Andersons pediatric cardiology 771
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FIG.29.11 Heartwithapersistentleftsuperiorcavalvein(SCV)draining
totheroofoftheleftatrium.Thewallsthatwouldnormallyseparatethe
cavityofthesinusfromtheleftatriumareabsent.Inthissetting,theright
atrialorificeofthesinusbecomesaninteratrialcommunication.
Mostoftenapersistentleftsuperiorcavalveinisalsoconnectedtotheleft
atrialroof.Insomeexamplesofthiscombination,filigreedremnantsoftheatrial
wallpersistbetweentheorificeofthecoronarysinusandtheterminationofthe
leftcavalveinintheleftatrium.Theopeningofthecoronarysinusisusually
large.ItsanteriormarginencroachesonthetriangleofKoch,approximatingthe
areaoftheatrioventricularnode.Thereforecaremustbetakenwhenthedefectis
closed.Unlesspreviouslydiagnosed,acoronarysinusdefectmaybedifficultto
recognizeasaninteratrialcommunicationintheoperatingroom,appearing
simplytobethemouthofthecoronarysinusitself.Suchdefectsaresignificant
inthesettingofhypoplasticleftheartsyndromewithintactatrialseptum,since
theyprovideanoverflowfortheobstructedleftatrialchamber.
Morphogenesis
Duringnormaldevelopment,thefirstsignofatrialseptationisthegrowthofthe
muscularprimaryseptumfromtheroofoftheatrialcomponentoftheprimary
hearttube.1Asitgrowsdowntodividetheprimaryatrium,itcarriesonits
leadingedgeacapofmesenchymaltissue(Fig.29.12,left).
FIG.29.12 Initialstagesofatrialseptation.(A)Initialgrowthoftheprimary
septumfromtheatrialroof,withthemesenchymalcaponitsleadingedge.
Atthisearlystage,thereisasyetnoformationofthepulmonaryvein,but
themarginsofthedorsalmesocardiumprotrudeintotheatrialcavityasthe
pulmonaryridges.Astheprimaryseptumgrowstowardtheatrioventricular
(AV)cushions,withtheprimaryforamenformedbetweenthecushionsand
themesenchymalcap,thepulmonaryveincanalizestoopenintotheatrial
cavitythroughthesiteofthedorsalmesocardium.(B)Bythisstage,the
originoftheseptumfromtheatrialroofisbeginningtobreakdowntoform
thesiteofthesecondaryforamen.
Thespacebetweenthecapandtheatrioventricularendocardialcushions,
whichdividetheatrioventricularcanalintotheeventualmitralandtricuspid
valvarorifices,istheprimaryatrialforamen.Priortoclosureoftheprimary
foramen,thepulmonaryveincanalizeswithinthepharyngealmesenchyme
behindthedevelopingheart,openingtotheatrialcavitybetweentheridges
formedatthesiteofthedorsalmesocardium,whichistheinitialconnection
betweenthehearttubeandthepharyngealmesenchyme(seeFig.29.12,right).
Thefusionofthecapwiththeendocardialcushionsthusobliteratestheprimary
atrialforamen.Thesiteofclosureisreinforcedbythegrowthofadditional
tissuesthroughtheright-sidedridgeofthetwopulmonaryridges,thusproducing
thevestibularspine(Fig.29.13,left).Bythetimetheprimaryforamenhas
closed,thecranialoriginoftheprimaryseptumhasbrokendowntoformthe
secondaryatrialforamen(seeFig.29.12,right,andFig.29.13,left).
FIG.29.13 Finalstagesofatrialseptation.(A)thegrowthofthevestibular
spinereinforcesthesiteofclosureoftheprimaryforamen,atthesame
timecommittingtheorificeofthepulmonaryveintotheleftatrium.The
vestibularspineandthemesenchymalcapmuscularizetoformthe
anteroinferiorbuttressoftheatrialseptum.Infoldingoftheatrialroof,
concomitantwiththetransferoftheincorporationofthepulmonaryveinsto
theleftatrium,producesthesuperiorrimoftheovalfossa.(B)This
infoldingusuallyoverlapstheupperedgeoftheprimaryatrialseptum,
beingofsufficientdimensionstoclosetheovalforamenwhenleftatrial
pressureexceedsrightatrialpressuresubsequenttobirth.
Inthefinalstagesofseptation,theupperedgeoftheremainingprimaryatrial
septumisusuallyoverlappedbytheinfoldingoftheatrialroofbetweenthe
superiorcavalandpulmonaryveins.Thisfoldformsthesuperiorrimoftheoval
fossa,oftendescribedastheseptumsecundum(seeFig.29.13,right).
Failureofseptationofthecommonatrioventricularjunctionduetolackof
growthofthevestibularspinepreventstheloweredgeoftheprimaryseptum
fromfusingwiththeventricularseptum.Thisresultsintheostiumprimum
