AnteriorChoroidalArteryAneurysm

Aneurysmsoftheposteriorwalloftheinternalcarotidartery(ICA,)namelytheanteriorchoroidalartery(AChA)andposteriorcommunicatingartery(PCoA)aneurysms,representthemostcommonsiteforintracranialaneurysms(upto35%oftotalaneurysms.)

TheAChAarisesafewmillimetersdistalandslightlylateraltotheoriginofthePCoAfromthesupraclinoidICA.Ithasaslightlysmallercaliber,canimmediatelybranchafteritsorigin,andsuppliesveryimportantterritories,suchastheposteriorlimboftheinternalcapsule,basalganglia,opticapparatus,cerebralpeduncle,lateralgeniculatebody,andlimbicsystem.

Thereisminimalcollateralsupportforthisvessel,anditspreservationisofparamountimportanceduringclipligationofposteriorcarotidwallaneurysms.Thedistalendsofthebladesshouldnotcompromisetheintegrityofthisvessel,whichisoftenserpentineandhidden,drapingovertheAChAaneurysmdome.

MostAChAaneurysmspresentwithsuprasellarandambiencisternsubarachnoidhemorrhage(SAH.)Theyrarelypresentwithintracerebral/intaventricularhemorrhageoroculomotornervedysfunction.

Inthischapter,IwilldescribetheoperativetechniquesforclipligationofAChAaneurysms.Thesimilarprocedureforposteriorcommunicatingarteryaneurysmsisdescribedinitsowndedicatedchapter.Ihaveabbreviatedtherelevantcontenthereandencourage

TheNeurosurgicalAtlas byAaronCohen-Gadol,M.D.

youtorefertothechapteronPCoAaneurysmsbeforeproceedingwiththefollowingdiscussion.

IndicationsforSurgery

ModerntreatmentoptionsforAChAaneurysmsincludeobservation,endovasculartreatment,ormicrosurgicalclipligation.Observationisareasonableoptionforsmallunrupturedaneurysmsorlargeronesinolderpatients(>75yearsold.)Patientswithunderlyingmedicalproblemsthataffecttheirsurvivalmayalsobetreatedconservatively.

AswithPCoAaneurysms,patientswithsignificantlifeexpectancyshouldbeconsideredformoreaggressiveinterventionthanthosewithothersmallanteriorcirculationaneurysms.Aneurysmschosenforobservationshouldbereconsideredforinterventioniftheyincreaseinsizeorcauseanoculomotornervepalsy.

TheAChAfrequentlyemergesfromtheneckoftheaneurysm.Thisneckconfigurationcomplicatesendovasculartherapyandplacesthesmall-caliberAChAatriskduringcoildeployment.Thus,microsurgicalclippingcontinuestoplayanimportantroleinthemanagementofAChAaneurysmsthatrequirerelativelyminimalmicrodissectionorbrainretraction.

Calcifiedaneurysms,identifiedonCTA,arebestmanagedviaendovascularmethodsbecauseclipligationhasareasonablelikelihoodofcompromisingtheoriginsoftheAChAandtheICAbythethickcollapsedwallsoftheparentvesselsafterclipdeployment.

PreoperativeConsiderations

InitialradiologicevaluationoftheseaneurysmsgenerallyincludeseitheraconventionalcatheterarteriographyorCT/MRangiography.CTangiography(CTA)hasbecomethemainstreaminitialstudyforevaluationbecauseofthefinevasculardetailitprovides.Moreover,it

showsthesurroundingskullbaseanatomy,includingthelocationoftheanteriorandposteriorclinoidprocesses,withrespecttotheaneurysmneck.

BonewindowingofaCTscancanrevealthepresenceofamiddleclinoidprocess,whichisrelevantwhenthesurgeonispreparingforproximalcontrol.Araresubsetoftheseaneurysmsprojectlaterallyintothetemporallobe.Thisanatomicvariationwarnstheoperatoragainstearlytemporalloberetractioninthesettingofarupturedaneurysmuntilproximalcontrolisreliablyestablished.

Figure1:AtypicalAChAaneurysmisshownoriginatingfromthejunctionoftheICAandAChA(leftimage).Theaneurysmarisingfromtheposteriorcarotidwall,justdistaltothePCoAoriginisconsideredanAChAaneurysmandistypicallymorelaterallyprojectingthanPCoAaneurysms.ClipligationiseffectiveforcompleteexclusionoftheaneurysmwhilemaintainingthepatencyoftheAChA(rightimage).

Intraoperativeneurophysiologicmonitoring,includingsomatosensoryandmotorevokedpotentials,canmonitorthepatient’stolerancetotemporaryocclusionandwarnthesurgeonaboutpotentiallyinvisiblecompromiseoftheperforatingvesselsafterclipligation.ThismonitoringstrategyisespeciallybeneficialfortheAChAarteryand

itsassociatedperforatingvesselsthatsupplythemotorsystem.

