Truncus arteriosus (TA) is an uncommon congenital cardiovascular anomaly that is characterized by a single arterial trunk arising from the normally formed ventricles by means of a single semilunar valve (ie, truncal valve). In addition, the pulmonary arteries originate from the common arterial trunk distal to the coronary arteries and proximal to the first brachiocephalic branch of the aortic arch. The common trunk typically straddles a defect in the outlet portion of the interventricular septum (ie, conal septum); however, in rare cases, it may originate almost completely from the right or left ventricle. In patients with a patent and normal caliber aortic arch, the ductus arteriosus is either absent or diminutive.
The anomaly is thought to result from incomplete or failed septation of the embryonic truncus arteriosus, hence the persistence of the Latin term truncus arteriosus and its variants. Aortopulmonary and interventricular defects are believed to represent an abnormality of conotruncal septation. Because the common trunk originates from both the left and right ventricles, and pulmonary arteries arise directly from the common trunk, a ductus arteriosus is not required to support the fetal circulation.
Accordingly, an inverse relationship between the caliber of the ductus arteriosus (derived from the sixth branchial arch) and that of the distal portion of the aortic arch (derived from the fourth branchial arch) is typically present. Although the hemodynamic consequences of a common arterial outflow may predispose to the development of the fourth or the sixth arch (but not both), anomalous development of the arch system is likely a fundamental aspect of the morphogenetic anomalies that produce truncus arteriosus.
Pulmonary arteries may arise from the common trunk in one of several patterns, which are often used to classify subtypes of truncus arteriosus. Several classification schemes have been proposed, none of which is ideal.
The earliest classification, developed by Collett and Edwards in 1949, includes truncus arteriosus types I-IV, as follows  :
Truncus arteriosus type I is characterized by origin of a single pulmonary trunk from the left lateral aspect of the common trunk, with branching of the left and right pulmonary arteries from the pulmonary trunk.
Truncus arteriosus type II is characterized by separate but proximate origins of the left and right pulmonary arterial branches from the posterolateral aspect of the common arterial trunk.
In truncus arteriosus type III, the branch pulmonary arteries originate independently from the common arterial trunk or aortic arch, most often from the left and right lateral aspects of the trunk. This occasionally occurs with origin of one pulmonary artery from the underside of the aortic arch, usually from a ductus arteriosus.
Type IV truncus arteriosus, originally proposed by Collett and Edwards as a form of the lesion with neither pulmonary arterial branch arising from the common trunk, is now recognized to be a form of pulmonary atresia with ventricular septal defect rather than truncus arteriosus.
Collett and Edwards describe variations of each of these types.
In 1965, Van Praaghs proposed the other commonly cited classification scheme that also includes 4 primary types, as follows  :
Type A1 is identical to the type I of Collett and Edwards.
Type A2 includes Collett and Edwards type II and most cases of type III, namely those with separate origin of the branch pulmonary arteries from the left and right lateral aspects of the common trunk.
Type A3 includes cases with origin of one branch pulmonary artery (usually the right) from the common trunk, with pulmonary blood supply to the other lung provided either by a pulmonary artery arising from the aortic arch (a subtype of Collett and Edwards type III) or by systemic to pulmonary arterial collaterals.
Type A4 is defined not by the pattern of origin of branch pulmonary arteries, but rather by the coexistence of an interrupted aortic arch. In the vast majority of cases of type A4, which fall into the type I of Collett and Edwards, the pulmonary arteries arise as a single pulmonary trunk that then branches. In any of these patterns, intrinsic stenosis, hypoplasia, or both may be present in one or both branch pulmonary arteries, which may have an effect on management and outcome.
The Van Praagh scheme is combined with Collett and Edwards types in the image below.
Associated cardiovascular anomalies
Various abnormalities may be associated with truncus arteriosus, some of which may have an impact on management and outcome.
Structural abnormalities of the truncal valve, including dysplastic and supernumerary leaflets, are frequently observed, and significant regurgitation (moderate or severe) through the truncal valve may be present in 20% or more patients.
Similarly, proximal coronary arteries are abnormal in many patients, with a single coronary artery and an intramural course as the most important variations.
The other major anomaly associated with truncus arteriosus in a substantial portion of cases is interruption of the aortic arch, which almost always occurs between the left common carotid and subclavian arteries.
Other relatively common but minor associations include right aortic arch, left superior caval vein, aberrant subclavian artery, and atrial septal defect. In addition to these defects found in the usual spectrum of truncus arteriosus, several other major but rare associated anomalies are reported, including complete atrioventricular septal defect, double aortic arch, and various forms of functionally univentricular heart.
