TechniquesofSylvianFissureSplit

Figure1:GenerousdissectionoftheSylvianfissureallowsatraumaticclipligationofcerebralaneurysms(inset)usingdynamicretraction.

Theevolutionofmicrosurgicaltechniqueshasfacilitatedpreservationofnormalbraintissuewhileprovidingsurgeonswithareasonableaccesstodeepbrainlesions.Theapplicationofthesetechniquestointra-cisternalandsubarachnoidaldissectionhascreatedsafesurgicalcorridors.Specifically,thedissectionoftheSylvianfissureallowsexposureofthestructuresaroundtheanteriorcircleofWillis,parainsularregions,mesiobasaltemporallobe,andinterpeduncularcisterns,withminimalinvasionintoorretractionofthenormalbrainparenchyma.

Asafeandefficient“splittingoftheSylvianfissure”requiresfamiliaritywiththesurgicalanatomyofthestructuressurroundingthefissure

TheNeurosurgicalAtlas byAaronCohen-Gadol,M.D.

andthefundamentalsofmicrosurgicaltechniques.AwideopeningoftheSylvianfissureistechnicallychallenging;thistaskfrequentlydoesnotreceivetherespectorattentionitdeserves.Masteryofefficientfissuredissectionisimperativesothattheoperatordoesnotarriveatthecriticalpartoftheoperationpartiallyfatigued.

Inthischapter,IwillreviewthesurgicaltechniquesapplicabletoexplorationoftheSylvianfossatogainaccesstothecarotid-opticcisternandinsula.Therearecertainlysome“easyfissures”inolderpatientswithbrainatrophy,and“difficultfissures”inyoungpatientswithaneurysmalsubarachnoidhemorrhage.Eachscenariorequiresadifferentsetofmicrosurgicalskillsandmaneuversforsafeandefficientfissuredissection

IndicationsfortheProcedure

ThereismuchcontroversyregardinghowmuchofthefissureshouldbedissectedtosafelyreachtheanteriorskullbaseoranteriorcircleofWillis.

Proponentsofextensivefissuredissectionbelievethatthedisconnectionofthefrontalandtemporallobeswillleadtomoreoperativespacewithlessbasalfrontalloberetractionalongtheanteriorskullbaseandbasalcisterns.Someoperatorsroutinelydissectthefissuregenerously,whereasothersdoaminimalanteriorfissuresplit.

IbelievetheextentofrequiredSylvianfissuredissectiondependsonthelocationandtextureoftheunderlyingpathologyaswellasthebraincompliance(theextentofbrainatrophyandabsenceofbrainswelling,)andtheskilloftheoperatorinusingdynamicretractionefficientlywhileavoidingfixedretraction.Essentially,thecomplexinteractionbetweenthedegreeoftechnicaldifficultyinmanagingtheaneurysmorremovingfibrousvasculartumorsandbrainresistance

tomobilizationdeterminestheneedforagenerousSylvianfissuresplit.

TherearetwoextremesintheneedforSylvianfissuresplit.Forlargeinsulartumorsorgiantmiddlecerebralartery(MCA)aneurysms,thefissuremustbedissectedaswidelyaspossibletothelevelofthesuperiorandinferiorperi-insularsulci.Theposteriorextentofdissectionisdeterminedbythelocationoftheposteriorpoleofthetumorandthesafetyofdissectionbetweenveryadherentposteriorinterdigitationsofthefissure,especiallyinthedominanthemisphere.

Formostpatientswhoharboranteriorskullbasetumorsoranteriorcirculationaneurysms,IdissectonlytheanteriorlimboftheSylvianfissure,exposingthecisternjustanteriortotheM1.Ifadditionalfissuresplitisdeemednecessaryduringhandlingofthelesion,Iredirectmyattentiontoattemptmoreextensivesplit.

Overall,somedegreeofatraumaticfissuredissectionismandatoryforthelateralsubfrontaltrajectory.

MicrosurgicalOperativeAnatomyoftheSylvianCistern

TheSylviancisterncontainsthreeparts:thefissure,theopercularsulci,andtheSylvianfossa.TheSylvianfossahousestheMCAthatisjustsuperficialtotheinsula.

