; Inf.Temporal A., inferior temporal artery;Inf. Vermian Vein, inferior vermianvein; Int. Cerebral V., internal cerebralvein;MPChA, medial posterior choroidalartery;Parahippo. Gyr., parahippocam-

pal gyrus;PICA, posterior inferior cere-bellar artery; Pineal, pineal gland; Sup.Colliculus, superior colliculus; Tent.Edge, tentorium;Transv. Sinus, transverse sinus;V. of Galen, vein of Galen.

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ambient cistern with minimal morbidity (16, 25, 26, 36). An-other limitation of the approach is the vein of Labb . In 4 of 10hemispheres, the vein of Labb extended far enough anteri-orly to limit the cortical incision in the temporal lobe.

Another route to access the choroidal fissure of the temporalhorn is through the inferior limiting sulcus of the insula (50).

The approach involves making a 2-cm incision in the inferiorlimiting sulcus to gain access to the temporal horn. The inci-sion is usually made between the insular branches of themiddle cerebral artery (Fig. 8D). The depth of the temporalhorn is approximately 5 mm from the insular surface. Once thetemporal horn is entered, the choroidal fissure is opened in an

FIGURE 6. Transtemporal transchoroi-dal approach.A, specimen showing righttemporal craniotomy exposing the inferiortemporal gyrus. Theinsetshows the siteof the scalp incision and craniotomy. B,specimen showing temporal horn of the

lateral ventricle entered through a 2-cmcorticotomy in the inferior temporal gyrusto expose the head of the hippocampus,choroid plexus, and choroidal fissure. C,specimen showing choroidal fissureopened on the forniceal side by separatingthe choroid plexus from the fimbria of the

fornix to preserve the draining veins andperforating arteries, which traverse thethalamic side of the fissure. Note the lat-eral geniculate body, anterior and lateral

posterior choroidal arteries, and basalvein. D, specimen showing uncal recess

positioned in the anterior tip of the tem-poral horn between the head of the hip-

pocampus and the amygdala. The anteriorchoroidal artery enters the temporal hornat the inferior choroidal point located justbehind the hippocampal head and suppliesthe choroid plexus.E, specimen showingenlarged view of the P2p, lateral posteriorchoroidal artery, and basal vein coursingin the ambient cistern.F, specimen show-ing quadrigeminal cistern and P3 ac-cessed through the posterior portion of theapproach directed through the choroidal

fissure. G, specimen showing retraction ofthe hippocampus exposing the inferiortemporal artery originating from the P2p.H, specimen showing transamygdala ex-tension of the transchoroidal approach.

An incision has been carried forwardthrough the amygdala to expose the struc-tures within the crural and interpeduncu-lar cisterns, including the P2a; amygdala;

posterior communicating, internal ca-rotid, anterior choroidal, and medial pos-terior choroidal arteries; and optic and oc-ulomotor nerves. AChA, anteriorchoroidal artery; Basal V., basal vein;Cer. Ped., cerebral peduncle; Chor. Fiss.,choroidal fissure; Chor. Plex., choroid

plexus;CN II, optic nerve; CN III, ocu-lomotor nerve;Hip. A., hippocampal ar-

tery; ICA, internal carotid artery; Inf.Temp. A., inferior temporal artery;Inf. Temp. Gyrus, inferior temporal gyrus;Lat. Gen. Body, lateral geniculate body; LPChA, lateral posterior choroidal artery;MPChA, medial posterior choroidal artery; Parahippo. Gyrus, parahippocampal gyrus; PCoA, posterior communicating artery;Pulvinar, pulvinar of thalamus;Quadrigeminal, quadrigeminal cistern;Sphenopar. Sinus, sphenoparietal sinus.




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identical manner to that described for the transtemporal tran-schoroidal approach (Fig. 8, AF). A variation of the transin-sular transchoroidal approach has been extensively used anddescribed in performing amygdalohippocampectomies fortemporal lobe epilepsy as well as for approaching lesions inthe ambient cistern (50). Given the extensive use of this ap-proach in clinical practice, one would expect minimal morbid-ity secondary to the incision along the anterior portion of theinferior limiting sulcus (12, 42, 50).

