ParietalCraniotomy

GeneralConsiderationsandOperativeAnatomy

Parietalcraniotomyisdesignedtoprovideanoperativeexposureofthemidtoposteriorhemispherewhilesparingthehighlyfunctionalanteriorlylocatedsensorimotorcorticesandtheposteriorlylocatedvisualcortex.Theapproachcanbedevisedtolateralandmesialparietallobelesionsaswellastointerhemisphericmedianorparamedianlesions.

Thevariationsofthiscorridorallowaccesstolesionsthroughthetranscorticalroute(throughthemorefunctionally“silent”superiorparietallobule)ortheinterhemisphericfissure.Theparasagittalveinsareoftenlessnumerousintheposteriorparietalregion,thereforeprovidinganopportunitytoreachdeeplesionsthroughtheinterhemispherictrajectory.

Therightornondominantparietallobe(seeWikipedia)isimplicatedinspatialawarenessandnavigation.Operativeinterventionsthatplacetheentirerightlobeatriskareassociatedwithhemibodyneglect.Thisneglectdoessignificantlyimproveovertime,butsomeresidualdisabilitypersists.

Theleftordominantparietallobe(seeWikipedia)isinvolvedinsymbolicfunctionsinlanguageandmathematics.Damagetotheleftloberesultsinproblemswithmathematics,longreading,writing,andunderstandingsymbols.Gerstmann'ssyndromeisassociatedwithlesionsinthedominantinferiorparietallobe,whereasBalint'ssyndrome(simultanagnosia,oculomotorapraxia,opticataxia)isassociatedwithbilaterallesions.

TheNeurosurgicalAtlas byAaronCohen-Gadol,M.D.

Theposteriorparietalcortexcanbesubdividedintothesuperiorparietallobule(Brodmannareas5+7)andtheinferiorparietallobule(39+40),separatedbytheintraparietalsulcus.

Figure1:Lateral(A),anterior(B),andposterior(C)viewsofthecerebrum.Notethelocationofthesuperiorandinferiorparietallobulesseparatedbytheintraparietalsulcus(C).Parasagittalbridgingveinsarevariableintheirsizeandlocationandplayanimportantroleindrainingtheparamedianhemispheres.Venouslakesalongthesuperiorsagittalsinuscanbeproblematicifthe

duralopeningisextendedclosetothemidline(B)(ImagescourtesyofALRhoton,Jr).

Parietallobeveinsareclassifiedaccordingtosurfaceofdrainage(medialorlateralgroup)andtothedirectionofdrainage(ascendinggroup:draintothesuperiorsagittalsinusordescendinggroup:draintotheinferiorsagittalsinusorthesylvianfissure).Onthelateralsurfaceofthelobe,theascendingveinsarethecentral,postcentral,anteriorandposteriorparietalveins,whileparietosylvianveinsformthedescendinggroup.Onthemedialsurfaceofthelobe,theascendingveinsaretheparacentral,anteromedialandposteriomedialparietalveins.Finally,thedescendinggroupisformedbytheposteriorpericallosalveins.

ThelateralgroupalsoincludestheveinofTrolard,alsoknownassuperioranastomoticvein,whichcrossesthefrontalandparietallobesonitswayfromthesylvianfissuretothesuperiorsagittalsinus.ThemostcommonlocationoftheveinofTrolardisthepostcentralregion,butitcanalsobefoundatthecentralorprecentralregion.Thecorticalveinsdraindirectlytothesuperiorsagittalsinusormayjoinaparasagittalmeningealsinusorlacunaeinthedura,whichisthedrainagechannelofmeningealveinscommonlylocatedattheparietalandposteriorfrontalareas.

Figure2:Superior(upperleft),oblique(upperright)andposteriorviews(lowerrow)ofthecerebrumdemonstratingtheascendinggroupofveinsthatdraintheparietallobe(central,postcentral,anteriorandposteriorparietalveins).Ant.:anterior;Cent.:central;Mid.:middle;Front.:frontal;Par.:parietal;Post.:posterior;Sag.:sagittal;Str.:straight;Sup.:superior;Temp.:temporal;V.:vein.(Modifiedwithpermission,courtesyofALRhoton,Jr.)

IndicationsfortheApproach

Theparietalcraniotomyisusedforbothintra-andextra-axiallesionsoftheregion,includingneoplasmssuchasmetastases,gliomas,andmeningiomas,andvascularlesionssuchasarteriovenous

malformationsandcavernousmalformations.Theparietalinterhemisphericcorridorisusedtoapproachparafalcine,medialparietal,andspleniallesions.

