Andersons pediatric cardiology 835
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Theholeswithinthesubstanceoftheseptum,andsomeofthosewithdivorceof
thesubpulmonaryinfundibulumfromtheapicalmuscularseptum,have
exclusivelymuscularrims.Thesechannelsarethereforedescribedsimplyas
beingmuscular(seeFig.32.3,left).Theycanbeassociated,aswillbeshown,
withmalalignmentoftheseptalcomponents.
PerimembranousDefects
Theperimembranousdefectsthatopenintothebaseoftherightventricleare
positioneddirectlybeneathitsinnercurvature.Whentheventricularseptumis
intact,thisareaisoccupiedbythemembranouspartoftheseptum,withthe
supraventricularcrestseparatingtheareafromtheleafletsofthepulmonary
valve.Themembranousseptumiscrossedbythehingeoftheseptalleafletof
thetricuspidvalve,whichdividesitintoatrioventricularandinterventricular
components.Theaxisofatrioventricularconductiontissuepenetratesthrough
thisseptumfromtheapexofthetriangleofKoch.Havingpenetrated,inthe
settingofseptalintegrity,theconductionbundlerunsonthecrestofthe
muscularseptum,sandwichedbetweenthefibrousandmuscularseptal
componentsandemergingintherightventriclebeneaththemedialpapillary
muscle,whichtakesitsoriginfromtheposterocaudallimboftheseptomarginal
trabeculation(Fig.32.5).Perimembranousdefectsarefoundwhenthe
ventricularseptumisdeficientinthisareaofnormalcontinuitybetweenits
membranousandmuscularcomponents.Suchdefects,ofnecessity,willopen
centrallyintotherightventricle.Indeed,centraldefectsfoundattheventricular
basecanonlybeperimembranous.Untilveryrecently,wehadpresumedthat
theirphenotypicfeaturewasfibrouscontinuitybetweentheleafletsoftheaortic
andtricuspidvalves(seeFig.32.3,right).Inmostinstances,thisisthecase.We
arenowaware,however,thatinasmallproportionofcasesfulfillingthecriteria
forthedefecttobeperimembranous,thefibrouscontinuityisfoundonly
betweentheleafletsofthemitralandtricuspidvalves,withmyocardiumofthe
supraventricularcrestinterposingbetweentheleafletsoftheaorticandtricuspid
valves.Thestrictdefinitionfortheperimembranousdefect,therefore,shouldbe
thepresenceoffibroustissueintheposteroinferiorborderofthedefect
producingcontinuitybetweentheleafletsofthemitralandtricuspidvalves,but
usuallyalsoproducingaortic-to-tricuspidfibrouscontinuity.Insomeinstances,
thetricuspid-to-mitralfibrouscontinuitywillbethroughthesubstanceofthe
centralfibrousbody.
FIG.32.5 Landmarksofthenormalventricularseptum.Itslargerpartis
muscular,butdirectlybeneaththeinnercurveoftherightventriclethereis
afibrouspart,knownasthemembranousseptum.Thisisdividedbythe
hingeoftheseptalleafletofthetricuspidvalveintoatrioventricularand
interventricularcomponents.Theatrioventricularconductionaxisisdirectly
relatedtothisseptum,penetratingtheatrioventricularcomponentatthe
apexofthetriangleofKoch,andbeingsandwichedbetweenthe
interventricularpartandthemuscularseptumuntilemergingontheseptal
surfaceoftheseptomarginaltrabeculationdirectlybeneaththemedial
papillarymuscle.
Perimembranousdefectsweretraditionallyconsideredtobe“membranous,”21
sincetheyrepresentfailuretoclosetheembryonicinterventricular
communicationbyformingthemembranousseptum.Thedefects,however,are
alwayslargerthanthenormaldimensionsofthemembranousseptum.The
embryonicinterventricularcommunicationalmostcertainlyfailstoclose
becausethemuscularventricularseptumisdeficientarounditsborders.Thatis
whyweprefertodescribethedefectsasbeingperimembranous.15
DoublyCommittedandJuxtaarterialDefects
Thefeatureofthethirdphenotypictypeofdefectisthattheyoccupytheregion
that,inthenormalheart,isformedbythefreestandingcomponentofthe
muscularsubpulmonaryinfundibulum.Thephenotypicfeatureofthisthirdtype,
whichisbothdoublycommittedandjuxtaarterial,isfibrouscontinuityinthe
roofbetweentheleafletsoftheaorticandpulmonaryvalves(Fig.32.6).Such
defectscannotexistinthenormalheart,inwhichthefreestandingmuscular
subpulmonaryinfundibulumhasdevelopedinnormalfashion.
FIG.32.6 Defectthatisdoublycommittedandjuxtaarterial.Itopenstothe
rightventricleintheareanormallyoccupiedbythesupraventricularcrest
(comparewithFig.32.5).Itsphenotypicfeatureisfibrouscontinuityinits
roofbetweentheleafletsoftheaorticandpulmonaryvalves.Inmost
instances,asinthisexample,itpossessesamuscularposteroinferiorrim.
SMT,Septomarginaltrabeculation.
PositionofDefects
WithinthecategorizationofICD-11,itisthepositionofthedefectsrelativeto
thelandmarksandcomponentsoftherightventricleor,inotherwordstheir
geography,thatservesastheinitialfeaturefordescription.Musculardefects,
whicharepunchedwithinthesubstanceofthemuscularseptum,canexistsoas
toopentotheinlet,oranywhereintheapicalpartsoftherightventricle,
includingthosethatopenanteriorlyrelativetothebodyoftheseptomarginal
trabeculation.Musculardefectsopeningtotherightventricularoutletcanalso
