Developmental disorders

III./1.4.: Transposition of principal vessels

This malformation involves faulty development of the spiraling aorticopulmonary (conotruncal) septum, which grows straight downward, instead of winding as normally. As a result of this disorder of cardiac development, the aorta emerges from the right ventricle, rather than the left ventricle, whereas the origin of the pulmonary trunk is shifted to the left ventricle.

Three forms of transposition can be distinguished:

• 1. Transpositio vera (total transposition), where the pulmonary trunk originates from the left ventricle and the aorta originates from the right ventricle, i.e. the position of principal vessels is switched around. This disorder occurs in 1 of 2000 cases. Here deoxygenated blood returning from the systemic circulation bypasses the lungs. This condition is compatible with viability only if it is accompanied by VSD (image 2) or ductus Botalli persistens (patent ductus arteriosus, image 1). In most cases, ASD can also be observed..

Image 1: Ductus Botalli apertum. A probe can be found in the opened Botallo’s duct. This duct plays an important role in the fetal circulation, but after birth the persistence of permeable Botallo’s duct (ductus Botalli persistens) is harmful, because it leads to mixture of oxygenated and deoxygenated blood, resulting in decreased oxygen levels in the periphery. (from the archive of Semmelweis University,2nd Department of Pathology –collected by Attila Kovács and István Kenessey).

Image 2: Atrial septal defect (ASD) and ventricular septal defect (VSD). Forms of septal and ventricular septal defects may vary from harmless to life threatening cases, occuring independently, or as part of a syndrome. (from the archive of Semmelweis University,2nd Department of Pathology –collected by Attila Kovács and István Kenessey).

• 2. Transpositio corrigata: the anatomical positions of the aorta and pulmonary trunk are reversed but they still originate from their normal sites (the aorta emerging from the left ventricle ventrally, and the pulmonary trunk emerging from the right ventricle, dorsally). This malformation does not affect viability of the fetus.

• 3. This category comprises those transposition cases, in which both main vessels arise from one or the other ventricle, occasionally from a supernumerary ventricle (accessory septating cavity.