Theparietalcraniotomyismostoftenperformedforconvexity,falcine,andparafalcinemeningiomas.Inthesecases,thepatentsuperiorsagittalsinusandtheassociateddrainingveinsareatriskandshouldbesparedtoavoiddisablingvenousinfarcts.Anydissectionaroundthetumorcapsuleshouldprotectenpassageveinsandarteries.Asdiscussedabove,vascularinjuriesintheparietallobecancausedeficitsinspatialawareness,sensorimotorfunction,andvisualprocessing,andalsoriskinjurytothenearbymotorcortexanddeepwhitemattertracts.

Parietalcraniotomycanalsobeusedtoapproachparamedian(periatrial)lesionsoftheatriumofthelateralventricle.Thetraditionalapproachtotheatriuminvolvesatranscorticalroutethroughthesuperiorparietallobulewithariskofdeficitsinspatialawarenesssuchasastereognosiaandspeechorvisualprocessing.Recentstudieshavesuggestedthat,dependingonthepatient’soccupationandactivities,qualityoflifemayindeedbesignificantlyimpactedbysuchdeficits.

Toavoidtheserisks,lesionsoftheatriumcanbeapproachedthroughaparamedianposteriorparietalcraniotomyandcontralateralinterhemispherictransfalcineapproachthroughtheprecuneus.Thisapproachprovidesalongerandmoretechnicallychallengingpathtotheatrium,butinvolveslesswhitemattertractdisruptionandbrainretraction.

PreoperativeConsiderations

Corticalstimulationmappingunder“awake,”“sleep”conditionsorphasereversalmappingmaybeconsideredforlocalizingthe

sensorimotorcortexforintraparenchymallesionssituatedalongtheanteriorparietalarea.Sinceearlyaccesstothebasalcisternsisnotavailableduringparietalcraniotomies,Ihavealowthresholdforplacingalumbardrain,evenforlargelesionswithsignificantmasseffect.Toavoidtranstentorialherniationinthecaseofmassivelesionswithmidlineshift,Iopenthedraintoremovecerebrospinalfluid(CSF)duringduralopening.ThisCSFdrainagesignificantlyassistswithbrainrelaxationandmanipulationofedematousbrain.

Iftheinterhemisphericcorridorisconsideredandlargeparasagittalveinsaresuspectedonpreoperativecontrast-enhancedmagneticresonance(MR)imaging,anMRorCTvenogramguidesthelocationofcraniotomy.Thevenogramwillalsoconfirmthepatencyofthesuperiorsagittalsinusinthepresenceofaninfiltratingmeningioma.Ifnumerousparasagittalveinsprohibittheipsilateralinterhemisphericcorridor,thecontralateralinterhemispherictransfalcineroutemaybeconsideredforparafalcinelesions.

Ifthetumorpartiallyinfiltratesthelumenofthevenoussinusandtheriskofairembolismissignificant,apreoperativecardiacdiagnosticworkupisnecessarytoexcludetheriskofaparadoxicalairembolism.AtransesophagealechocardiogramandtransthoracicDopplermaybeusedandthereshouldbealowthresholdofsuspicionforairembolismduringtheprocedure.

PARIETALCRANIOTOMY

Figure3:Thepatientispositionedthree-quartersproneontheoperatingroomtable.Thispositionprecludestheneedtoturnthepatient’sheadintoanonphysiologicposture,aswouldbethecaseifthepatientwerepositionedsupine.Moreover,thelateralpositionpromotestheextra-axiallesionstoremainreadilyaccessibleandgravityretractioncanbeexploitedforreachingtheinterhemisphericfissure.Thepatientmustbefirmlysecuredtothetablebecausetiltingthetableduringsurgerycanriskpatientdisplacement.

Thedegreesofthepatient’sheadturnandtiltaredependentontheexactlocationofthelesionwithrespecttothemidline,coronal,andlambdoidsutures.Forparafalcineparietallesions,thesideofthelesioncanbeplacedinthedependentpositiontousegravityretractionwhiletiltingtheheadawayfromthefloortopermitamoreergonomicsittingpositionfortheoperatorduringmicrosurgery.Similarly,whenapproachingtheatriumorperiatrialregionthroughthecontralateraltransfalcineroute,Iprefertoplacethepatientinathree-quarterspronepositionandthenormalhemisphereonthedependentside.

