transtentorial approach to parahippocampal lesions · i prefer the paramedian supracerebellar...
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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.
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Anteriorhippocampalandparahippocampallesions(anteriortothecerebralpeduncleandwithintheuncus)areapproachedviaalimitedanteromedialtemporalloberesection.
Thenuancesoftechniqueoftheparamediansupracerebellartranstentorialapproachforresectionofextra-axiallesionsaredescribedintheParamedianSupracerebellarCraniotomychapterandarealsoreviewedhere.
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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.
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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
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thisprocedure.Cerebrospinalfluiddrainagethroughthelumbardrain(intheabsenceofobstructivehydrocephalus)providesfurtherdecompressionformobilizationofthecerebellum.
Thepreoperativestudiesshouldalsoevaluatetheveinsandvenoussinusesofthetentorium.ThevenousphaseonMRangiographyorcatheterangiographycanguidethesafetyoftentorialtransectionoravoidanceofthistechnique.Iflargeveinsarepresentwithinthetentorium,thetentorialincisionsaretailoredtoavoidexcessivebleedingorriskofvenousinfarctionduetoobstructionofdeepveins(veinsofRosenthal)thatrarelydrainintothevenouschannelsofthetentorium.
OperativeAnatomy
Adetailedfamiliaritywiththeregionalanatomyofthetentoriumisnecessarytoavoidcomplications.
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Figure3:Theregionalanatomyforapproachingthetentoriumanditstransectionisshown.Thewindowwithinthetentoriumistailoredbasedonthelocationofthelesionandtheextentofexposurenecessaryusingnavigation.Abilateralcraniotomyisunnecessary.Themediobasalsupratentorialregionsbecome
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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
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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).
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Figure6:Asingleburrholeismadeattheinferioredgeofthetransversesinus,approximately2cmlateraltothemidlineandtorcula.Asmallboneflapiselevatedwhiletheentirewidthofthetransversesinusisexposedtoallowroomforlaterrostralmobilizationofthissinus(leftupperimage).Theduraisopenedasasinglecurvedflapbasedonthesinus.Tworetractionsuturesmaybeplacedalongtheposterioraspectofthetentoriumtomobilizeandgentlyrotatethetransversesinussuperiorlytoexpandtheoperativespacethroughthesupracerebellarcorridor(rightupperimage).
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Figure7:Oneortwoparamedianbridgingveinsmayhavetobesacrificed.Largemidlinebridgingveinsareleftintact.Notetheretractionsuturesplacedthroughtheposteriortentorium.Thesesuturesgentlyrotateandmobilizethetransversesinusessuperiorly.Microdopplerultrasonographycanconfirmthepatencyofthesinusandgaugethesafedegreeofretractiononthesinus.
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Figure8:Gradualreleaseofcerebrospinalfluidthroughthelumbardrainoraventriculostomycatheterallowsgentlecaudalmobilizationofthelateralcerebellarhemisphere.Theduramaybeincisedina“T”-shapedpattern(hashedline)forsmallerlesionsora“U”-shapedfashion(seeFigure10)forlargerlesions.Intraoperativenavigationguidesthelocationofthetentorialincisions.
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Figure9:Thestepsincompletingthetentorialincisions(“T”-shapedopening)forthetumorinFigure2.Theedgeofthetentoriumiselevatedwithafineright-angleddissector,andmicroscissorsareusedtocontinuethetransectionprocess(leftupperimage).Asmallcurvedknifecanfacilitatecuttingtowardtheoperator(rightupperimage).Thelowerimagesdemonstrateplacementofoneoftheretractionsutureswithinthetentorium(leftlowerimage)andthefinalextentoftheoperativecorridorusingretentionsuturesovercottonoidpattiestoprotectthesurfaceofthehemisphere(rightlowerimage).
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Figure10:Theduramaybeincisedina“U”-shapedconfigurationforexposingwiderregionsofthemediobasalsurfaceandresectionoflargerlesions.Retractionsuturesalsomobilizetheincisedsectionofthetentoriumalongwiththecerebelluminferiorly.Notethelocationoftheunderlyingtemporalhorn(blue)andhippocampus(yellow).Dissectionofthearachnoidmembranesoverthemedialdorsolateralmesencephalonwillmobilizethecerebelluminferiorlyandexpandtheoperativecorridor.Thetrochlearnerveisprotectedalongtheedgesoftentorium.
Whenincisingthelateraltentorium,thesurgeonshouldfollowthe
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borderofthepetrosalsinusorpetrousridgeuntilthetrochlearnerveisexposedenteringthefreeborderofthedura.Thetentoriumshouldthenbecutbeforethisentrypointwhilethenervecanbeseendirectly.Theposteriorpetrosalveinandotherbridgingveinsareprotectedduringtentorialsectioning.
Figure11:Intraoperativenavigationguidesthebordersoftentorialsectioningbasedontheexactlocationofthelesion.Becauseofunfamiliaroperativeangles,thesurgeoncaneasilymisinterpretorbedisorientedregardingthelocationofthelesioninrelationtothesurfaceofthetentorium.Afteradequatesurfaceoftheposteriorbasaltemporallobeisexposed,navigationcanguidethelocationofthecorticotomyifthelesionisnotapparentonthepialsurface.Distalposteriorcerebral
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arteryenpassagebranchesarenumerousinthisregionandshouldbemeticulouslypreserved.Thethalamoperforatingarteriescanbeinjuredduringtumormanipulation.Inaddition,indiscriminatecoagulationleadstoundesirablethalamicandoccipitallobeischemia.Smallcorticalarteriesoverlyingthelesionmayhavetobesacrificed.