The transchoroidal approaches provide excellent exposure ofthe lateral posterior choroidal artery, which enters the temporalhorn through the choroidal fissure. In addition, the P2p wasexposed transchoroidally in 9 of 10 hemispheres examined. Inone hemisphere, the P2p was located on the undersurface of theparahippocampal gyrus and could not be exposed transchoroi-dally. Both transchoroidal approaches exposed the upper half ofthe ambient cistern, the posterior third of the crural cistern, andthe adjacent part of the quadrigeminal cistern (Figs. 7and9). Thetransinsular variation, given the superior-to-inferior trajectory ofthe approach, allowed for greater exposure of the inferior portionof the ambient cistern (Figs. 7F and 9F). In addition, we were able

to gain access to the posterior P2aand anterior P3 through bothvariations. The plexal portion ofthe anterior choroidal artery wasexposed within the temporalhorn (Table 1).

Limitations of the approachesthrough the choroidal fissure in-clude an inability to access themedial posterior choroidal ar-tery, the origin and cisternalsegments of the anterior choroi-dal artery, the P1, and most ofthe P2a and P3. The medial pos-terior choroidal artery lies deepwithin the ambient cistern be-low the PCA and is intermin-gled with multiple brainstemperforators, making identifica-

tion through these approachesdifficult (Table 1).

DISCUSSION

Lesions in and around the per-imesencephalic cisterns presentunique challenges to the neuro-surgeon. Success requires a thor-ough knowledge of the anatomyand an understanding of the lim-itations of the approaches used toaccess the region. The deep loca-

tion, narrow confines, and den-sity of critically important vascu-lar st ruct ur es add t o the

challenges. In the present study, we have examined six ap-proaches to the region in an attempt to define the surgical anat-omy of the region and describe the limitations of the variousapproaches (Fig. 10).

The PCA is the dominant vascular structure within the per-imesencephalic cisterns. The segments of the PCA are namedaccording to the cistern in which they reside, and each segmentrequires a unique surgical approach (6, 16, 25, 36, 44, 52). Basilarapex, P1-posterior communicating artery, and posterior commu-nicating artery-P2a aneurysms can be accessed through the trans-sylvian or anterior subtemporal approach (47, 11, 15, 20, 3335,38, 49). The occipital transtentorial and infratentorial supracer-ebellar approaches have been considered the best approaches foraneurysms of the P3 segment (36, 44).

Aneurysms of the P2, particularly the P2p, have historicallybeen the most challenging lesions to approach (16, 25, 36, 44).The P2p runs within the ambient cistern in close associationwith the parahippocampal gyrus. The P2 typically ascendswithin the ambient cistern and usually lies on the superiorsurface of the parahippocampal gyrus. The ambient cistern isCshaped in coronal cross section because it extends around

FIGURE 7. Stealth magnetic resonance imaging scans demonstrating theexposure gained through the transtemporal-transchoroidal approach. Arrows,the limits of the exposure. A, trajectory through the inferior temporal gyrusand temporal horn to the ambient cistern. B, distance from the lateral cortex tothe temporal horn averaged 2 to 2.5 cm.C, trajectory to the posterior ambientcistern from the posterior margin of the corticotomy through the inferiortemporal gyrus. The distance to the ambient cistern from the cortical surfacewas approximately 4.0 to 4.5 cm.D, right-sided anterior limit of exposure atthe inferior choroidal point. E, left-sided posterior limit of exposure in the

proximal quadrigeminal cistern.F, inferior limit of exposure within the ambient cistern. Note the inability to access thelower ambient cistern.G, upper limit of exposure at the choroidal fissure.

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the rounded medial edge of the parahippocampal gyrus.When the P2p lies within the upper portion of the C, thegyrus limits the exposure of this segment when approachedfrom below through the subtemporal, occipital transtentorial,and infratentorial supracerebellar approaches. Only the prox-imal and inferior surface of the P2p was accessible through thesubtemporal route in 3 of 10 hemispheres. In the other 7hemispheres, a portion of the parahippocampal gyrus wouldhave to be resected to expose the P2p subtemporally (14). TheP2p was successfully exposed in 4 of 10 hemispheres throughthe occipital transtentorial and infratentorial supracerebellarapproaches. In 2 hemispheres, the P2p divided into an inferiortrunk, which could be exposed below the parahippocampalgyrus, and a superior trunk, which was hidden above theparahippocampal gyrus. In the remaining 4 hemispheres, theP2p was inaccessible by the occipital transtentorial and infrat-entorial supracerebellar approaches because it coursed on theupper surface of the parahippocampal gyrus.

