TranstentorialApproachtoParahippocampalLesions

GeneralConsiderations

Operativeaccesstotheposteriormedialtemporallobeandparahippocampalregionsischallengingbecauseoftheoverlyingvitalcorticesandunavailabilityofsafeskullbasecorridors.

Subtemporal,transtemporal,transsylvian/transinsular/transcisternal,andpotentialinterhemisphericparieto-occipitalapproacheshavebeenconsideredreasonabletrajectoriestotheregion.ThesubtemporalapproachrequiresexcessivetemporalloberetractionwithanassociatedrisktotheveinofLabbé.Thetranstemporalapproachtransgressestheposteriortemporalneocortex,avitalterritoryonthedominantside;italsoleadstodisruptionoftheopticradiations.Thetranssylvian-transcisternalalternativeprovidesadeepandnarrowworkingchannelwithunacceptablylimitedaccesstotheposteriorpartofthemedialtemporalloberegion.

Iprefertheparamediansupracerebellartranstentorialapproachtotheposteriormediobasaltemporallobe.Thisapproachprovidesthenecessaryoperativeaccessforintraparenchymallesions.Thetransectionofthetentoriumviatheparamediansupracerebellarrouteoffersauniqueopportunitytoreachthebasalposteromedialtemporallobewhileleavingthesupratentorialstructuresunharmed.Thisexposurehasatechnicallychallenginglongworkingdistance,butfavorableworkingangles.Theexposureisalsosomewhatlimited;thereforethisrouteshouldbejudiciouslyselected.

TheNeurosurgicalAtlas byAaronCohen-Gadol,M.D.

Anteriorhippocampalandparahippocampallesions(anteriortothecerebralpeduncleandwithintheuncus)areapproachedviaalimitedanteromedialtemporalloberesection.

Thenuancesoftechniqueoftheparamediansupracerebellartranstentorialapproachforresectionofextra-axiallesionsaredescribedintheParamedianSupracerebellarCraniotomychapterandarealsoreviewedhere.

Figure1:Thesupracerebellartranstentorialapproachhasnumerousadvantagesoverotheralternativesupratentorialoperativecorridorstoreachtheposteriorbasaltemporallobe:1)asmallparamediancraniotomyisminimallydisruptive,and2)onlyonecerebellarhemisphereismanipulatedandthesupratentorialcorticesareleftintactandnotplacedunderretraction.Thegreenarrow(upperillustration)pointstotheroadmaptrajectoryforthisoperativecorridor,andthegreenandpurplecoloredsectionsofthehippocampusillustratethereachofthisapproach(upperimage).Thelowerimagesshowtheoperativeviewofthetypicallocationsofthelesioninrelationtothehippocampus(yellowshading)andtemporalhorn(blueshading).

IndicationsfortheApproach

Thesupracerebellartranstentorialapproachcanexposelesionsintheposteriorhippocampalandparahippocampalregions.Theselesionsaretypicallyatorjustposteriortothelevelofthecerebralpeduncleorposteriortotheuncus.Intraparenchymaltumors,arteriovenous/cavernousmalformations,andmoredistalposteriorcerebralarteryaneurysmsarereasonablecandidatesforthisapproach.Thisapproachisalsousefulforexposingmulticompartmentalpinealregionmasses.

Theexposureisdeepandnarrow;specialexpertiseinmicrosurgicaltechniquesisrequiredforitsuse.Largelesions,extendingsuperiorlyandanteriorly,maynotbesuitablecandidates.

Figure2:Posteriorparahippocampallesions,suchasthismetastaticadenocarcinoma,maybeexposedthroughthesupracerebellartranstentorialapproach.Thelocationofthistumorisatthemostanteriorreachofthisapproach.

PreoperativeConsiderations

AstudyofthesurroundingarteriesonT2-weightedimages,includingtheposteriorcerebralarterybranches,isimportant.Thisarteryisatriskanditsrouteshouldbecarefullystudied.

Thetransverseandsigmoidsinusesmayhaveslightlyvariablecourses,andtheirpreoperativestudycanenhancethesafetyofthecraniotomy.Factorssuchasasteeptentorialangleandaveryobesepatientwithashortneck,althoughnotcontraindicationstotheuseofthesupracerebellarroute,canmaketheoperationmorechallenging.Intheseraresituations,thepatient’sneckflexionmayamelioratethedifficultworkinganglesoverthecerebellum,anditisrecommendedthatthepatientbeplacedinthesittingposition.