Figure2:TheSylvianfossacontainstheMCAandisthespacejustlateraltotheinsula(imagecourtesyofALRhoton,Jr).

Thefissureisdividedintoanterior(stem)andposterior(insulo-opercular)compartments.Thestemoriginatesinferiorlyattheanteriorperforatedsubstanceandextendslaterallytothetemporalpole.Thestemreacheslaterallyanddividesintotheascending,horizontal,andposteriorrami;theconfluenceoftheseramihasbeenreferredtoasthe“Sylvianpoint.”

Figure3:TheSylvianpointismarkedwith*.Thehorizontalandascendingramidividetheinferiorfrontalgyrusintotheparsorbitalis,parstriangularis,andparsopercularis.TheSylvianfissurecontainsseveralinteropercularsulcibetweentheopercularsurfacesoflateralorbital,inferiorfrontal,inferiorparietal,andsuperiortemporalgyri.Thesesulciareoftenobliqueandcurvedduetooppositionoftheadjacentgyri.Thisconfigurationofthesulcimakesfissuredissectionademandingtaskrequiringthesurgeon’spatience(imagecourtesyofALRhoton,Jr).ThelengthofthefissureproximaltotheSylvianpointisconsideredthe“proximal”fissureandthecorrespondinglengthdistaltothispointiscalledthe“distalfissure.”MostcasesneedaproximalSylvianfissuredissection.

Figure4:Theproximalsectionofthefissure(vallecula)housestheinternalcarotidarterybifurcationandlimeninsula,wheretheMCAbifurcatesintoitssuperiorandinferiortrunks.ThevalleculaalsocontainslaterallenticulostriateperforatorsandthedeepSylvianvein.OpeningofthevalleculawillprovidespacetoreachtheproximalMCAandinternalcarotidarterybifurcationterritories.

Theproximal(sphenoidal)section(A)alsoincludesthearea(3-4

cm)overtheplanumpolarewherethepialsurfacescanbehighlyadherent,requiringgentlemicrodissection.Thepaucityofthevesselsinthissectionallowsadherenceofthefrontotemporalopercula.

Themiddle(insular)section(B)is6to7cminlengthandextendsfromthelimeninsulatotheposteriorinsularpoint.Inthissectionofthefissure,thesulciarelessinterdigitated,possiblysimplifyingfissuredissection.

Theposterior(retroinsular)section(C)isshort(4–5cm)butdeep,andcoveredbythesupramarginal,transversetemporal,andtransverseparietalgyri.Thedissectioncanbeespeciallychallengingatthissegmentbecauseofcomplexinterdigitationsoftheopercula.

Thereisadensenetworkofpia-arachnoidfibersaroundthearteries,veins,andpialsurfacesoftheadjacentoperculaandinsulargyrioftheentirefissure.

TheMCAisdividedinfoursegments:theM1segmentcoursesposteriorandparalleltothesphenoidridge,theM2segmentresidesonthelimeninsula,theM3segmentspreadsoverthefrontotemporoparietalopercula,andtheM4segmentiscomposedofthebranchestothecerebralconvexity.

ThecourseoftheM1segment(3–4cminlength)withintheproximalSylvianfissureisC-orS-shaped.ThereareanumberofanatomicvariationsinMCAbranching,andfamiliaritywiththeseanatomicvariationsisimportantforsurgeryofneighboringlesions.PleaserefertothechapteronClipLigationofMCAAneurysmsforareviewofthesevariations.

AdequateexplorationoftheMCAatthepointofitsoriginfromtheinternalcarotidarterythroughitsbifurcationwillallowappropriate

identificationofthesurgicalvascularanatomyandpreservationofnormalstructuresduringsurgeryinthisregion.ThedissectionshouldbeconductedalongtheinferiorandanterioraspectoftheM1topreventinadvertentinjurytothelenticulostriatearteries.

SYLVIANFISSUREDISSECTION

Thepatientisusuallypositionedsupineontheoperatingtablewithhisorherheadrotatedawayfromthesideofthesurgeryapproximately30degrees.Turningtheheadmorethan30degreesmayobstructfissuredissectionbyrotatingthetemporaloperculaintotheangleoffissuredissection.