The transchoroidal approach provides access to the upperambient cistern. In 9 of 10 hemispheres, the P2p was exposed

transchoroidally. In 1 hemisphere, the P2p was positioned in thelower half of the ambient cistern and could not be exposedtranschoroidally, because the upper part of the parahippocampalgyrus blocked access to the P2p. Some authors have suggestedthat the location of the P2 segment aneurysm in relation to theinferior choroidal point, as revealed on angiography, is a criticallandmark for selecting the appropriate surgical approach to theselesions (16, 44). Ikeda et al. (16) recommend using the transcho-roidal route for distal P2 aneurysms when the PCA is at or abovethe level of the inferior choroidal point . On the basis of ourcurrent study, we think that the midpoint of the rounded medialedge of the parahippocampal gyrus provides an equally impor-tant landmark. Moving superiorly from the midpoint of themedial edge improves the exposure through the transchoroidalapproach but worsens the exposure through the subtemporal,occipital transtentorial, and supracerebellar approaches. Con-versely, moving inferiorly from the midpoint of the gyrus im-proves the exposures gained through the subtemporal, occipital,and supracerebellar approaches but worsens the exposure of thetranschoroidal approaches (Fig. 10).

FIGURE 8. Transinsular transchoroidalapproach to the perimesencephalic cis-terns. A, specimen showing right or-bitozygomatic craniotomy with exposureof the frontal and temporal lobes along thesylvian fissure. The inset shows the site of

the scalp incision and craniotomy. B, thesylvian fissure is widely opened to exposethe insular cortex and insular branches ofthe M2 segment of the middle cerebralartery. C, specimen showing enlargedview. The inferior limiting sulcus of theinsula has been exposed. D, specimenshowing cortical incision through the an-terior portion of the inferior limiting sul-cus parallel to M2 branches opening intothe temporal horn and exposing the cho-roid plexus and head of hippocampus. Thetemporal horn usually lies approximately0.5 cm deep to the inferior limiting sulcus.E, specimen showing choroidal fissure

opened by dividing the attachment of thechoroid plexus to the fimbria of the fornixand retracting the choroid plexus towardthe thalamus to expose the lateral genicu-late body, basal vein, and P2p. F, speci-men showing P2p elevated to expose thedepths of the ambient cistern and an infe-rior temporal branch of the P2p passing tothe lower surface of the temporal lobe.Basal V., basal vein;Cent. Insular Sul-cus, central insular sulcus; Chor.Plexus, choroid plexus;Fimbria, fimbriaof fornix; Hippo., hippocampus; Inf.Limiting Sulcus, inferior limiting sulcusof insula;Inf. Temp. A., inferior tempo-ral artery;Lat. Gen. Body, lateral genic-ulate body;LPChA, lateral posterior choroidal artery;M2, M2 segment of middle cerebral artery; Sylvian Fiss., sylvian fissure;Sylvian V., sylvian vein.




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The anterior, medial posterior, and lateral posterior choroidalarteries are intimately associated with the perimesencephaliccisterns (8, 24, 45). Specific surgical approaches are required toexpose these arteries. The anterior choroidal artery arises fromthe internal carotid artery just distal to the PCA and traverses theanterior two-thirds of the crural cistern before entering the tem-poral horn at the inferior choroidal point. The origin and cisternalsegments can only be approached through the transsylvian pre-temporal and anterior subtemporal routes. The plexal portion isonly accessible transchoroidally. The medial posterior choroidalartery has the longest course of the three choroidal arteries, andno single approach can expose all the segments of the vessel. Theartery typically arises as a single trunk from the P1 and traversesthe crural, ambient, and quadrigeminal cisterns medial to and

below the PCA before entering the posterior roof of the thirdventricle. The transsylvian pretemporal approach can expose theorigin of the vessel and the crural segment. In 1 of 10 hemi-spheres studied, the origin of the vessel was from the P2p andwas not accessible through the transsylvian route. The subtem-poral approach exposed the origin as well as the crural, ambient,

