The Conotruncus

The conotruncus, also known as the cardiac outflow tract, is formed from two cardiac segments whose borders are imperceptible in mammals: the conus arteriosus and the truncus arteriosus.

The conus arteriosus, also known as the infundibulum (Latin for funnel), is the proximal portion of the outflow tract. It remodels and forms the subpulmonary infundibulum on the right and disappears on the left.

The truncus arteriosus is the distal portion of the developing cardiac outflow tract that divides into the aortic and pulmonic valves.

Here, we limit ourselves to the abnormalities of the conotruncus seen in common types of conotruncal heart defects.

• Tetralogy of Fallot: Most common cyanotic cardiac defect. Anterior deviation of the conal septum with infundibular hypoplasia, resulting in subpulmonary obstruction. In some cases, the conal septum is absent. On the truncus side of things, there is stenosis or atresia of the pulmonic valve. Less commonly there may be absence of the pulmonary valve leaflets. There is also aortic override.
• D-loop transposition of the great arteries: Second most common cyanotic congenital heart condition diagnosed in the 1st year of life. There is atrioventricular concordance and ventriculoarterial discordance, meaning the atria and ventricles connect properly, but the ventricles and outflow vessels connect improperly. The truncus abnormality consists the aortic valve situated to the right of and anterior to the pulmonary valve. The Rastelli procedure can be performed in the presence of a ventricular septal defect, connecting the right ventricle to the pulmonary artery.
• L-loop transposition of the great arteries: Also known as congenitally or physiologically corrected transposition. There is atrioventricular and ventriculoarterial discordance, meaning the atria and ventricles connect improperly, and the ventricles and outflow vessels connect improperly. This results in two parallel circulations with systemic venous return reaching the pulmonary circulation through the right-sided left ventricle, and the pulmonary venous return reaching the aorta through the left-sided right ventricle.
• Double-outlet right ventricle: Both the pulmonary artery and aorta arise predominantly from the right ventricle. The aorta is usually directly to the right of the pulmonary artery. The mitral and pulmonic valves are not continuous and the majority of patients have pulmonic and mitral valve anomalies.
• Truncus arteriosus: Single-vessel outflow from the heart. This vessel overrides the ventricular septum and supplies the systemic, pulmonary, and coronary circulation. The conal septum is usually absent. A ventricular septal defect is seen in almost all patients. Classified according to the branching pattern of the pulmonary artery. Associated with DiGeorge syndrome.
  
  Babies with truncus arteriosus present with a right-sided arch and commonly have cardiomegaly at birth. There are increased pulmonary vascular markings due to increased arterial blood flow.
• Interrupted aortic arch: As its name suggests, the aortic arch is interrupted between the ascending and descending portions of the thoracic aorta, either completely or incompletely (spanned by a fibrous band).

References

• Frank L, Dillman JR, Parish V, Mueller GC, Kazerooni EA, Bell A, Attili AK. Cardiovascular MR imaging of conotruncal anomalies. Radiographics. 2010 Jul-Aug;30(4):1069-94.
• Restivo A, Piacentini G, Placidi S, Saffirio C, Marino B. Cardiac outflow tract: a review of some embryogenetic aspects of the conotruncal region of the heart. Anat Rec A Discov Mol Cell Evol Biol. 2006 Sep;288(9):936-43.