Aftertheduralopening,furtherbrainrelaxationmaybeimmediatelyachievedbygentleelevationoftheanteriorfrontallobeandopeningthearachnoidmembranesovertheopticocarotidcisterns.Asmallcottonballmaybeinsertedunderneaththefrontallobetomaintaintheoutflowofcerebrospinalfluidfromthesecisternsduringfissuredissection.

Icoverthesurfaceofthebrain,excepttheperi-insularareas,withpiecesofmoistTelfatoavoidheatinjurytothecortexfromtheintenselightofthemicroscope.

TheSylvianfissureiscoveredalongitsentirelengthwithathickbandofarachnoidmembrane.ThesuperficialSylvianveinsoutlinethecourseofthefissure.Intheabsenceoftheveins,identificationofthefissuremaybedifficult;insuchcases,recognitionoftheM4branchesexitingthefissureontothecortexmaybehelpful.Thefissureis10to14cminlength,longerthanoftenappreciated.Minororsubtleformsofcorticalmalformationcantransformthefissureintoaseriesofsulci,makingtheoperator’sjobverydifficult.

Figure5:ThetraditionalSylvianfissureopeninginvolvessplittingapproximatelytheanteriorone-thirdofthefissure(namely,Sylvianstem,proximalfissure:betweentheinternalcarotidarterybifurcationandparstriangularis)alsoknownastheanteriorlimb,exposingtheM1andmedial2cmoftheM2segments.Theextentofthistraditionalsplitisdemonstrated.Thearrowsdefinetheneckoftheposteriorcommunicatingarteryaneurysmapproachedviathetranssylvianroute.

TheSylvianfissureismorereadilysplitbyconductingdissectionaboveratherthanbelowthesuperiorSylvianveinsincetheveintravelsapproximately4mmbelowthefissureinmorethan80%ofthehemispheres.Thearachnoidofthefissureshouldpreferablybeopenedonthefrontalsideoftheveins,sothattheveinswillnotcrossthefissurewhenthefrontallobeinelevated.

IfmorethanonesuperficialSylvianveinispresent,Iprefertodissectbetweenthetwoveinsbecausetheseveinsareencasedbyarachnoidbandsthataretypicallymorerobustthanthepia.ThereisoftennobridgingveinbetweenthesuperficialSylvianveins,andthereforethesmallfronto-orbitaltributaryveinstothesuperficial

Sylvianveinarelesslikelytobesacrificedduringfissureopening.Preservationoftheveinsduringfissuredissectionprotectscerebralvenousdrainageandvoidsvenousinfarction;thisisespeciallyimportantiftheseveinsseemtobedominantbasedontheirlargecaliberandparasagittalveinsarelessprominentonpreoperativeangiography.

ThenaturalsuperiorretractionoftheapexoftheparstriangularisalongtheSylvianpointtypicallyprovidesthesurgeonwiththewidesttransfissurecorridorwherethesuperficialarachnoidmembraneisdemarcated;IresumetheSylvianfissureopeningatthisspecificlocation.

Furthermore,explorationofthefissureatthispointwillexposetheinsularapex,animportantlandmarkforsurgicalorientation.Usingaroundblade(beaverknife),Imakeasmall(3-mm)openingalongandabovethesuperficialSylvianvein.Thesuperficialarachnoidmembranealongthefissuremaybeincisedatseveralpointsinasimilarfashion.Next,Iextendtheopeningthroughthearachnoidbyholdingtheedgesofthearachnoidwithtwoshort,finebipolarorjewelerforceps,strippingthearachnoidovertheveinandsplittingthesuperficialfibersofthefissure.

Ifslightpialbleedingisencountered,hemostasisisachievedusingaminutepieceofgelfoamcoveredwithasmallcottonpledget,andbipolarcoagulationisavoidedduringtheentirefissuredissection,ifpossible.Explorationmaybeconductedafewmillimetersfurtherintheneighborhoodoftheoozingpialsurface.Thesurgeonthenmayreturntothisregioninashortperiodoftimetoappreciatethespontaneoushemostasisobtainedusingthistechnique.