and proximal quadrigeminal segments of the medial posteriorchoroidal artery in all 10 hemispheres studied. Access to thedistal quadrigeminal portion of the artery was limited because ofthe need for excessive temporal lobe retraction and/or the posi-tion of the vein of Labb. Exposure of the medial posteriorchoroidal artery through the two transchoroidal approaches wasdifficult because of the fact that the artery has a relatively low-lying position within the cistern and visualization is obstructedby the overlying PCA and multiple brainstem perforators. Thelateral posterior choroidal artery proved to be the most difficultartery to identify and expose. The artery typically arises as one orseveral branches off the distal P2 and courses directly lateral onthe superior surface of the parahippocampal gyrus to enter the

temporal horn. Only the transchoroidal approaches could reli-ably expose the artery.

When approaching lesions involving the P3 or lesions ex-tending for any considerable distance into the quadrigeminalcistern, the occipital transtentorial or infratentorial supracer-ebellar approach should be selected. None of the other ap-proaches reliably exposed these structures.

All the approaches examined have limitations in their ability toexpose critical structures within the perimesencephalic cisterns.These limitations may not be readily apparent on preoperativeimaging studies but may become apparent during the course ofan operation. Several of these approaches can be combined usingthe same craniotomy to extend the operative field. For example,

combing the transsylvian pretemporal approach with thetransinsular transchoroidal approach allows the surgeon to gainaccess to the ambient cistern. In addition, through this combinedtranssylvian approach, the surgeon would have access to theorigin, cisternal, and plexal portions of the anterior choroidalartery and lateral posterior choroidal artery as well as access tothe P1, P2a, and P2p. The combined exposure could be criticalwhen dealing with a complex arteriovenous malformation ortumors receiving blood supply from multiple choroidal arteries(13, 14, 17, 21, 41, 42, 48).

Another combined approach providing extended exposureand flexibility during surgery is the combination of the subtem-poral and transtemporal transchoroidal approaches. The subtem-poral approach is limited in its ability to expose the upper am-bient cistern, the P2p, the lateral posterior choroidal artery, andthe plexal portion of the anterior choroidal artery. However, theapproach provides excellent exposure of the interpeduncularcistern, P1, P2a, lower ambient cistern, medial posterior choroi-dal artery, and origin of the anterior choroidal artery. Having theability to add the transchoroidal exposure by opening throughthe lower part of the temporal lobe and temporal horn during thecourse of a subtemporal approach could greatly facilitate theability to deal with complex arteriovenous malformations, tu-mors, and P2p aneurysms.

FIGURE 9. Stealth magnetic resonance imaging scans of the transinsulartranschoroidal approach. A and B, trajectory of the approach through the

sylvian fissure and inferior limiting sulcus to the ambient cistern. C,superior limit of the exposure through the choroidal fissure to the ambientcistern. D , anterior limit of the exposure at the inferior choroidal point inthe posterior end of the crural cistern. E, posterior limit of exposure, inmost hemispheres, at the posterior ambient cistern. F, inferior limit ofexposure within the lower part of the ambient cistern. Note how thesuperior-to-inferior trajectory of the transinsular approach allows for

greater inferior exposure of the ambient cistern than through the transtem-poral transchoroidal approach as illustrated in Figure 7. Arrows, the lim-its of the exposure.

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The transtemporal transchoroidal approach can be extendedduring the course of an operation to provide access to theanterior midbrain. Making an incision through the amygdala,as would be done for a temporal lobectomy, provides access tothe internal carotid artery, P1, P2a, PCA, anterior choroidal

artery, and medial posterior choroidal artery as well as thecrural and interpeduncular cisterns (Fig. 6H). The extensionthrough the amygdala can be used to deal with unexpecteddifficulties in exposing pathological findings or to provideproximal control when dealing with aneurysms.

