Andersons pediatric cardiology 793
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PrevalenceandEtiology
Earlyreportsoftheprevalenceofatrioventricularseptaldefectswithacommon
atrioventricularjunctionhavevariedmarkedly.6,7Thesedifferencesalmost
certainlyreflecttheproblemsinherentinmostepidemiologicstudiesduetothe
biasinselection.Themostaccuratedataarethoseprovidedbythestudyofa
stablepopulationbackedupwithverificationatclinicalstudyorautopsy.Using
suchanapproach,Samaneketal.8calculatedaprevalenceof0.19per1000live
births,whichaccountedfor2.9%ofthepatientsexaminedwithcongenitally
malformedheartsinBohemiafrom1952to1979.Ofthesecases,three-fifthshad
shuntingconfinedattheatriallevel,theso-calledostiumprimumdefects,
whereastheremainderhadassociatedventricularshunting.Regionaldifferences
werealsofound,withmostoftheafflictedchildrenbeingborninindustrialareas
andgirlspredominatingintheoverallnumbers.Thelesionwasevenmore
frequentinstillborns,accountingfor6.2%ofallcongenitallymalformedhearts.8
Thereisastrongassociationbetweendeficientatrioventricularseptationwith
Downsyndrome(seeChapter4).InToronto,aboutone-thirdofpatientswith
Downsyndromehadanatrioventricularseptaldefectwithacommonvalvar
orifice,whereasonlyone-twentiethhadtheso-calledostiumprimumvariant.9In
theBohemianpopulation,halfofthosewithdeficientatrioventricularseptation
hadDownsyndrome.8Thiscloseassociationwithtrisomy21iscitedas
evidenceagainsttheusualmultifactorialmodelputforwardtoexplainthe
inheritanceofcongenitalcardiacdisease.10Evidencehasbeenfoundof
autosomaldominantinheritance,notlinkedtochromosome21,inlargefamilies
involvingmanypatientswithatrioventricularseptaldefect,11andthereis
appreciableevidenceoffamilialrecurrence.12,13Itispossible,therefore,to
discernatleastthreedifferentgeneticpatterns:onefoundinassociationwith
Downsyndrome,asecondemergingasanautosomaldominanttrait,andthe
thirdbeingisolated.Thereisahighrateofrecurrence,particularlyinfemales.
Majoranatomicdifferencesarepresentwithintheoverallpopulationofpatients
havingatrioventricularseptaldefectswithorwithoutDownsyndrome,suchas
theatrioventricularvalvebeingcommonordividedintoseparateleftandright
valvarorifices.Itisintuitivetosuggestthatthesedifferencesmustbea
reflectionofthedifferentgeneticmechanismsinvolved.
Anatomy
UnderstandingAtrioventricularSeptal
DefectsWithaCommonAtrioventricular
Junction
Asshowninthefollowingtext,theabnormalstructureanddevelopmentof
atrioventricularjunctionsarethephenotypicfeaturesofthegroupoflesions
formingthefocusofthischapter.Byatrioventricularjunctions,wemeanthe
areasoftheheartwheretheatrialmyocardiumbecomescontiguouswiththe
ventricularmyocardium.Tounderstandtheabnormalities,itisnecessaryto
emphasizethefeaturesofnormality.Inthenormalheart,themyocardial
segmentswithinthesejunctionalareasareseparatedfromoneanothersaveatthe
siteofpenetrationofthebundleofHis,whichispartofthemuscularaxis
responsibleforatrioventricularconduction.Theseparationwithinthejunctions,
providingthenecessaryelectricalinsulation,isproducedlargelybythefibrofatty
tissuesoftheatrioventriculargrooves.Thesetissuesformthegreaterpartofthe
so-calledvalvarannuli,whichalsosupporttheattachmentsoftheleafletsofthe
atrioventricularvalves.Inthenormalheart,therearetwoatrioventricular
junctionsthatsurroundthetricuspidandmitralvalvarorifices.14Thereisa
centralcomponentpresentseparatingthejunctions,whichalsoabutsonthe
subaorticoutflowtract(Fig.31.1,left).
FIG.31.1 Cutsreplicatingthefour-chamberechocardiographicplanes
takeninanormalheart(left)andaheartwithatrioventricular(AV)septal
defectandcommonatrioventricularjunction(right).Theessenceofthe
normalheartisthepresenceofseparaterightandleftatrioventricular
junctions,withtheatrioventricularcomponentofthemembranousseptum
separatingtherightatriumfromtheposteriorextentoftheleftventricular
outflowtract.Theabnormalhearthasacommonatrioventricularjunction,
withanatrioventricularseptaldefectbetweentheleadingedgeoftheatrial
septumandthecrestofthemuscularventricularseptum.Relativetothe
planeoftheatrioventricularjunction,theseptaldefecthasatrialand
ventricularcomponents.
Inthenormalheart,partofthisseparatingcomponentismadeupofatrue
atrioventricularseptum,thisbeingtheatrioventricularcomponentofthe
membranousseptum,whichisrelativelysmall(redarrowinFig.31.1,left).Itis
thelackofthisseptalcomponent,alongwithadditionalmyocardialseparating
structures,thatunderscoresthemorphologyofatrioventricularseptaldefects
foundinthesettingofacommonatrioventricularjunction(Fig.31.1,right).In
heartswithseparatejunctions,theseptalleafletofthetricuspidvalveisusually
attachedataconsiderablymoreapicallevelthanisthecorrespondingleafletof
themitralvalve(seeFig.31.1,left).Theposteroinferiorpartoftheareabetween
thehingesoftheatrioventricularvalvarleaflets,however,isnotstrictlyseptal.
Thisisbecause,inthisarea,theatrialmyocardialwalloverlapsthecrestofthe
muscularventricularmass,withanextensionoftheinsulatinginferoposterior
fibrofattyatrioventriculargrooveseparatingthetwomuscularmasses(Fig.31.2,
left).Ineffect,itisasandwichofmuscularandfibrofattytissuesthatis
interposedbetweenthecavitiesoftherightatriumandtheleftventricle.The
fibrousatrioventricularseptumisfoundanterosuperiortothismusculararea(see