Anaxillaryrollsupportsthecontralateralaxilla.Theipsilateralshoulderisgentlypulledanteriorlyandinferiorlyandsecuredwithtapetokeepitoutoftheoperator’sworkingzone.Forconvexity

lesions,itisadvantageoustotiltthepatient’sheadenoughtoplacethelesionatthehighestpointintheoperativefield.

Figure4:Thepatient’sheadissecuredinaskullclamp.Theapplicationofaskullclampshouldsatisfycertainprinciples.First,alineconnectingthesinglepinwiththemidpointbetweentheoppositetwopins(swivelrockerarm)mustcrosstheequatorofthepatient’sheadtopreventskullclampfixationfailureandheadslippage.Second,thepinsshouldnotbeplacedclosetothevertex.Thispositioningoftheheadwillallowthegravitytoretractthedependenthemisphereandfacilitateamoreexpandedcorridorthroughtheinterhemispherictrajectory.Thisheadpositionisincontrasttothepositioninthenextsketchwhereacorticalorconvexitylesionisexposedandthelesionisplacedclosetothehighestpointoftheoperativefield.

Figure5:Variousincisionstyleshavebeenmarked.Thelinearincision(red)oftenprovidesampleexposure.Thehorseshoeincision(blue)isreservedforlargeconvexitymeningiomas.Theparamediancraniotomyisoutlined(black).IuseneuronavigationorpreoperativeMRvenogram/angiogramstopositionthecraniotomy.Thistoolassistswithlocalizingandavoidingparasagittalbridgingveins,especiallyforproceduresrequiringinterhemisphericdissection.

Figure6:Forlargelateralconvexitymeningiomasandgliomas,atraditionalhorseshoeincisionisreasonable.Theheadisrotateduntilthelesionisplacedatthesummitoftheoperativefield.

Figure7:Forparamedianinterhemisphericlesions,Iplacetwoburrholesoverthesuperiorsagittalsinusasguidedbyneuronavigation.Thesinusistypicallydeviatedtotherightofthesagittalsutureinmostpatients;themaximumdeviationisusuallynomorethan11mm.Earlyidentificationofthesinushelpsmeplanthesizeandlocationoftheboneflap.

APenfield#3dissectorisusedtogenerouslydissectbetweentheinnertableofthecalvariumandthewallofthesuperiorsagittalsinus.Ifthewallofthesinusisadherent,athirdburrholeshouldbeplaced;allburrholesshouldbereadilyincontinuitywithintheepiduralspace.Cerebrospinalfluiddrainagethroughthelumbardrainmobilizesthewallofthevenoussinusandtheduraawayfromthebone,thereforepreventingtheirinjurybythefootplateofthedrill.Thelastbonycutshouldbemadeoverthevenoussinus.Thismaneuver

allowsatimelyelevationoftheboneflapifbleedingisencounteredandaninjurytothesinushasoccured.

Uponelevationoftheboneflap,mildtomoderatebleedingfromthesinuswallmaybecontrolledwiththrombin-soakedgelfoamorSURGICELFibrillar(Somerville,NJ).Thelatterisleftinplaceuntouchedduringclosure.PleaserefertothechapterontheRepairofDuralVenousSinusInjuryinthePrinciplesofCranialSurgeryVolumeforfurtherdetailsregardingmanaginginjuriestothesinus.

Figure8:Forconvexityorintraparenchymallesions,theduraisopenedcircumferentiallyaroundthetumorwitha2-cmmarginawayfromthecontrast-enhancingregionasguidedbyintraoperativenavigation(leftimage).

IfIplantoreachtheinterhemisphericspace(rightimage),Iopenthedurainacurvilinearfashionandcreateaduralflapbasedonthesuperiorsagittalsinus.Careistakentoavoidinjuringthelargedrainingveins.Occasionally,asmalldrainingveinmayneedtobesacrificed.Ifaparasagittalveinisencountereddrainingintothesinus,theduralopeningmustbeadjustedto

protectthevein’sinletintothesinus(leftimage,inset).

Notethatthebridgingveinsmoveintheposterior-to-anteriordirectiontodrainintothesinusandmayhavemultipletributaries.Paramedianextensionsofthesuperiorsagittalsinusorvenouslakesarefrequentlyencounteredinthisregion.Theirpresencemaylimitopeningtheduraclosetothemidlineandrestricttheinterhemisphericexposure.Inthissituation,theduralincisionnearthemidlinemaybeextendedparallelratherthanperpendiculartothevenoussinus.