FIGURE 10. Composite diagram dem-onstrating the exposure gained throughsix approaches to the perimesencephaliccisterns. Axial (left) and coronal (right)images are shown.A,left imagedemon-strates the axial exposure gained through

the pretemporal (pink) and occipital tran-stentorial and supracerebellar transtento-rial (blue) approaches. The right imagedemonstrates the coronal exposure of theambient cistern provided by the occipitaltranstentorial and supracerebellar tran-stentorial approaches. The pretemporalapproach exposes the interpeduncular andcrural cisterns but not the ambient orquadrigeminal cisterns (left, pink). Theoccipital transtentorial and supracerebel-lar transtentorial approaches expose thequadrigeminal and ambient cisterns butnot the crural or interpeduncular cisterns(left, blue). The occipital transtentorial

and supracerebellar transtentorial ap-proaches provide access to the lower two-thirds of the ambient cistern (right im-age), whereas the upper one-third of theambient cistern is obscured by the medialedge of the parahippocampal gyrus. B, leftimagedemonstrates the axial exposure ofthe perimesencephalic cisterns throughthe subtemporal approach. The subtempo-ral approach consistently exposes the in-terpeduncular, crural, and ambient cis-terns (orange solid). Exposure of the

posterior ambient and quadrigeminal cis-terns is more variable and depends on the

position of the vein of Labb (orangehatched). Theright image, in the coro-nal plane, demonstrates the extent of ex-

posure of the ambient cistern through thesubtemporal approach. The lower one-halfof the ambient cistern was exposed sub-temporally in all specimens (orangesolid). The exposure of the upper one-halfof the ambient cistern was blocked in thesubtemporal approach by the medial edgeof the parahippocampal gyrus and the po-sition of the vein of Labb. Excessive re-traction would be required to expose theupper part of the ambient cistern.C,leftimage, transinsular and transtemporal

transchoroidal approaches provided access to the ambient cistern in all hemispheres (green solid). Exposure of the quadrigeminal cistern was more variable and waslimited by the position of the tail of the hippocampus (green hatched). In the right image, both the transinsular (upper green arrow) and transtemporaltranschoroidal (lower green arrow) approaches exposed the upper one-half to two-thirds of the ambient cistern (solid green). The transinsular transchoroidalapproach accessed the lower ambient cistern (green hatched). The added inferior exposure gained through the transinsular approach is a result of the superior-to-inferior trajectory through the inferior limiting sulcus of the insula ( right image, upper green arrow).




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When planning surgical approaches to the perimesencephaliccisterns, the surgeon must anticipate the critical structuresneeded in the exposure. With this understanding, the appropri-ate approach or combination of approaches can be selected. Thetranschoroidal extensions of the transsylvian pretemporal andsubtemporal approaches can be added during the course of

surgery to extend the exposure of these more standardapproaches.

Impact and Importance

Approaching lesions within the perimesencephalic cisternspresents unique challenges to the neurosurgeon. The anatomy ofthis region is complex, with numerous arteries, perforating ves-sels, deep venous drainage pathways, and cranial nerves cours-ing through its confines. The perimesencephalic cisterns lie deepwithin the brain, and portions are shielded by overlying struc-tures, including the temporal lobe, parahippocampal gyrus, li-men insula, tentorial incisura, and vein of Labb. No single

surgical approach can provide access to the entire perimesence-phalic cistern, but approaches must be tailored to a particularpatients pathological findings with a thorough understanding ofthe anatomy of this region. We present the microsurgical anat-omy of six approaches to the region, paying particular attentionto the exposure of the PCA and its major branches. The ap-proaches are compared using a novel application of imageguidance.

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COMMENTS

Ulm et al. present an excellent study on the dissection of sixcadaveric heads infused with colored silicone and, inaddition, neuronavigation of three cadaveric heads. Theydemonstrate the extent of exposure of the perimesencephaliccisterns by navigation, with excellent photographs of micro-surgical anatomy, tables, and schematic drawings. Special at-tention is drawn to the accessibility of vascular lesions of thechoroidal arteries and the posterior cerebral artery.

Six different approaches to the perimesencephalic cisternsare presented: the transsylvian, subtemporal, occipital tran-stentorial, infratentorial supracerebellar, transtemporal tran-schoroidal, and transinsular transchoroidal approaches. Eachapproach is described and documented in detail. In this tuto-rial, efforts are made to document limitations, such as tempo-ral lobe retraction or the position of the vein of Labb in thesubtemporal approaches. Whether this is comparable in a stiffcadaver brain remains to be discussed. It should be pointedout, as the authors concluded, that surgical approaches tolesions in that area must be tailored to the site of the patho-

logical findings. Pathological changes, such as dilated ven-tricles, edema/bleeding, or tumor extension, can offer addi-tional important information regarding the most suitableapproach. Three-dimensional imaging for neuronavigation ofthe cerebral cortex with preoperative demonstration of thevenous drainage and the cerebral surface is important in theplanning of temporal approaches to perimesencephalic andtemporal lesions.