OperativeAnatomy

TheAChAarisesslightlymorelaterallythanthePCoAfromtheposteriorwallofthesupraclinoidICA.Identificationofbothvesselsattheposteriorcarotidwallisimportantbeforeoperativemanipulationbegins.

TheAChAmayconsistofuptothreebranches.Itcoursesthroughthecruralcisternenroutetoitsentrypointintothetemporalhornofthelateralventricle,providingseveralcriticalperforatorsalongitsway.Therefore,theAChAisdividedintocisternalandplexalsegments.

TheAChAclassicallyarisesjustdistaltothePCoA,butitcanoccasionallyarisemoredistally,eitherfromtheICAbifurcationortheM1segment.ItcanevenemergefromthePCoAitself.BoththePCoAandAChAprojectposteriorlyandmediallyfromthedorsalaspectoftheICA.Theanterior-to-posteriorsurgicalviewofthepterionalapproachplacesthesevesselscoursingawayfromthesurgeon’slineofsight.AllthesurgeonseesisaknuckleoftheveryproximalarteryatitsoriginbeforeitbecomeshiddenbehindtheICA.

Figure2:AlateralnonoperativeviewacrosstheposteriorICAdemonstratestheanatomicrelationshipsbetweenthePCoA(mobilizedwiththerightdissector,)itsperforatingvessels,andtheAChA(inbrightred.)Theoculomotornerveisvisible(imagecourtesyofALRhoton,Jr.)

Figure3:AviewoftheAChA(brightred)throughtherighttransventriculartranschoroidalapproachisshown.Notetheedgeofthetentoriumandtheoculomotornerveforanatomicorientation(imagecourtesyofALRhoton,Jr.)

ThereareinconsistentanastomosesbetweenthePCoAandAChAalongthelateralgeniculatebody.ThisfindingexplainsthevariabilityinthesizeoftheischemicterritoriescausedbyAChAocclusion.

MICROSURGICALCLIPLIGATIONOFANTERIORCHOROIDALARTERYANEURYSMS

PleaserefertotheCranialApproachesvolumefordescriptionoftheextendedpterionalcraniotomythatissuitableforalmostallAChAaneurysms.Aposterolateralorbitotomymaybeaddedforlarge

aneurysms.Inthissection,Ielaborateonthespecificsofthepterionalapproachasitpertainstotheexposureofthisaneurysm.

Themainobjectivesofpatientpositioningaretopreventthetemporallobefromobstructingyourdissectionoftheaneurysmwhileexploitinggravitytomovethefrontallobeawayfromtheanteriorcranialfossa.Thesemaneuversexpandtheposteromedialsubfrontaloperativecorridorandareaccomplishedbyextendingthepatient’sheadandlimitingitsrotationto20degrees.

Thesphenoidwingmustbeaggressivelydrilleduntilthesuperiororbitalfissureisreached.Themostmedialextentofthewingisthemostimportantbecauseitcoversthecarotidcistern.Ifthislastobstructionisnotremoved,substantiallymorefrontallobeelevationisneededtoexposethecistern.Theroofoftheorbitisalsodrilledsothataflatoperativetrajectoryovertheorbitisavailable.

Figure4:Schematicrepresentationoftheposteriorsubfrontaloperativepathway(greenarrow)towardtheAChAaneurysm.ThispathwayispossibleviaaproximalSylvianfissuresplit,leadingdirectlytotheanteriorandlateralaspectsofthesupraclinoidICA.EarlyexposureoftheproximalICAwithouttemporalloberetractionisnecessarytoavoidprematuresacrupture.

INTRADURALPROCEDURE

InitialExposure

Therearethreemaingoalsthatmustbeaccomplishedbeforetheaneurysmcanbedirectlyhandled.

ThefirstgoalisaproximalSylvianfissuresplit.

Thesecondgoalismobilizationofthefrontallobeanddissectionofitsarachnoidbandsoverthechiasmandfloorofthefrontalfossa.

ThethirdgoalisestablishingproximalcontroloverthesupraclinoidICA.Theaneurysmmustbeexposedsolelyviafrontalloberetraction.Thecruxoftheinitialexposureisfreeinguptheposteriorsubfrontallobesothatitcanbesafelyandgentlymobilizedwithoutanytransmissionofforcetotheaneurysmdome.

Figure5:Theinitialintraduralexposureisillustrated.Adequatesphenoidwingremovalisevidentwhentheduracanbemobilizedflatovertheorbitalroofandalongtheanterioredgeofthecraniotomy.

