Andersons pediatric cardiology 981
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muscle)orbecauseoftheirproximitytovitalstructures,suchastheleft
ventricularconductiontissuesortheleftcoronaryarteryinthetransversesinus.
Ifremovalisattempted,thesafestareaforresectionistheareaoccupiedbythe
muscularoutletseptum.
AssociatedMalformations
Othermalformationscancoexisteitherwhentheventricularseptumisintactor
inassociationwithaventricularseptaldefectorobstructionoftheleft
ventricularoutflowtract.Persistentpatencyofthearterialductisparticularly
significantbecauseitloadstheleftventricle,producingathickerwall.Other
significantanomaliesareinfrequentbutcanoccur.Theyincludestenosisofthe
subaorticoutflowtract,aorticcoarctation,oranomalouspulmonaryvenous
connections.
Morphogenesis
Twoconceptshavebeenadvancedtoexplainthedevelopmentoftransposition.
Thefirstsuggeststhattheanomalyistheconsequenceofinappropriate
separationofthearterialpoleoftheheart.Asaresult,thesixthaorticarches,
destinedtosupplythepulmonaryarteries,areconnectedtothemorphologically
leftventricle,whilethefourthaorticarchestaketheiroriginfromthe
morphologicallyrightventricle.Itwasproposedthatthiscanoccurifthe
cushionsseptatingtheintermediateandproximalpartsofthedevelopingoutflow
tractfuseinstraightratherthanintheirnormalspiralfashion.18Thesecond
theoryputstheseatofmaldevelopmentintheproximalpartsofthedeveloping
outflowtracts.Thishypothesissuggeststhat,ratherthanthesubaorticportion
becomingconnectedtotheleftventricle,asoccursinnormaldevelopment,itis
thesubpulmonarycomponent,togetherwiththepulmonarytrunk,whichisthus
connected.ThisconceptwasfirstproposedbyKeith,19whoinvokeddifferential
absorptionoftheinnerheartcurvatureasthecausativemechanism.Theconcept
wasrevitalizedundertheguiseofdifferentialconalgrowth.2Soastoexplainall
theknownvariantsoftransposition,includingthearrangementinwhichthe
aortaarisesposteriorlyandtotheright,itisnecessarytoinvokemaldevelopment
withinboththeproximaloutflowtractsandtheoutflowcushions.Initial
evidencefromanimalmodelsshowingdiscordantventriculoarterialconnections,
suchasthePitx2knockoutmouse,supportsthisnotionofthecombinationof
fusionofthecushionsinstraightratherthanspiralingfashion,withthe
subpulmonarycomponentoftheproximaloutflowtractthentransferredtothe
developingleftventricle(Fig.37.9).
FIG.37.9 Frontalsectionsfromawild-typemouse(left)andaPitx2
knockoutmouse(right)atcomparablestagesofdevelopment.They
illustratethemarkedlydifferentorientationofthecushionsinthedeveloping
outflowtract.Theknockoutmiceproceedtodevelopeitherdiscordant
ventriculoarterialconnectionsordouble-outletrightventriclewith
subpulmonaryinterventricularcommunicationbecauseitisthe
subpulmonarycomponentoftheproximaloutflowtract,ratherthanthe
subaorticcomponentasinthewild-typemouse,thatistransferredtothe
developingleftventricle.(CourtesyProfessorNigelBrown,St.George's
MedicalUniversity,London.)
