Andersons pediatric cardiology 1307
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FIG.49.7 MagneticresonanceimaginginthepatientshowninFig.49.5
confirmstheherniationoftheleftatrialappendage(LAA),albeitthrougha
thinnedareaofpericardiumratherthanatruepericardialdefect(arrow).
Ao,Aorta;LA,leftatrium;PA,pulmonarytrunk;RA,rightatrium;RAA,right
atrialappendage.
ConjoinedTwins
Amalformationintheprocessofmonozygotictwinning,whichusuallyproduces
separatebutidenticalindividualsbycleavageofthesinglefertilizedegg,can
resultinthetwinsbecomingincompletelydivided.Thisproducestherare
examplesofconjoinedtwinsthat,forcenturies,havefascinatedbothmedical
andlaypersons.Theincidenceiscalculatedatapproximately1conjunctionin
every50,000births.18Thefancifulaccountsofmonstersandprodigiesthat
appearedincenturiespastarenotthatfarremovedfromreality.Thepossibilities
andsitesforconjunctionarelegion.Accordingly,thecategoriesprovidedfor
classificationareformidable.ThefamousSiamesetwins,EngandChang,who
survivedintooldage,werejoinedonlyattheabdomen,sharingnomorethana
commoncordofliversubstance.Twinswithcardiacinvolvementarealljoined
atthechest,althoughnotnecessarilywithcommoncardiacchambers.Indeed,
thosewithseparateheartsinacommonthoraciccavityhavethebestchanceof
survival.Evenwhentheheartsthemselvesarequiteseparate,therecanbe
extensiveinterminglingofthecirculationsbetweenthetwins.Wehavehadthe
opportunitytostudythreesetsofconjoinedtwinsatautopsy.
Inoneset,thetwinsfacedeachotherandwerejoinedatthethoraxand
abdomenwithacommonribcage,withasternumateachsidecommontoboth
twins.Therightsideofonetwinfacedtheleftsideoftheother,andeachhadits
organsarrangedintheusualfashion.Therewereseparatelungs,butacommon
liver.Eachtwinpossessedaspleeninitsownleftside.Theheartswerejoinedat
ventricularandatriallevel,eventhougheachhearthadtwoatriumsandtwo
ventricles.Thelatterwereunconnected,althoughcontainedinacommon
ventricularmass.However,theinterconnectionsofthegreatveinswerecomplex
intheextremeandwouldhaveprecludedanyattemptatsurgicalseparation.
Thesecondsetoftwinsstudiedwasjoinedattheatriallevelinafascinating
manner.Theheartswerejoinedinthefashionofleavesofabook,ratherthan
facetoface.Withinthisarrangement,thetwinssharedamidline
morphologicallyrightatrium(Fig.49.8).Consequently,whiletheonetwinhad
usualatrialarrangement,theotherexhibitedisomerismoftherightatrial
appendages.19Inkeepingwiththerightisomerism,theright-sidedtwinalsohad
lungsandbronchusesbilaterallyofrightmorphology,andtherewasnospleenin
theabdominalcavity.Severalotherexamplesofconjoinedtwinshavebeen
recordedwheretheonetwinhadtheusualarrangementofitsorgansandthe
otherhadrightisomerism.Itisalwaystheright-sidedtwinthatisafflictedbythe
isomerism.Recentinvestigationshavesuggestedtheconceptofcross-talk
betweentheconjoinedembryosasthebasisforthefailureoflateralisation.20
Theleft-sidedtwinishypothesizedtosynthesizeaninhibitorofthegenesonic
hedgehog,activinbeingproposedastheinhibitor.Itissuggestedthatdiffusion
oftheinhibitortotheright-sidedtwinpreventstheinductionofnodal,thislatter
genebeingconsiderednecessaryfortheformationofmorphologicallyleft
structures(seealsoChapter26).
FIG.49.8 Heartsfromconjoinedtwinsjoinedtogetheratatrialand
ventricularlevels.Intheright-sidedheart,theright-sidedappendage,
althoughnotwellseenintheimage,wasofrightmorphology.Thereisa
sharedmidlineappendage,alsoofrightmorphology,whiletheleft-sided
appendageisofleftmorphology.Thustheheartoftheright-sidedtwin
exhibitsrightisomerism,whereastheatrialappendagesintheheartofthe
left-sidedtwinareusuallyarranged.
Clinicaldiagnosisofconjunctionisnotnowlikelytobeaproblem.
Intrauterinecross-sectionalultrasonographywillrevealthediagnosisinthe
majorityofcases.Althoughanalysiswillbedifficult,anassessmentcanbemade
ofthedegreeofcardiacinvolvement.Ifsurgicalseparationistobeattempted,a
fullinvestigationwillbeneededafterbirth.Echocardiographyshouldalwaysbe
performed,alongwithresonanceimagingorcomputedtomography.Itis