Soft,moist,cottonpledgets(1Ï1mm)orballsaregentlyglidedbetweenthepialmembranesoftheadjacentgyri.Gradualandgentlecompressionoverthepledgetsbythefinesuctiontube,inadditionto

thespreadactionofthebipolarforceps,willallowgradualextensionoftheinitialopeningdowntotheSylvianfossa.Aslightlylargerpledgetisintroducedintotheinitialwindowtoreplacethesmallerpledgetasthefissureopeningisenlargedanddeepened.Thelargerpledgetcankeepthissegmentofthefissureopenasdissectioniscontinuedanteriorlywithouttheneedforretractors.

IsplitthefissuredeepenoughtoidentifytheMCAbranchesandinsulaintheSylvianfossaatthesiteoftheinitialfissureopening.Thedissectionthenproceedsintheanterogradefashionwhileopeningthefissurefromdeeptowardthesurfaceor“inside-to-outside.”

Figure6:TheSylvianfissureissplitusingtheinside-to-outsidetechnique.ThedissectionisstartedattheSylvianpointandextendedtothedepthofthefissuresothatIcanidentifydistalMCAbranchesonthesurfaceoftheinsula.Ithenpursuedissectionfromdeeptosuperficial(upperimage,inset,arrow).Thismethodofopeningthefissurehasananalogy.Yasargilcomparedthismethodtoradialsplittingofpeeledorangewedges.Itisdifficulttoseparatetheedgesfromtheoutside(leftlowerimage),butitiseasytoputmyfingerintothemiddleoftheorangeandradiallyseparatethewedges(rightlowerimage).

Itisoftenmoredifficulttoseparateorangeslicesfromoutsidetheorange,butifonestartsbyenteringintothecenteroftheorangeandidentifyingthedividingplanesofthewedgesfrominside,onecansplittheslicesmorereadilywithoutcompressingtheindividualslicesandreleasingtheirjuice.

Theinside-to-outsidetechniqueallowsearlyidentificationoftheMCAbranches,thereforeallowingthesurgeontoadjusttheplaneofdissectionalongtheinterdigitatingoperculawhilemaintainingthe

depthofthefissureasalandmarkfororientation.

Thecontoursoftheinterdigitatingoperculararelyfollowastraightverticallineasthelateralorbitalgyrusindentstheproximalsuperiortemporalgyrusandtemporalpole,causingaC-orS-shapedcourseofproximalfissureinthecoronalplane.Earlyidentificationoftheundulatingpialandarachnoidplanesfrominsidethefissuresimplifiesdissectiontremendously.Asthefissureisopenedtowardthesurface,thethickersuperficialarachnoidmembranesmaybecutusingmicroscissors.

FollowingidentificationoftheMCAbranchesandinsuladeepattheSylvianpoint,anterogradeinside-to-outsidedissectioncontinues.InjectionofsalinesolutionusingasyringedeepintoandalongtheSylvianfossacanexpandthisfossa,facilitatingtheidentificationofthearachnoidplanesbetweenthepialbanks.

Iliketousejeweler’sforcepswithfinetipstograbandseparatethemoresuperficialthickarachnoidbands,whenneeded.

Icontinuedeeperdissectionusingstraight(nonbayoneted)microscissorsandbipolarforceps(5-mmtips).Throughthealternatinguseofbipolarforcepstospreadthethinarachnoidlayers,andmicroscissorstotransectthickones,theinitialfissureopeningisenlarged.Forcefulbluntdissectionorseparationofthethickarachnoidbandsoradherentpialbanksusingthespreadingactionoftheforcepswillleadtopialinjuryandbleeding.Theuseofbipolarcoagulationtostoppialbleedingmayactuallyleadtofurthercorticalinjuryandfurtherbleeding.

Theplanumpolareortheflattersurfaceoftheanteriorfissureonthetemporalsidemaybeveryadherenttothefrontallobe.Patientmicrosurgicaldissectioniswarranted.Thickarachnoidlayerscoverthemostanteriorlimbofthefissurejustbehindthesphenoidwing;

sharpdissectioninthisareamayrequiresacrificeofoneofthesuperiorSylvianveinbranches.

