Embryology and Congenital Abnormalities of the Aorta 23

Chapter Embryology and Congenital Abnormalities 2 of the Aorta Jean Philippe Guibaud and Xavier Roques Contents tal arteries enlarge to formthe proximalportions of the subclavian arteries, they migrate cephalad and in the 2.1Introduction ....................... 21 embryo of 5±6-mm length, they detach separately from 2.2 Classification of Vascular Rings right and left dorsal aortas, upstreamfromfusion of andRelatedMalformations............... 21 these two vessels. In the embryo of 10-mm length, the third, fourth 2.3 Description of Main Aortic Arch Abnormalities . 25 and sixth arches are well formed. The primitive aorta is 2.3.1 Coarctation of the Aorta ............ 25 now divided in trunks of aorta and pulmonary artery, 2.3.2 Interrupted Aortic Arch (Group IV) ...... 25 2.3.3 Aberrant Right Subclavian Artery so that the third and fourth arches are detached from or Arteria Lusoria (subgroup IIB1) ...... 26 the aorta, while the sixth one follows upon the trunk of the pulmonary artery. At stage of 15-mm length, the embryo has lost its symmetrical aspect (Fig. 2.2). This transformation was 2.1 Introduction the result of interruption and displacement of segments and the descent of the heart in the thorax. The proxi- The complex evolution of the vascular system from the mal part of the third arch moved laterally, so that it human embryo to the definitive pattern of the aortic rose at the union of the fourth arch and the ventral aor- arch has been provided by Congdon [1] and by Barry ta. The third arch forms the common carotid artery. At [2]. The ventral aortic root is in front of the oesopha- this stage the dorsal aorta was interrupted between the gus. It follows upon the conotroncus, last segment of third and the fourth arch. Circulation occurs in two di- the primitive cardiac tube, and is prolonged by two ves- rections: to the head by the third arch and to the rest sels, the first aortic arches, which cross the intestine lat- of the body by the fourth arch. On the right side, the erally, to join in the dorsal part of the embryo, the two distal part of the sixth arch disappears, while the proxi- dorsal aortas. Six pairs of arches will develop, one for mal one forms the right pulmonary artery. On the left, each branchial cleft, connecting the ventral aorta with the proximal part of the sixth arch forms the left pul- the two dorsal aortas [3]. All are not present at any one monary artery, while its distal part persists until the time, the first regresses when the following appears; birth and forms the ductus arteriosus. With the regres- some disappear completely, others persist, but are nota- sion of the eighth segment of the right dorsal root and bly modified [4]. The two dorsal aortas meet and merge the right ductus arteriosus, the basic pattern of the nor- in a single vessel, on the median line at the inferior mal left aortic arch is formed (Fig. 2.3). part of the embryo. This fusion goes up until the seventh somite, at the level of the inferior part of the ve- nous sinus (Fig. 2.1). When the length of the embryo is 3 mm, the first 2.2 Classification of Vascular Rings and second pairs of primitive aortic arches are the first and Related Malformations to be formed, and the first to disappear. The third aortic arch is well developed when the After different attempts of classification (Krauss, length of the embryo is 4 mm, and the fourth and sixth Rathke, Neuhauser, etc.), Stewart et al. [5] provided a arches are outlined. The fifth pair of arches makes only pertinent system explaining malformations. a brief appearance and then disappears. At this same Most vascular rings and related malformations of stage, the dorsal aortic roots and dorsal aorta give off the aortic arch result fromeither a lack of regression or intersegmental arteries which supply blood to spinal an abnormal regression of segments. The formation of cord and developing somites. The seventh intersegmen- the normal aortic arch system is dependent primarily 22 I. State of the Art Fig. 2.1. Schematic diagram indicating the various components are indicated by broken outlines. The Arabicnumbers indicate of the embryonic aortic arch complex in the human embryo. the segments of each dorsal aorta. (From Barry [2] with per- Those components which do not precisely persist in the adult mission) Fig. 2.2. Diagrammatic view of the aortic arch complex as it appears in the human embryo of 15-mm crown±rump length. The various components are indicated by the same shading as was used in Fig. 2.1. (FromBarry [2] with permission) J.P Guibaud and X. Roques Chapter 2 Embryology and Congenital Abnormalities of the Aorta 23 Fig. 2.3. Diagrammatic ventral view of the resultant normal aortic arch complex. Scheme