Contents
2.1 Introduction . . . . 21 2.2 Classification of Vascular Rings
and Related Malformations . . . . 21 2.3 Description of Main Aortic Arch Abnormalities . . . 25 2.3.1 Coarctation of the Aorta . . . . 25 2.3.2 Interrupted Aortic Arch (Group IV) . . . . 25 2.3.3 Aberrant Right Subclavian Artery
or Arteria Lusoria (subgroup IIB1) . . . . 26
2.1 Introduction
The complex evolution of the vascular system from the human embryo to the definitive pattern of the aortic arch has been provided by Congdon [1] and by Barry [2]. The ventral aortic root is in front of the oesopha- gus. It follows upon the conotroncus, last segment of the primitive cardiac tube, and is prolonged by two ves- sels, the first aortic arches, which cross the intestine lat- erally, to join in the dorsal part of the embryo, the two dorsal aortas. Six pairs of arches will develop, one for each branchial cleft, connecting the ventral aorta with the two dorsal aortas [3]. All are not present at any one time, the first regresses when the following appears;
some disappear completely, others persist, but are nota- bly modified [4]. The two dorsal aortas meet and merge in a single vessel, on the median line at the inferior part of the embryo. This fusion goes up until the sev- enth 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 and second pairs of primitive aortic arches are the first to be formed, and the first to disappear.
The third aortic arch is well developed when the length of the embryo is 4 mm, and the fourth and sixth arches are outlined. The fifth pair of arches makes only a brief appearance and then disappears. At this same stage, the dorsal aortic roots and dorsal aorta give off intersegmental arteries which supply blood to spinal cord and developing somites. The seventh intersegmen-
tal arteries enlarge to formthe proximal portions of the subclavian arteries, they migrate cephalad and in the embryo of 5±6-mm length, they detach separately from right and left dorsal aortas, upstreamfromfusion of these two vessels.
In the embryo of 10-mm length, the third, fourth and sixth arches are well formed. The primitive aorta is now divided in trunks of aorta and pulmonary artery, so that the third and fourth arches are detached from 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 the result of interruption and displacement of segments and the descent of the heart in the thorax. The proxi- mal part of the third arch moved laterally, so that it rose at the union of the fourth arch and the ventral aor- ta. The third arch forms the common carotid artery. At this stage the dorsal aorta was interrupted between the third and the fourth arch. Circulation occurs in two di- rections: to the head by the third arch and to the rest of the body by the fourth arch. On the right side, the distal part of the sixth arch disappears, while the proxi- mal one forms the right pulmonary artery. On the left, the proximal part of the sixth arch forms the left pul- monary artery, while its distal part persists until the birth and forms the ductus arteriosus. With the regres- sion of the eighth segment of the right dorsal root and the right ductus arteriosus, the basic pattern of the nor- mal left aortic arch is formed (Fig. 2.3).
2.2 Classification of Vascular Rings and Related Malformations
After different attempts of classification (Krauss, Rathke, Neuhauser, etc.), Stewart et al. [5] provided a pertinent system explaining malformations.
Most vascular rings and related malformations of the aortic arch result fromeither a lack of regression or an abnormal regression of segments. The formation of the normal aortic arch system is dependent primarily
Embryology
and Congenital Abnormalities of the Aorta
Jean Philippe Guibaud and Xavier Roques
2
Chapter
Fig. 2.1.Schematic diagram indicating the various components of the embryonic aortic arch complex in the human embryo.
Those components which do not precisely persist in the adult
are indicated by broken outlines. The Arabicnumbers indicate the segments of each dorsal aorta. (From Barry [2] with per- 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)
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 bilat- eral ductus arteriosiº (Fig. 2.4). Some of the malforma- tions were described before their discovery. The pres- ence or the absence of one or both ductus arteriosi and the upper descending aorta is pertinent to the classifi- cation.
When the separation of the proximal outflow tract displaces the aorta and the pulmonary artery towards the left, the upper descending aorta and ductus arterio- sus will be at the left. When the separation displaces these same vessels towards the right, the upper des- cending aorta and the ductus arteriosus will be at the right.