Wolf LdemannMadjid SamiiHannover, Germany

Lesions in the perimesencephalic cisterns represent the mostdifficult surgical challenges for the operating neurosur-geon. In these locations lie the basilar apex aneurysms, poste-rior cerebral artery aneurysms, pineal region tumors, upperbrainstem tumors, and blood supply to medial temporal arte-riovenous malformations and intraventricular tumors. There-fore, a command of the anatomy of and possible surgicalapproaches to the perimesencephalic cisterns is critical to safeoperative neurosurgery of these lesions.

Ulm et al. have prepared a superbly illustrated and carefullyexplained guide to the regional anatomy and surgical ap-proaches of the perimesencephalic cisterns. The addition of theimage-guided analysis of the approaches helps to bring clinicalrelevance to the discussion and serves as an aide to selecting anapproach based on magnetic resonance imaging findings.

Neurosurgical lesions in these locations are relatively rare,and surgical approaches to the perimesencephalic cisterns areuncommon procedures. Therefore, surgeons will find this ar-ticle an excellent reference for review before any surgicalapproach in this area.

Robert A. SolomonNew York, New York

Ulm et al. present an exhaustive analysis of microsurgicalapproaches to the perimesencephalic cisterns, with em-phasis on vascular pathology. They provide the reader with avery thorough review of the surgical anatomy of this complexregion. A very clear description of cisterns, arteries, veins,parenchymal relationships, and surgical landmarks is given,with the support of outstanding pictures. The plus point ofthis article is the comparison of the six different surgicalapproaches, their pros and cons, and the possibility to exposethe different structures in each one on the basis of the micro-

surgical analysis of 12 hemispheres from six cadaveric headsinfused with colored silicone. They also analyzed three headswith magnetic resonance imaging and registered in a StealthImage Guidance workstation (Medtronic Surgical NavigationTechnologies, Louisville, CO). They found that some vascularstructures can be exposed by more than one surgical approach(P1, P2a, P3, anterior choroidal artery, etc.), whereas otherstructures are safely exposed by only one approach (P2p,lateral posterior choroidal artery). When we try to extrapolatethese results to the living patient, we have to remember first




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that, obviously, the consistency of the formalin-fixed brain isnot the same as that of the fresh brain. Conversely, the degreeof exposure needed will depend on anatomic variations of theposition of arteries in a particular cistern (i.e., low or highP2p). Another important anatomic variation that can limit thedegree of exposure we can achieve is the dominant venous

drainage pattern (i.e., dominant Labbs vein). For these rea-sons, it is very important to analyze carefully the magneticresonance imaging and magnetic resonance angiography orconventional digital subtraction angiography beforehand toselect the best approach to the patient, maximize the exposure,and minimize the risk of postoperative venous infarction.Another issue mentioned in this article is the possibility of

combining different approaches (i.e., transchoroidal with sub-temporal, transinsular-transchoroidal with pretemporal or-

bitozygomatic). This will allow optimization of the surgical

corridor and gain extra exposure to deep structures. This isespecially important in arteriovenous malformation surgery,

in which multiple feeders from different segments of the pos-

terior cerebral artery or anterior choroidal artery have to bereached. In conclusion, this is an excellent analysis of this topic

and is a must-read for residents and neurosurgeons.

David Rojas-ZalazarEvandro de OliveiraSo Paulo, Brazil

Cyanobacteriawere among the first organisms to use fermentation to produce adenosine triphosphate and introduce oxygen into the atmosphere throughphotosynthesis. Oxygen permitted aerobic respiration and helped bring about the evolution of larger, more sophisticated, eukaryotic cells. Please see para-meciaon page 1312. (Photograph courtesy of Hewlett-PackardsA Walk Through Time exhibit.)

ULM ET AL.

rhoton - perimesencephalic cistern and pca

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