ObstructivehydrocephalusrequirespreparationoftheKeen’spointorapreoperativefrontalventriculostomy.Aparamedianlinearincision(seebelow)canreadilyuncoverthebonyareacorrespondingtotheburrholefortheKeen’spoint.

Iusethemodifiedpark-benchpositionforpatientpositioningduring

thisprocedure.Cerebrospinalfluiddrainagethroughthelumbardrain(intheabsenceofobstructivehydrocephalus)providesfurtherdecompressionformobilizationofthecerebellum.

Thepreoperativestudiesshouldalsoevaluatetheveinsandvenoussinusesofthetentorium.ThevenousphaseonMRangiographyorcatheterangiographycanguidethesafetyoftentorialtransectionoravoidanceofthistechnique.Iflargeveinsarepresentwithinthetentorium,thetentorialincisionsaretailoredtoavoidexcessivebleedingorriskofvenousinfarctionduetoobstructionofdeepveins(veinsofRosenthal)thatrarelydrainintothevenouschannelsofthetentorium.

OperativeAnatomy

Adetailedfamiliaritywiththeregionalanatomyofthetentoriumisnecessarytoavoidcomplications.

Figure3:Theregionalanatomyforapproachingthetentoriumanditstransectionisshown.Thewindowwithinthetentoriumistailoredbasedonthelocationofthelesionandtheextentofexposurenecessaryusingnavigation.Abilateralcraniotomyisunnecessary.Themediobasalsupratentorialregionsbecome

availableaftertransectionofthetentorium(A-upperimage):Supracerebellartranstentorialapproachontheleftsideaftertentorialresection,demonstratingtheoperativecorridortowardthebasalsurfaceofthetemporallobe(B-lowerimage).FromdeOliveiraJG,etal.Supracerebellartranstentorialapproach-resectionofthetentoriuminsteadofanopening-toprovidebroadexposureofthemediobasaltemporallobe:Anatomicalaspectsandsurgicalapplications.JNeurosurg116:764-772,2012.

Figure4:Sectioningawindowofthelefttentoriumthroughaparamediansupracerebellarcraniotomyexposestheposteriorambientcisterns,basaltemporallobe,andtherelevantarterialanatomy.Notethegenerousexposureoftheposteriorparahippocampusanddistalposteriorcerebralarterybranchesthroughthisroute(imagescourtesyofALRhoton,Jr).

PARAMEDIANSUPRACEREBELLARTRANSTENTORIAL

APPROACHFORINTRA-AXIALPARAHIPPOCAMPALLESIONS

Iroutinelyusethelateralorpark-benchpatientpositionforthisroute.Theinitialstagesoftheexposurearethesameastheonesforthepineallesions.Forfurtherdetails,pleaseseeParamedianSupracerebellarCraniotomy.

Figure5:Aleft-sidedsuboccipitalsupracerebellarcraniotomyisperfomedtoexposethelefttentorium.Askullclampisusedwiththepatient’sneckflexedandheadturnedslightly(15-20degrees)towardthefloor.Thepatient’sipsilateralshoulderisallowedtofallforwardandistapedawayfromthesurgeon’sworkingzone.Intraoperativeneuronavigationidentifiesthelocationofthemidline,aswellasthetransverseandsigmoidsinuses.Aparamedianverticallinearincisionismadehalfwaybetweentheinionandmastoidgroove.Thisincisionextendsone-thirdaboveandtwo-thirdsbelowthetransversesinusandisabout7–8cminlength.NotethattheKeen’spointisunderneaththeupperedgeoftheincision.Thelocationofthetransversesinusismarkedwiththeshorthorizontalline(leftimage).

Figure6:Asingleburrholeismadeattheinferioredgeofthetransversesinus,approximately2cmlateraltothemidlineandtorcula.Asmallboneflapiselevatedwhiletheentirewidthofthetransversesinusisexposedtoallowroomforlaterrostralmobilizationofthissinus(leftupperimage).Theduraisopenedasasinglecurvedflapbasedonthesinus.Tworetractionsuturesmaybeplacedalongtheposterioraspectofthetentoriumtomobilizeandgentlyrotatethetransversesinussuperiorlytoexpandtheoperativespacethroughthesupracerebellarcorridor(rightupperimage).

