Video explanation In healthy individuals, the human heart develops around the 20th day of gestation, when the outer endocardial tubes merge into a single cardiac tube. Thereafter, the cardiac tube begins to fold, developing into the atrium and ventricle. The right ventricle is dominant prior to birth, receiving 65% of the venous return to the heart, and is the main contributor of blood flow to the lower part of the body, the placenta, and the lungs. Though the exact cause of<unk>  is unknown, an association that has been observed is an anterior deviation of the infundibular septum that results in a misaligned VSD, with an overriding aorta causing a subsequent right ventricular obstruction. Different factors such as pulmonary stenosis can also contribute with the right ventricular outflow obstruction. During tet spells, a decrease in systemic vascular resistance or an increase in pulmonary resistance would be physiologically observed. The main anatomic defect in<unk>  is the anterior deviation of the pulmonary outflow septum. This defect results in narrowing of the right ventricular outflow tract (RVOT), override of the aorta, and a VSD. Four malformations<unk> " denotes four parts, here implying the syndrome's four anatomic defects. This is not to be confused with the similarly named teratology, a field of medicine concerned with abnormal development and congenital malformations (including<unk> . Below are the four heart malformations that present together in<unk> : Heart with<unk>  A: Pulmonary stenosis; B: Overriding aorta; C: Ventricular septal defect (VSD); D: Right ventricular hypertrophy Normal heart Condition Description Pulmonary Infundibular Stenosis A narrowing of the right ventricular outflow tract. It can occur at the pulmonary valve (valvular stenosis) or just below the pulmonary valve (infundibular stenosis). Infundibular pulmonic stenosis is mostly caused by the overgrowth of the heart muscle wall (hypertrophy of the septoparietal trabeculae), however, the events leading to the formation of the overriding aorta are also believed to be a cause. The pulmonic stenosis is the major cause of the malformations, with the other associated malformations acting as compensatory mechanisms to the pulmonic stenosis. The degree of stenosis varies between individuals with<unk>  and is the primary determinant of symptoms and severity. This malformation is infrequently described as sub-pulmonary stenosis or subpulmonary obstruction. Overriding aorta An aortic valve with biventricular connection, that is, it is situated above the ventricular septal defect and connected to both the right and the left ventricle. The degree to which the aorta is attached to the right ventricle is referred to as its degree of "override." The aortic root can be displaced toward the front (anteriorly) or directly above the septal defect, but it is always abnormally located to the right of the root of the pulmonary artery. The degree of override is extremely variable, with 5-95% of the valve being connected to the right ventricle. Ventricular septal defect (VSD) A hole between the two bottom chambers (ventricles) of the heart. The defect is centered around the most superior aspect of the ventricular septum (the outlet septum), and in the majority of cases is single and large. In some cases, thickening of the septum (septal hypertrophy) can narrow the margins of the defect. Right ventricular hypertrophy The right ventricle is more muscular than normal, causing a characteristic boot-shaped (coeur-en-sabot) appearance as seen by chest X-ray. Due to the misarrangement of the external ventricular septum, the right ventricular wall increases in size to deal with the increased obstruction to the right outflow tract. This feature is now generally agreed to be a secondary anomaly, as the level of hypertrophy tends to increase with age.<unk> 's<unk>  specimen, from the UCT Pathology Learning Centre There is anatomic variation between the hearts of individuals with<unk> . Primarily, the degree of right ventricular outflow tract obstruction varies between patients and generally determines clinical symptoms and disease progression. Presumably, this arises from an unequal growth of the aorticopulmonary septum (aka pulmonary outflow septum). The aorta is too large, thus "overriding," and this "steals" from the pulmonary artery, which is therefore stenosed. This then prevents ventricular wall closure, therefore VSD, and this increases the pressures on the right side, and so the R ventricle becomes bigger to handle the work. Additional anomalies In addition,<unk>  may present with other anatomical anomalies, including: * stenosis of the left pulmonary artery, in 40% * a bicuspid pulmonary valve, in 60% * right-sided aortic arch, in 25% * coronary artery anomalies, in 10% * a patent foramen ovale or atrial septal defect, in which case the syndrome is sometimes called a pentalogy of<unk>  * an atrioventricular septal defect * partially or totally anomalous pulmonary venous return<unk>  with pulmonary atresia (pseudotruncus arteriosus) is a severe variant in which there is complete obstruction (atresia) of the right ventricular outflow tract, causing an absence of the pulmonary trunk during embryonic development. In these individuals, blood shunts completely from the right ventricle to the left where it is pumped only through the aorta. The lungs are perfused via extensive collaterals from the systemic arteries, and sometimes also via the ductus arteriosus.
Tetralogy of Fallot
Cœur en sabot
Trilogy of Fallot
Double aortic arch
Transposition of the great vessels
Levo-Transposition of the great arteries
Dextro-Transposition of the great arteries
Persistent truncus arteriosus