of identification same as in Figs. 2.1 and 2.2. (From Barry [2] with permission) on regression of the eight segment of the right dorsal aortic root. ªThe point of departure for this classification of mal- formation of the aortic arch is a hypothetic specimen in which there is no regression at any of these sites. This hypothetic formis a double aortic arch with bilateral ductus arteriosiº (Fig. 2.4). Some of the malformations were described before their discovery. The presence or the absence of one or both ductus arteriosi and the upper descending aorta is pertinent to the classification. When the separation of the proximal outflow tract displaces the aorta and the pulmonary artery towards the left, the upper descending aorta and ductus arteriosus will be at the left. When the separation displaces these same vessels towards the right, the upper descending aorta and the ductus arteriosus will be at the right. Regression or development of a segment can be explained by the intensity of blood flow in the vessels [6]. When blood flow decreases, the segment regresses or disappears. Edwards described four main groups of malforma- Fig. 2.4. Ventral view of Edwards` hypothetic double aortic arch tions, and for each of them, there are subgroups: and bilateral ductus arteriosi. The ascending and descending l Group I is the group of the complete double aortic aorta are each depicted in midline positions. Arrows point to arch; there is no interruption at any point in the the four key locations where regression occurs and are num- double aortic arch pattern. One or both arches may bered from 1 to 4. Arrow 1 indicates the eighth segment of the right dorsal aortic root, arrow 2 the right fourth arch, and ar- be patent or not (subgroup A or B), associated with rows 3 and 4 the corresponding two positions on the left [5] the presence of left, right or bilateral ductus arteriosi 24 I. State of the Art Fig. 2.5. Subgroup IIA1. The normal aortic arch [5] Fig. 2.7. The aberrant right subclavian artery arises fromthe posterior of the uppermost part of the descending aorta and ascends at an angle of about 708 fromleft to right behind the oesophagus [5] Fig. 2.6. Subgroup IIA1. The normal aortic arch system is formed when the right dorsal aortic root (region 1) and the right ductus arteriosus regress [5] Fig. 2.8. Interruption at region 2 (right fourth arch) causes the (subgroups 1, 2, 3). If one arch is not patent, the right subclavian artery to arise fromthe right dorsal aortic atretic segment may be region 1 (eighth segment of root [5] the right dorsal aortic root), region 2 (right fourth arch), region 3 (eighth segment of the left dorsal aortic root) or region 4 (left fourth arch). branching, the interruption occurs at region 1 l Group II is characterized by the presence of an in- (Figs. 2.5, 2.6). The second subgroup (B) concerns tact left aortic arch. There are three main subgroups the aberrant right subclavian artery and the inter- (A, B, C) according to the location of the interruption is at region 2 (Figs 2.7, 2.8). The third sub- ruption. The first subgroup (A) concerns normal group (C) concerns the isolation of the right subcla- J.P Guibaud and X. Roques Chapter 2 Embryology and Congenital Abnormalities of the Aorta 25 vian artery fromthe aorta; the interruption occurs at both regions 1 and 2. Each of these subgroups may be associated with the presence of a left, right or bilateral ductus arteriosi (subgroups 1, 2, 3). l Group III is characterized by the presence of a right aortic arch. The anomalies of this group are the mirror of the anomalies of group II: mirror-image branching, aberrant left subclavian artery and isolation of the left subclavian artery fromthe aorta (subgroups A, B, C). l Group IV concerns unusual malformations explained by complex combinations of interruptions at the four sites. 2.3 Description of Main Aortic Arch Abnormalities 2.3.1 Coarctation of the Aorta Coarctation of the aorta is a congenital narrowing of the upper descending thoracic aorta, adjacent to the site Fig. 2.9. Group IV. Interruption of aortic arch type B in the of attachment of the ductus arteriosus [7]. Preductal or classification of Celeria and Patton. The ascending aorta termi- nates in the common carotid arteries. The descending aorta postductal, this shelf is usually juxtaductal. Variability arises fromthe pulmonary systemby way of a large patent in coarctation morphology, associated lesions, differ- ductus arteriosus. There is always an aberrant right subclavian ences between neonatal, infant and adult coarctations, artery [5] and influence of the use of prostaglandin E1 in the pre- operative management are many reasons underlying the complexity of this abnormality.

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