Figure7:Oneortwoparamedianbridgingveinsmayhavetobesacrificed.Largemidlinebridgingveinsareleftintact.Notetheretractionsuturesplacedthroughtheposteriortentorium.Thesesuturesgentlyrotateandmobilizethetransversesinusessuperiorly.Microdopplerultrasonographycanconfirmthepatencyofthesinusandgaugethesafedegreeofretractiononthesinus.

Figure8:Gradualreleaseofcerebrospinalfluidthroughthelumbardrainoraventriculostomycatheterallowsgentlecaudalmobilizationofthelateralcerebellarhemisphere.Theduramaybeincisedina“T”-shapedpattern(hashedline)forsmallerlesionsora“U”-shapedfashion(seeFigure10)forlargerlesions.Intraoperativenavigationguidesthelocationofthetentorialincisions.

Figure9:Thestepsincompletingthetentorialincisions(“T”-shapedopening)forthetumorinFigure2.Theedgeofthetentoriumiselevatedwithafineright-angleddissector,andmicroscissorsareusedtocontinuethetransectionprocess(leftupperimage).Asmallcurvedknifecanfacilitatecuttingtowardtheoperator(rightupperimage).Thelowerimagesdemonstrateplacementofoneoftheretractionsutureswithinthetentorium(leftlowerimage)andthefinalextentoftheoperativecorridorusingretentionsuturesovercottonoidpattiestoprotectthesurfaceofthehemisphere(rightlowerimage).

Figure10:Theduramaybeincisedina“U”-shapedconfigurationforexposingwiderregionsofthemediobasalsurfaceandresectionoflargerlesions.Retractionsuturesalsomobilizetheincisedsectionofthetentoriumalongwiththecerebelluminferiorly.Notethelocationoftheunderlyingtemporalhorn(blue)andhippocampus(yellow).Dissectionofthearachnoidmembranesoverthemedialdorsolateralmesencephalonwillmobilizethecerebelluminferiorlyandexpandtheoperativecorridor.Thetrochlearnerveisprotectedalongtheedgesoftentorium.

Whenincisingthelateraltentorium,thesurgeonshouldfollowthe

borderofthepetrosalsinusorpetrousridgeuntilthetrochlearnerveisexposedenteringthefreeborderofthedura.Thetentoriumshouldthenbecutbeforethisentrypointwhilethenervecanbeseendirectly.Theposteriorpetrosalveinandotherbridgingveinsareprotectedduringtentorialsectioning.

Figure11:Intraoperativenavigationguidesthebordersoftentorialsectioningbasedontheexactlocationofthelesion.Becauseofunfamiliaroperativeangles,thesurgeoncaneasilymisinterpretorbedisorientedregardingthelocationofthelesioninrelationtothesurfaceofthetentorium.Afteradequatesurfaceoftheposteriorbasaltemporallobeisexposed,navigationcanguidethelocationofthecorticotomyifthelesionisnotapparentonthepialsurface.Distalposteriorcerebral

arteryenpassagebranchesarenumerousinthisregionandshouldbemeticulouslypreserved.Thethalamoperforatingarteriescanbeinjuredduringtumormanipulation.Inaddition,indiscriminatecoagulationleadstoundesirablethalamicandoccipitallobeischemia.Smallcorticalarteriesoverlyingthelesionmayhavetobesacrificed.

Next,thesurgeoncanbeginmicrosurgicalremovalofthetumor.Thisinferior-to-superiortrajectoryisbeneficialforremovingtumorsthatextendtothelevelofthetemporalhornandCalcaravis.

Dynamicretractionofthecerebellumusingthesuctiondeviceallowsexposureandresectionofthetumorwithouttheuseoffixedretractors.Thesuctionapparatusallowsamorecontrolled,expandedviewoftheworkingzoneattheexactlocationofthedissection.Incontrast,ifretractorsareused,theretractor’swideblademayinfactcompromisethedeepexposurebecauseofitslessflexiblevectorofretraction.

Figure12:Thefinalappearanceoftheoperativespaceafterresectionofthetumor.