Asthemoreproximalpartofthefissureissplit,theMCAbifurcationandtheM1branchisidentifiedandamoremedialdissectionalongthisarterywillallowexpansionofthevalleculaandcreationofasurgicalcorridortowardtheopticocarotidcisterns.Theposteriorfrontaloperculumoftentendstoherniateintothetranssylviancorridor;itsaggressiveretractionleadstoitsvenouscongestion,andspontaneouscorticalbleedingandmustbeavoided.

AlongthemedialaspectoftheSylvianfissureandjustbeforereachingtheopticocarotidcisternsanteriorly,athickarachnoidbandtethersthefrontalandtemporallobestoeachother.Thisbandanditsoccasionalencasingsmallveinaretransected.A“T”shapedarachnoidincisionismadeovertheopticocarotidcisternswiththeanteriorlimboftheincisionjustlateraltotheopticnerveandoverthecarotidartery.Thisincisionisfurtherextendedmediallytodisconnecttheposteriorgyrusrectusfromthechiasm.Theposteriorlimbofthe“T”incisioncanparalleltheapproximatecourseoftheposteriorcommunicatingarteryandconnectswiththearachnoidopeningalongthemedialsylvianfissure.

ThefollowingstepssummarizetheprinciplemaneuversforSylvianfissuredissection.

Step1:ThesuperficialfissureisdissectedopenattheleveloftheSylvianpointwheretheinteropercularspacethroughthearachnoidbandismostprominent.ThisdissectionisextendedtotheleveloftheM2branchesandinsula.Notetheuseofthesmallcottonpledgetdiscussedabove.

Step2:Theverysuperficialthickarachnoidlayerencasingtheveinsmaybedisruptedusingfineforcepsormicroscissors.Amicrocottonballisusedtokeepthedissectionplanesopenandavoiddirectcontactbetweenthesuctiontipandthepialsurfaces.

Step3:Next,IusethedistaldeepopeningwithinthefissureandidentifythedistalMCAbranchesasalandmarktofurtheropenthefissurefrominside-to-outsideordeep-to-superficialintheposterior-to-anteriordirection.

Step4:Theplanumpolaremaybeveryadherent,andaroundknifemaybeusedtosharplyidentifythedissectionandpialplanes.Pleasenotetheroadmapfortheinside-to-outsidetechnique(insetimage,greenarrows).

Iavoidthecommonlyused“outside-to-inside”technique,whichismoredifficulttoperformbecauseoftheadherenceofthefrontotemporaloperculaandthelackofanylandmarkstoguidedissectionwithintheinterdigitatingopercula.

AfterextensionandcompletionofSylvianfissuredissectionmoremediallyalongthedistalM1segment,thetemporaloperculumismobilizedawayfromtheinsula.ThepresenceofMCAbranchesbetweenthetemporaloperculumandtheinsulafacilitatesmobilizationofthetemporaloperculummorethanthefrontaloperculum.Thesuperiorandinferiorperi-insularsulcicanbeidentified,ifnecessary.TheM1branchmaybeusedasalandmarktoreachtheopticocarotidcisternsandtheinternalcarotidarterybifurcation.

DistalFissureDissection

Distalfissuredissectionisoftenlimitedbecauseofadherenceoftheposterioroperculaatthislevel;aggressivemanipulationinthisareawillplacethesuperiortemporalgyrusatriskofinjury;thiscanbeafactorespeciallyinthedominanthemisphere.Dissectionoftheposteriorfissuresisnecessaryonlyforlargeinsulartumors,M2/M3aneurysms,andgiantMCAbifurcationaneurysms.

Aroundarachnoidknifemaybeusedtoworkwithintheadherentpialsurfaces.

Gentledynamicretractionofthefrontotemporaloperculaandanterior-to-posteriorworkinganglesoftenprovidegoodexposureoftheretrosylvianfossa,posteriorinsularcortex,andtheperi-insularsulci,aswellastheposteriorM2branches.

PearlsandPitfalls

Idonotdissectthefissurefromproximaltodistal.ThismaneuverrequiressignificantfrontalloberetractionastheM1isidentifiedearlyandfollowedtowarditsdistalpathway.However,distal-to-proximaldissectiondoesnotprovideearlyproximalcontrolandposescertainrisks,especiallyinthecase

ofarupturedaneurysm.

DOI:http://dx.doi.org/10.18791/nsatlas.v3.ch01.5

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