Becauseofunpredictablelateralreachofthevenouslakes,asmalltearalongthelateralwallofthesinuscanbeencounteredduringtheparamedianduralincision.Thetearshouldbeclosedusingfinesutures.Bipolarcoagulationleadstoshrinkageoftheduraandexpansionofthetear.

Figure9:Toreachtheparafalcinespace,Ireleasetheveinsthroughtheirarachnoidadhesionsanduntethertheminpreparationfortheirmobilization.Thismaneuvermaybetediousbecausethearachnoidmembranescanbethickandhighly

adherent.CSFlumbardrainageaffordsearlymobilizationofthehemisphereawayfromthemidlineandfalx(leftimage).

Iplaceretractionsutureswithinthesuperiorfalxandgentlymobilizeandrotatethesuperiorsagittalsinus,therebyexpandingtheoperativecorridorandworkingangleswithintheinterhemisphericspace(rightimage).

Figure10:Thenextstepsofmicrodissectionwithintheinterhemisphericcorridorcannowbegin.Theparasagittalveinsshouldnotbeplacedundersignificanttension.

Figure11:Toreachthecontralateralperiatrialregion,IcreateaT-shapedincisionwithinthefalx(leftimage).Thefalcineflapsarereflectedandheldinplacewithretractionsutures.Acorticotomythroughthecontralateralprecuneusandobliquewhitematterdissectionwithinthemedialcontralateralhemisphereallowentryintotheatrium(greenarrow,rightimage).Divisionofthefalx,cortex,andwhitematterareperformedusingnavigation.

Closure

Oncethepathologyishandled,hemostasisisachievedandthesurgeon’sattentionturnstoclosure.Iftheventricleisentered,aventriculardrainagecathetermaybeplacedtocleardebriswithintheventriclesduringtheimmediatepostoperativeperiod.

Idonotroutinelyclosethedurainawatertightfashionforsupratentorialcraniotomies.Iavoidallograftduralsubstitutesfortheirriskofasepticinflammationorinfection.Duralclosureshouldnot“kink”orcompromiseflowwithintheparasagittalveins.

PearlsandPitfalls

Thethree-quarterspronepositionisareasonableoptionforparietallesionsasitfacilitatesaccesstotheipsilaterallesion

andallowsgravityretractiontoexpandtheinterhemisphericoperativecorridor.

Injurytothesuperiorsagittalsinusduringaparamediancraniotomyshouldbepreventedatallcosts.Keepalowthresholdofsuspicionforairembolism.

Parasagittalbridgingveinsoftendonotreceivetherespecttheydeserve.Avenousinfractioninthisregioncanbecatastrophic.

Contributor:MarcusAcioly

References

Al-MeftyO.OperativeAtlasofMeningiomas.Philadelphia:Lippincott-Raven,1998.

AlverniaJE,LanzinoG,MelgarM,SindouMP,MertensP.Isexposureofthesuperiorsagittalsinusnecessaryintheinterhemisphericapproach?Neurosurgery.2009;65(5):962-965.

RazaS,Quinones-HinojosaA,OliviA.Convexitymeningiomas,inDeMonteF,McDermottM,Al-MeftyO(eds):Al-Mefty’s

Meningiomas,2nded.NewYork:ThiemeMedicalPublishers,2011.

RhotonALJr:Thecerebrum.Neurosurgery.2002;51(Suppl1):S1-51.

RhotonALJr.Thecerebralveins.Neurosurgery.2002;51(4Suppl):S159-205.

SteinmetM,KrishnaneyA,LeeJ.Surgicalmanagementofconvexity,eningiomasInBadieB.(ed):NeurosurgicalOperativeAtlas:

Neuro-oncology,2nded.RollingMeadows,IL:ThiemeMedicalPublishersandtheAmericanAssociationofNeurologicalSurgeons,2007.

TewJMJr,vanLoverenHR.AtlasofOperativeMicroneurosurgery,Vol1.Philadelphia:Saunders,1994.

TewJMJr,vanLoverenHR,KellerJT.AtlasofOperativeMicroneurosurgery,Vol2.Philadelphia:Saunders,2001.

DOI:http://dx.doi.org/10.18791/nsatlas.v2.ch03

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