SUPRACEREBELLARTRANSTENTORIALAPPROACHFOREXTRA-AXIALLESIONS

Thismodificationofthesupracerebellarapproachcanalsobeperformedwiththepatientinthepark-benchposition.

http://www.neurosurgicalatlas.com/uploads/volumes/ATLAS/2-CRANIAL_APPROACHES/14a-Transtentorial_Approach_To_Parahippocampal_Lesions/_lightbox/CA_TranstentParahippo_13.jpg

Figure13:Notetheroleofthisrouteforresectionofmedialtentorialextra-axialtumorsand,morespecifically,meningiomas.Thetrochlearnervemustbeprotectedalongthelateraledgeoftheincisuraduringtentorialtransection(upperinsetimage).Incisionalongtheredhashedlinewillsacrificethenerve—incisionalongtheblackhashedlineisappropriate.Alternatively,a“T”-shapedincisionmaybemadewithinthetentoriumforintraparenchymallesionswithintheposteriorbasaltemporallobe(seetheabovesection).Anintraoperativephotographduringresectionofaleft-sidedpetrousapexmeningiomademonstratesthelocationofthenerveasitenterstheduraattheanterioredgeofthecoagulatedtumorandtentorium(lowerimage).

Figure14:Earlyexposureofthedorsolateralbrainstemand

surroundingneurovascularstructuresatthetentorialincisuraallowsfortheirprotectionbymicrodissectionawayfromthetumorbeforesignificanttumordebulkingisundertakenandthesurgicalfieldisobscuredbybleeding.Extra-axialtumorscanbedevascularizedearlyinsurgerythroughcauterizationoftheundersurfaceofthetentorium.

Figure15:Agenerousportionofthetentoriumisthenincisedfromthepetrousridgetothemidlinewhiletheoperatoridentifiesandpreservesthetrochlearnervealongtheentireanterioredgeofthetentorium.Occasionalbridgingveinsdrainingtheoccipital

lobeandenteringthesuperioraspectofthetentoriummaybesacrificed.Themedialtentorialcutshouldpreservethestraightsinusanditstributaries.Venouslakesmaybepresent,andvenousbleedingthroughtheleafletsofthetentoriumshouldbecontrolledusingthrombin-soakedgelfoampacking.Bipolarcauterizationwillexacerbatethebleedingbyshrinkingandtearingthetentorialedges.

Sectioningofthetentoriumasdescribedabovewillfurtherdevascularizethetumorandfurnisharelativelybloodlessfieldtodebulkthetumorandmicrosurgicallymobilizeitfromthesurroundingcortex.

Figure16:Thistentorialresectioncreatesawidecorridortothebasaloccipitalandposteromedialtemporalregions.Anintra-axialtumorinthisregioncanbesimilarlyresected.

Closure

Thetentoriumisgentlyreflectedbackinitsoriginalpositionandnotsutured.Theduramustbeclosedinawatertightfashionbecausetheoccurrenceofpostoperativecerebrospinalfluidfistulaeisasignificantriskaftertumoroperationswithintheposteriorfossa.Iprefertoavoidtheuseofanallografttoreconstructtheduraldefectandinsteaduseapieceofpericranialautograft.

Thebonemaybereplacedusingcranialplates.Iminimizethestrangulationofthesuboccipitalmusclesbydeepsuturestoavoidmusclenecrosisanduncontrolledpostoperativepain.Theneckmusclesaregentlyapproximated.Thefasciaisclosedinawatertightfashion.

PostoperativeConsiderations

Thepatientisobservedintheintensivecareunitforadayortwoaftersurgeryandthentransferredtotheward.Steroidsareadministeredprophylacticallytominimizetheriskofasepticmeningitis.Ifpreoperativehydrocephaluswaspresentandaventricularcatheterwasimplantedintraoperatively,thiscathetershouldbeleftinplaceduringsurgeryandremovedduringthepostoperativerecoveryperiod.

Aggressiveretractionofthecerebellumcanleadtoretractionedema.Thiscanbeseenonpostoperativeimagingandcanoccasionallycausesymptomaticposteriorfossatensionandaneedfordecompression.Therefore,cautionshouldbeexercisedduringduralclosureandboneflapreplacement.Ifthebrainappearsswollen,theduralclosureshouldnotcausemoretensionandtheboneflapshouldnotbereplaced.Thisbrainswellingcanbepotentiallycompoundedbypartialtransversesinusthrombosisandparavermianveinsacrifice.

PearlsandPitfalls

Comparedwithothermorecommonlyusedapproaches,thesupracerebellartranstentorialcorridorprovidesnumerousadvantages,butisassociatedwithlongandnarrowworkingdistances.

Aggressivecerebellarretractionshouldbeavoidedandenpassagevesselsalongthemediobasalsurfaceofthetemporallobe,includingthethalamoperforatingarteries,shouldbeprotected.

DOI:https://doi.org/10.18791/nsatlas.v2.ch14

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