Theinitialstepsoftheintraduralprocedure,includingtheexposureoftheposteriorcarotidwall,arediscussedinthePosteriorCommunicatingArteryAneurysmchapter.ThefollowingparagraphsspecificallyrefertoAChAaneurysms.

Figure6:Theposterolateralwalloftheleftcarotidarteryisexposed.TheoriginofthePCoAisapparentonlyasasmallknuckleafewmillimetersinferiortotheaneurysmsac.TemporaryclippingoftheICAisoftennecessary,especiallyforrupturedaneurysms,duringthenextstepsofneckdissection.

Withthebrainrelaxed,thefrontallobecompletelyreleased,andproximalcontrolsecured,thefocusofdissectioncannowshifttowardthecircumdissectionoftheneck,themostcriticalpartoftheoperation.Applicationoftheclipisarelativelystraightforwardaffair.Thecreationofspacefortheclipbladesisthechallenge,andpreclippingdissectionrequiressurgicalintellectandappreciationofthree-dimensional(3D)anatomybasedonadetailedstudyofpreoperativeimages.

AneurysmDissection

TheaneurysmsacandAChAtravelawayfromtheoperator.Asaresult,onlyasmallknuckle,representingtheoriginoftheAChA,isvisible.IprefertousebriefperiodsoftemporaryproximalICAocclusion;IdonotbelieveetomidateburstsuppressionisnecessaryduringtemporaryproximalICAocclusionifcollateralsupportisrobustviaafunctionalcircleofWillis.Thelossofturgorintheaneurysmremarkablyfacilitatesitsneckdissectionduringhigh-riskmaneuvers.

TheneckdissectionproceedsbetweenthisknuckleandtheproximalneckoftheaneurysmandproceedsdistallyalongthelateralaspectoftheICA.Althoughthismaneuvercanbetedious,sharpdissectionunderhighmagnificationwillalmostalwaysfindaplanebetweentheAChAandtheaneurysmneck.

Onlyafewmillimetersoftheaneurysmneckisisolated,justenoughtoplaceaclipblade.Thereisnoneedtodissectmoreofthearteryordissectoutontotheaneurysmdome.IftheAChAcannotbereadilyidentified,theICAcanbegentlyretractedmediallytoallowthesurgeontoexploretheoriginofthearterymoredistallyalongtheneck.

Figure7:ThedistalneckisthespacebetweentheICAandtheaneurysm;similarmicrosurgicalprinciplesmentionedaboveapplytothisspaceaswell.TheAChAissometimesadherenttothemidbodyorfundusoftheaneurysm.TemporaryproximalICAclippingisespeciallybeneficialtosoftentheaneurysm,allowingdissectionoftheAChAawayfromtheneckwithout

placingtheAChAatrisk.OncetheproximalanddistalneckshavebeendissectedandthecoursesofthePCoAandAChAidentified,definitiveclippingoftheaneurysmcancommence.

WithtemporaryICAocclusioninplace,Imobilizetheproximalanddistalneckanddissecttheportionoftheneckthatisturningawayfrommylineofsight,ensuringthatIcanseewheretheneckisturningaroundtoformitsmedialborderwithrespecttowalloftheICA.Thismaneuverisimperativebecausealackofunderstandingofthe3DneckanatomywillleadtheoperatortoapplytheclipatawrongangleawayfromtheICAaxis,partiallyclippingtheneckandprecipitatinganintraoperativerupture.

Completingdissectionblindlyusingtheclipbladesand“guessingatthedeeperaneurysmneckborders”arerecipesfordisaster.Thesenovicemaneuversarearesultofanervoussurgeonwhoisallowinghisorheremotionstocontrolmicrosurgery.

Inrupturedaneurysmcases,thesubarachnoidclotaroundthedomeisleftundisturbeduntiltheaneurysmissecured.ThedissectionshouldbelimitedalongtheposterolateralwallofthesupraclinoidICAandtheneck;theoperatorshouldnotwanderandunintentionallypunctureoruncovertheaneurysmdome.Thesequenceofdissectionshouldproceedmethodicallyfromproximaltodistal,revealingtheknuckleofthePCoAorigin,theproximalneckandAChAorigin,thesacitself,thedistalneckandthenthedistalsectionoftheAChAawayfromthepathwayoftheclipblades.Next,theclipisdeployed.

ClipApplication

MostAChAaneurysmsaresmallandhaverelativelynarrownecks.Iligatetheseaneurysmswithabayonetedsimplestraightclip,angledperpendiculartotheICAaxiswiththetipspointinglaterally.AlthoughanaccordioneffectcanpotentiallycausestenosisoftheICAlumen,

thisislessofaconcerniftheaneurysmhasanarrowneck.