Regression or development of a segment can be ex- plained 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- tions, and for each of them, there are subgroups:
l Group I is the group of the complete double aortic arch; there is no interruption at any point in the double aortic arch pattern. One or both arches may be patent or not (subgroup A or B), associated with the presence of left, right or bilateral ductus arteriosi 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)
Fig. 2.4.Ventral view of Edwards` hypothetic double aortic arch and bilateral ductus arteriosi. The ascending and descending aorta are each depicted in midline positions. Arrows point to the four key locations where regression occurs and are num- 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- rows 3 and 4 the corresponding two positions on the left [5]
(subgroups 1, 2, 3). If one arch is not patent, the atretic segment may be region 1 (eighth segment of 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).
l Group II is characterized by the presence of an in- tact left aortic arch. There are three main subgroups (A, B, C) according to the location of the inter- ruption. The first subgroup (A) concerns normal
branching, the interruption occurs at region 1 (Figs. 2.5, 2.6). The second subgroup (B) concerns the aberrant right subclavian artery and the inter- ruption is at region 2 (Figs 2.7, 2.8). The third sub- group (C) concerns the isolation of the right subcla- Fig. 2.5.Subgroup IIA1. The normal aortic arch [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. 2.7. 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. 2.8.Interruption at region 2 (right fourth arch) causes the right subclavian artery to arise fromthe right dorsal aortic root [5]
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 isola- tion 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 of attachment of the ductus arteriosus [7]. Preductal or postductal, this shelf is usually juxtaductal. Variability in coarctation morphology, associated lesions, differ- ences between neonatal, infant and adult coarctations, and influence of the use of prostaglandin E1 in the pre- operative management are many reasons underlying the complexity of this abnormality. Two embryologic factors will cause aortic obstruction at or near the isthmus.
One is the underdevelopment or hypoplasia of the aor- tic arch or the isthmus. If this is present, tubular hypo- plasia will be important. In this case the amount of flow across the distal aortic arch and the isthmus is an im- portant factor of growth of this vascular structure [8].
Coarctation is usually most common when there are proximal lesions which decrease ascending aortic flow such as aortic stenosis or atresia, mitral stenosis or in- competence. The second factor is the presence of ecto- pic ductal tissue in the aorta at the aortic insertion of the ductus. This ectopic tissue tends to develop when ductal flow increases such as froman atrial or ventricu- lar septal defect.
2.3.2 Interrupted Aortic Arch (Group IV)
This is the complete luminal and anatomic discontinu- ity between two segments of the aortic arch. Three types are described in the Celoria and Patton classifica- tion.
In type A interruption occurs at the level of the isth- mus between the left subclavian artery and the ductus arteriosus or between the fourth and the sixth left aor- tic arch after migration of the left subclavian artery.
Type B (Figs. 2.9, 2.10) is the commonest type (55±
69%). The interruption occurs between the left common artery and the left subclavian artery and concerns re- gression of the segment between the fourth and the J.P Guibaud and X. Roques Chapter 2 Embryology and Congenital Abnormalities of the Aorta 25
Fig. 2.9. Group IV. Interruption of aortic arch type B in the classification of Celeria and Patton. The ascending aorta termi- nates in the common carotid arteries. The descending aorta arises fromthe pulmonary systemby way of a large patent ductus arteriosus. There is always an aberrant right subclavian artery [5]
Fig. 2.10.Group IV. Interruption of the aortic arch type B. Re- gression at regions 2 and 4. The right ductus arteriosus disap- pears [5]
sixth left aortic arches but in this case before migration of the left subclavian artery. This type is often asso- ciated with an aberrant right subclavian artery.
Type C is extremely rare (less than 4%). The inter- ruption occurs between the innominate artery and the left common carotid.
2.3.3 Aberrant Right Subclavian Artery or Arteria Lusoria (Subgroup IIB1)
The aberrant subclavian artery arises as the fourth branch of the left aortic arch and passes behind the oe- sophagus to reach the right arm. In the abnormality the interruption occurs at region 2 or the right fourth arch and the right ductus disappears (Figs. 2.7, 2.8).
References
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3. Bellot J. Embryologie des arcs aortiques. Nouv Presse Med 1972; 35:2321.
4. Mathey J, Binet JP, Denis B. Anomalies de dveloppement des arcs aortiques. J Chir 1959; 77:505±527.
5. Stewart JR, Kincaid OW, Edwards JE. An atlas of vascular rings and related malformations of the aortic arch system.
Springfield (IL): Thomas; 1964.
6. Rudolph AM, Heymann MA, Spitznas U. Hemodynamic considerations in the development of narrowing of the aor- ta. AmJ Cardiol 1972; 30:514±525.
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New York: Grune and Stratton; 1975.
8. Shinebourne EA, Elseed AM. Relation between fetal flow patterns, coarctation of the aorta, and pulmonary blood flow. Br Heart J 1974; 36:492±498.
