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Incidence, Prevalence, and Etiology Of infants born with congenital heart disease, approximately 3.5% will have TOF, which is 0.28 per 1000, or 1 in 3600, live births Males and females are equally affected As with so many congenital cardiac anomalies, precise etiology is unknown The majority of cases are sporadic Rubella in the first trimester of pregnancy has been implicated in a small number of cases, whereas viruses have been isolated from cases with severe pulmonary arterial hypoplasia Keeshond dogs were bred to produce a spectrum of inherited malformations of the ventricular outflow tracts, which included anomalies similar to TOF.5 The findings indicated a polygenic model of inheritance in which the genes act additively to produce the spectrum of maldevelopment The lesion is also known to be linked with abnormal migration of cells from the neural crest.6 A significant proportion of cases have microdeletions of the q11 region of chromosome 22, known to produce Di George syndrome, and the velocardiofacial syndrome, also known as the conotruncal anomaly face syndrome Almost all babies born with TOF, no matter what its variant, survive surgical intervention and reach adult life Consequently, questions of inheritability in offspring are important It has been known for some time that the incidence of congenital cardiac disease is higher among children born to women with congenitally malformed hearts For parents with tetralogy (mother or father), the risk of heart disease in their children is approximately 5% If, however, the affected parent has a sibling with the same or a similar cardiac anomaly or already has a child with heart disease, the risk is significantly increased Careful genetic counseling and testing, if not already performed, should be offered prior to pregnancy Anatomy and Morphogenesis As already discussed, the phenotypic feature of the lesion is anterocephalad deviation of the insertion of the muscular outlet septum relative to the limbs of the septomarginal trabeculation coupled with abnormal septoparietal trabeculations, which are usually, but not always, hypertrophied (see Fig 35.1, right).4 The narrowing between these components produces stenosis of the subpulmonary infundibulum In the normal heart, however, it is not possible to recognize a muscular outlet septum, although such an entity is found during cardiac development With normal development, as the aorta is transferred to the left ventricle, the structures that produced the embryonic septum are buried between the limbs of the prominent septomarginal trabeculation, with the latter feature formed by compaction of the initial right ventricular trabeculations.2 Thus in the normal heart it is possible to recognize only a muscular supraventricular crest (Fig 35.2, left) formed by the ventriculoinfundibular fold, which supports the freestanding infundibular muscular sleeve distally The posterior wall of the sleeve separates the infundibular cavity from the aortic root (see Fig 35.2, right) FIG 35.2 Left, Subpulmonary outflow tract of the normal heart opened to show the supraventricular crest, the muscular fold separating the attachments of the tricuspid and pulmonary valves It is inserted between the limbs of the septomarginal trabeculation (yellow bars) Note the location
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