Figure8:TheaneurysmcanbebestmanipulatedanddissectedunderbriefperiodsoftemporaryICAocclusion.Clipapplicationfollows;theoriginoftheAChAshouldnotbecompromised.Invariably,asmallneckremnantisnecessarytopreservetheAChA’sorigin.BayonetedclipsareidealforallowingmetovisualizetheoriginoftheAChAaroundthebulkoftheclipappliers.ThelowerphotodemonstratesoneoftheperforatingbranchesoftheAChA(arrow)justdistaltotheblades.

Figure9:TheclipwasdeployedandtheAChA(redarrows)atthetipofthebladeswasinspectedforitspatencyviaintraoperativefluorescenceangiogram.

Astheclipbladesaregraduallyclosed,attentionisfocusedonthetipsofthebladestoensurethattheyarefreeoftheAChAasitturnsaroundthedome.

Figure10:Oncetheclipisplaced,IgentlyrotatethebladestoensurethatthebladesarenotcompromisinganysegmentoftheAChA.Thisinspectioniscrucialforavoidingcomplications;cliprepositioningmaybenecessary.ThearrowontheupperphotopointsattheoriginoftheAChAbeforeclipdeployment,whilethearrowinthelowerintraoperativephotopointsatthemoredistalrouteofthevesselaroundtheaneurysm.

FluorescencevideoangiogramhassomelimitationsforevaluationofAChAaneurysmsaftertheirligation.Mostoften,becauseofthedeepoperativecorridorandthehiddenaneurysm,enoughexcitationlightcannotreachthefluorescentagentwithinthesacforadequateemissionsignals.Thisphenomenonleadstofalsenegativeresults.Therefore,punctureofthedomeorintraoperativecatheterangiogramisnecessaryforconfirmationofaneurysmexclusion.

AdditionalConsiderationsandAneurysmVariations

Iprefertouseasimplebayonettedstraightclip.Ifthisisnotanatomicallyfeasible,asinthecaseofaposteriorly-projectingAChAaneurysms,Iuseanangledfenestratedclip,encasingtheICA.Thisconstructavoids“dogears”andismosteffectiveforcollapsingthe

neck.Furthermore,delayedclipdisplacementisunlikely.

Figure11:Iclipposteriorly-projectingAChAaneurysmsusinganangledfenestratedclip.ThefenestrationencirclestheICA.TheAChAisseenjustdistaltotheheeloftheclip,proximaltotheICAbifurcation.ThisconstructdoesnotroutinelyrequireisolationoftheAChAincaseofitsattachmenttothedome.

Figure12:Theupperintraoperativephotosdemonstratetheconfigurationofaright-sidedposteriorlyprojectingAChAaneurysm.Thetipoftheball-tippeddissectorpointsattheoriginoftheAChA(upperimage).TheangledfenestratedclipwaseffectivewhileobviatingtheneedforsignificantmanipulationoftheAChA(middleimage).IntraoperativefluorescenceangiographydemonstratedpatencyoftheAChA(arrow)andexclusionoftheaneurysm.

Figure13:AnoperativeviewofafusiformAChAaneurysmwithoutadefinableneckisdemonstrated.Thisaneurysmisnotamenabletoclipligation.Wrappingtheaneurysmwithmuslinisthemostreasonablestrategyinthiscase.TheAChAshouldnotbesacrificed.

PostoperativeConsiderations

InadvertentsacrificeoftheAChAleadstoananteriorchoroidalsyndromethatconsistsofcontralateralhemiplegia,hemianesthesia,andhemianopsia.ThissyndromeisverydisablingandnearlyalwaystheresultoftrappingtheAChAorperforatorsarisingfromthePCoAandAChA.Everyeffortshouldbemadetoavoidthisunfortunateoutcome.

Standardpostoperativecareisused.Anticonvulsantsarerecommendedandtaperedoffaboutoneweekaftersurgery.

PearlsandPitfalls

ThetakeoffoftheAChAcanbemistakenfortheproximalneckoftheaneurysm.ThisanatomymustbecarefullydissectedtoavoidocclusionoftheAChAduringaneurysmclipping.

TheproximalsegmentoftheAChAtypicallydrapesovertheaneurysmsac.CarefulmicrodissectionisnecessarytosavetheAChAduringclipdeployment.

DOI:http://dx.doi.org/10.18791/nsatlas.v3.ch01.11

References

LawtonM.SevenAneurysms:TenetsandTechniquesforClipping.NewYork:ThiemeMedicalPublishers,2011

SamsonD,BatjerHH.Aneurysmsoftheposteriorinternalcarotidwall,in:IntracranialAneurysmSurgery:Techniques.MountKisco,NY:FuturaPublishingCompany,1990.

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