techniques of sylvian fissure split...and the fundamentals of microsurgical techniques. a wide...
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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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