Sepsis is probably the most important noncardiac problem in the differential diagnosis of neonates with truncus arteriosus, as well as other forms of complex congenital heart disease. Young infants with truncus arteriosus frequently present in shock because of high output heart failure with significant pulmonary overcirculation. This scenario may resemble the presentation of neonatal sepsis, especially when the ratio of pulmonary-to-systemic blood flow is sufficiently high that the patient is not cyanotic.
Pathophysiology of truncus arteriosus is typified by cyanosis and systemic ventricular volume overload. Outflow from both ventricles is directed into the common arterial trunk. Pulmonary blood flow is derived from this combined ventricular output, and its magnitude depends on the ratio of resistances to flow in the pulmonary and systemic vascular beds. Because of the mixing (although not complete) of left and right ventricular output that occurs primarily during systole and at the level of the common arterial trunk, subnormal systemic arterial oxygen saturation is common. Similarly, because the systemic and pulmonary circulations are essentially in parallel, pulmonary blood flow typically is at least 3-fold higher than systemic blood flow, with pulmonary overcirculation and increased myocardial work that results in increased resting oxygen demand and decreased metabolic reserve.
Truncus arteriosus represents 1-2% of congenital heart defects in liveborn infants. Based on an estimated incidence of congenital heart disease of 6-8 per 1,000 liveborn children, truncus arteriosus occurs in approximately 5-15 of 100,000 live births. Among aborted fetuses and stillborn infants with cardiovascular anomalies, truncus arteriosus represents almost 5% of defects.
A population-based review of all cases of live births from 1999 to 2008 identified as having severe congenital heart disease from the Nationwide Inpatient Sample (NIS) database indicated a decrease of the conditions over the study period.  There was a significant decreased incidence of truncus arteriosus as well as tetralogy of Fallot, pulmonary atresia, and hypoplastic left heart syndrome; however, these trends varied with sociodemographic factors. The investigators suggested a possible reason for the decreasing prevalence trend was the increased numbers of terminated fetuses with prenatally diagnosed congenital heart disease. 
No significant difference in the incidence of truncus arteriosus is noted among those born in the United States compared with other countries.
However, a Canadian longitudinal study (1983-2010) of all individual with congenital heart disease noted a more than 50% increase of severe and other congenital heart disease after the year 2000, with adults comprising two thirds of the cases by 2010.  The prevalence of congenital heart disease in the first year of life between 1998 and 2005 was 8.21 per 1000 live births; in 2010, the overall prevalence was 13.11 per 1000 in children and 6.12 per 1000 in adults. A temporal increase in prevalence of congenital heart disease and severe congenital heart disease was noted for children and adults. 
The natural history of truncus arteriosus without surgical intervention is not well characterized. In numerous earlier series, the median age at death without surgery ranged from 2 weeks to 3 months, with almost 100% mortality by age 1 year. Cases of patients surviving into adulthood with unrepaired truncus arteriosus are reported, but they are extremely uncommon. Cause of death in unrepaired patients is usually cardiac arrest or multiple organ failure in the face of systemic perfusion that is inadequate to meet the body's metabolic demands; progressive metabolic acidosis and myocardial dysfunction results.
Currently, for patients undergoing complete repair in the neonatal or early infant periods, early postoperative mortality is generally less than 10%. This represents a substantial improvement from earlier eras; as recently as 20 years ago, the early mortality rate after complete repair was higher than 25% in most series. Among patients surviving the initial postoperative period, the survival rate at a 10- to 20-year follow-up is higher than 80%, with most deaths resulting from sequelae of late repair (pulmonary vascular obstructive disease), reinterventions, or residual/recurrent physiologic abnormalities.
Although rarely used today, surgical palliation by banding of the pulmonary artery to protect the pulmonary vascular bed was a frequently used strategy until the 1970s and early 1980s. This practice resulted in only minor improvement in the natural history of the disease, with substantial early and intermediate mortality rates.
Based on limited data, no racial predilection is apparent.
Although many series report a slight male predominance, no significant predilection based on sex is apparent.
Truncus arteriosus is a congenital anomaly that is present from early in embryonic gestation. Currently, truncus arteriosus is diagnosed using prenatal ultrasonography in a small percentage of patients. Among patients diagnosed after birth, the median age at presentation is generally a few days, which is significantly earlier than was the case 20 or more years ago. Occasionally, patients are not diagnosed until later in infancy, childhood, or even adulthood, although such cases are exceedingly rare in the United States and Europe.
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