Next,thesurgeoncanbeginmicrosurgicalremovalofthetumor.Thisinferior-to-superiortrajectoryisbeneficialforremovingtumorsthatextendtothelevelofthetemporalhornandCalcaravis.
Dynamicretractionofthecerebellumusingthesuctiondeviceallowsexposureandresectionofthetumorwithouttheuseoffixedretractors.Thesuctionapparatusallowsamorecontrolled,expandedviewoftheworkingzoneattheexactlocationofthedissection.Incontrast,ifretractorsareused,theretractor’swideblademayinfactcompromisethedeepexposurebecauseofitslessflexiblevectorofretraction.
Figure12:Thefinalappearanceoftheoperativespaceafterresectionofthetumor.
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SUPRACEREBELLARTRANSTENTORIALAPPROACHFOREXTRA-AXIALLESIONS
Thismodificationofthesupracerebellarapproachcanalsobeperformedwiththepatientinthepark-benchposition.
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Figure13:Notetheroleofthisrouteforresectionofmedialtentorialextra-axialtumorsand,morespecifically,meningiomas.Thetrochlearnervemustbeprotectedalongthelateraledgeoftheincisuraduringtentorialtransection(upperinsetimage).Incisionalongtheredhashedlinewillsacrificethenerve—incisionalongtheblackhashedlineisappropriate.Alternatively,a“T”-shapedincisionmaybemadewithinthetentoriumforintraparenchymallesionswithintheposteriorbasaltemporallobe(seetheabovesection).Anintraoperativephotographduringresectionofaleft-sidedpetrousapexmeningiomademonstratesthelocationofthenerveasitenterstheduraattheanterioredgeofthecoagulatedtumorandtentorium(lowerimage).
Figure14:Earlyexposureofthedorsolateralbrainstemand
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surroundingneurovascularstructuresatthetentorialincisuraallowsfortheirprotectionbymicrodissectionawayfromthetumorbeforesignificanttumordebulkingisundertakenandthesurgicalfieldisobscuredbybleeding.Extra-axialtumorscanbedevascularizedearlyinsurgerythroughcauterizationoftheundersurfaceofthetentorium.
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Figure15:Agenerousportionofthetentoriumisthenincisedfromthepetrousridgetothemidlinewhiletheoperatoridentifiesandpreservesthetrochlearnervealongtheentireanterioredgeofthetentorium.Occasionalbridgingveinsdrainingtheoccipital
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lobeandenteringthesuperioraspectofthetentoriummaybesacrificed.Themedialtentorialcutshouldpreservethestraightsinusanditstributaries.Venouslakesmaybepresent,andvenousbleedingthroughtheleafletsofthetentoriumshouldbecontrolledusingthrombin-soakedgelfoampacking.Bipolarcauterizationwillexacerbatethebleedingbyshrinkingandtearingthetentorialedges.
Sectioningofthetentoriumasdescribedabovewillfurtherdevascularizethetumorandfurnisharelativelybloodlessfieldtodebulkthetumorandmicrosurgicallymobilizeitfromthesurroundingcortex.
Figure16:Thistentorialresectioncreatesawidecorridortothebasaloccipitalandposteromedialtemporalregions.Anintra-axialtumorinthisregioncanbesimilarlyresected.
Closure
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Thetentoriumisgentlyreflectedbackinitsoriginalpositionandnotsutured.Theduramustbeclosedinawatertightfashionbecausetheoccurrenceofpostoperativecerebrospinalfluidfistulaeisasignificantriskaftertumoroperationswithintheposteriorfossa.Iprefertoavoidtheuseofanallografttoreconstructtheduraldefectandinsteaduseapieceofpericranialautograft.
Thebonemaybereplacedusingcranialplates.Iminimizethestrangulationofthesuboccipitalmusclesbydeepsuturestoavoidmusclenecrosisanduncontrolledpostoperativepain.Theneckmusclesaregentlyapproximated.Thefasciaisclosedinawatertightfashion.
PostoperativeConsiderations
Thepatientisobservedintheintensivecareunitforadayortwoaftersurgeryandthentransferredtotheward.Steroidsareadministeredprophylacticallytominimizetheriskofasepticmeningitis.Ifpreoperativehydrocephaluswaspresentandaventricularcatheterwasimplantedintraoperatively,thiscathetershouldbeleftinplaceduringsurgeryandremovedduringthepostoperativerecoveryperiod.
Aggressiveretractionofthecerebellumcanleadtoretractionedema.Thiscanbeseenonpostoperativeimagingandcanoccasionallycausesymptomaticposteriorfossatensionandaneedfordecompression.Therefore,cautionshouldbeexercisedduringduralclosureandboneflapreplacement.Ifthebrainappearsswollen,theduralclosureshouldnotcausemoretensionandtheboneflapshouldnotbereplaced.Thisbrainswellingcanbepotentiallycompoundedbypartialtransversesinusthrombosisandparavermianveinsacrifice.
PearlsandPitfalls
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Comparedwithothermorecommonlyusedapproaches,thesupracerebellartranstentorialcorridorprovidesnumerousadvantages,butisassociatedwithlongandnarrowworkingdistances.
Aggressivecerebellarretractionshouldbeavoidedandenpassagevesselsalongthemediobasalsurfaceofthetemporallobe,includingthethalamoperforatingarteries,shouldbeprotected.
DOI:https://doi.org/10.18791/nsatlas.v2.ch14
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