Andersons pediatric cardiology 71
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FIG.2.8 Intactmorphologicallyrightatriumphotographedincomparable
orientationtotheopenedchamber,asshowninFig.2.7.Theappendage
formstheentiretyoftheanteriorwall.Notetheprominentgroovebetween
theposteriorextentoftheappendageandthesystemicvenoussinus,
markedbythedashedline.Thecavalveinsenterthesystemicvenous
sinussuperiorlyandinferiorly.
Asmallpartoftheinitialatrialcomponentofthedevelopingheartisretained
withinthedefinitiveatrium.Thispart,thebody,isnotusuallyrecognizablein
mostnormalheartssinceitislocatedbetweentheleftvenousvalveandthe
septum.Theleftvenousvalveitselfdoesnotremainasadiscretestructurein
mostpostnatalindividuals.Itistheappendagethatisthemostconstantpart.
Thereforethisfeatureshouldbeusedtopermitrecognitionofthechamberasthe
morphologicallyrightstructurewhentheheartiscongenitallymalformed.
Recognitionofstructuresaccordingtotheirmorphologyratherthantheir
location,andusingtheirmostconstantpartinfinalarbitration,iscalledthe
“morphologicalmethod.”2AsdiscussedinChapter1,thisprincipleisthebasis
oflogicalanalysisofcongenitallymalformedhearts.3Thecharacteristicexternal
featureoftherightappendageisitsbroadtriangularshape(seeFig.2.8),along
withitsextensivejunctionwiththesmooth-walledvenouscomponent,thisbeing
markedbytheterminalgroove.Internally,thegroovematcheswiththestrap-like
terminalcrest(seeFig.2.7).
Takingorigininparallelfashionfromthecrest,andextendinglaterallyinto
theappendage,arethepectinatemuscles.Inthemorphologicallyrightatrium,
thesemusclesextendallaroundtheatrioventricularjunction,reachingintothe
recesslocatedinferiortotheorificeofthecoronarysinus.Thisrecess,also
describedasasinus,issub-thebesianwhentheheartisseenintheattitudinally
appropriateposition(seeFig.2.7).Theextentofthepectinatemusclesrelativeto
thevestibuleoftherightatrioventricularvalveisthesinglemostcharacteristic
featureoftherightatriumincongenitallymalformedhearts.4Inmanyhearts,the
flap-likemuscularorfibrouseustachianandthebesianvalves,remnantsofthe
embryonicrightvenousvalve,takeoriginfromtheterminalcrestandguardthe
orificesoftheinferiorcavalveinandthecoronarysinus,respectively(seeFig.
2.7).Theirsizevariesmarkedly,andtheyarenotuniformlypresent.An
importantstructurecanusuallybefoundincontinuationwiththeeustachian
valve.ThisisthetendonofTodaro,5whichrunsthroughthewallthatseparates
thecoronarysinusfromtheovalfossa,theso-calledsinusseptum,toinsertinto
thefibrousrootoftheaorta.ItformsoneofthebordersofthetriangleofKoch
(seebelow).Insomeheartsitispossibletorecognizeremnantsoftheleft
venousvalve,whichwhenpresentisusuallyadherenttotherimsoftheatrial
septum.
Atfirstsight,therightatriumseemstohaveanextensiveseptum,surrounding
theovalfossaandincorporatingtheorificesofthesuperiorcavalveinand
coronarysinus.Thisappearanceisdeceptive.Onlytheflooroftheovalfossa,
derivedfromtheprimaryatrialseptum,alongwithitsanteroinferiorrim,
separatethecavitiesofthetwoatriums.Theremainingrimsoftheovalfossa,
oftendescribedastheseptumsecundum,orthesecondaryseptum,arethe
infoldedwallsoftheatrialchambers.6Theinfoldingisparticularlyprominent
superiorly,whereitformsWaterston's,orSondergaard's,groove(Fig.2.9).
FIG.2.9 Adultheartsectionedthroughthelongaxisoftheovalfossaand
photographedfromthefront.Thesectionshowswellthatthesuperiorrim
ofthefossa,oftendescribedastheseptumsecundum,isnomorethanthe
infoldedwallsbetweentherightsuperiorpulmonaryveinsandthesuperior
cavalvein.Theflooroftheovalfossaisderivedfromtheprimaryatrial
septum,whileitsanteroinferiorbuttressisasecondtrueseptalstructure,
formedbymuscularizationduringdevelopmentofthemesenchymalcap
carriedontheprimaryseptumandthevestibularspine(seeChapter3).
Theanteroinferiormarginofthefossarepresentingthepartformedby
muscularizationofthevestibularspine,alsoknownasthedorsalmesenchymal
protrusion,isanadditionaltrueseptalstructure(seeChapter3).Itcontinues
anteriorlyastheatrialcomponentoftheatrioventricularmuscularsandwich.
Thisexistsbecauseofthemoreapicalattachmentoftheleafletofthetricuspid
relativetothemitralvalve(seebelow).Themeatinthesandwichisanextension
oftheinferioratrioventriculargroove,whichseparatestheoverlappingsegments
ofatrialandventricularmuscle,andisparticularlywelldemonstratedbyvirtual
dissection(Fig.2.10).
