Methodology of echocardiographic analysis of morphological variations of the aortic arch and its branches in children – own experience
Article Category: review
Published Online: Apr 30, 2019
Page range: 24 - 42
Received: Nov 04, 2018
Accepted: Jan 22, 2019
DOI: https://doi.org/10.15557/jou.2019.0004
Keywords
© 2019 Wojciech Mądry et al., published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
Anomalies of the aortic arch are common and well-defined developmental abnormalities varying in anatomy and clinical importance(1–10). They may form vascular rings encircling the esophagus and the trachea, which may cause dysphagia and respiratory difficulties, or remain asymptomatic and thus undiagnosed or diagnosed accidentally. An unexpected course of mediastinal arteries may cause difficulties during cardiac, thoracic or laryngological surgeries, as well as during catheterization of large vessels and various types of interventions. Therefore, the anatomy of the aortic arch and its branches as well as pulmonary arteries should be precisely defined before any planned chest and neck procedures, even in the absence of symptoms suggestive of a conflict between the respiratory tract, esophagus and arteries.
Mediastinal vessels are obscured by bony structures of the chest and the aerated lung tissue/respiratory tract throughout most of their course, which makes ultrasound imaging difficult. Computed tomography (CT) and nuclear magnetic resonance (NMR) allow for much more precise imaging. These techniques offer the possibility of obtaining 3-dimensional reconstructions, which accurately reflect anatomical details of vessels and their spatial relationships with other thoracic organs. It is worth emphasizing that CT/NMR is not necessary in all patients as preliminary vascular assessment may be performed with a completely non-invasive ultrasonography. Despite significant limitations, careful and systematic ultrasonography of mediastinal arteries allows for obtaining a clear anatomical picture of most of these vessels(2–9). The paper presents echocardiographic methodology used in the Echocardiographic Laboratory in the Department of Cardiology in the Medical University of Warsaw to assess the aortic arch and its potential abnormalities. The methodology is supported with exemplary images of different types of anomalies.
Anatomical variations of the aortic arch (AA) are due to the variable course of the aortic arch, the arrangement and the course of its branches and, finally, the course of the ductus arteriosus (DA) or the ligamentum arteriosum(1,2).
The most common types of aortic arch are as follows(1–3):
Typical branches: the right brachiocephalic trunk (RBCT), the left common carotid artery (LCCA), the left subclavian artery (LSA) (other variants are also possible)
An aberrant right subclavian artery (ARSA)
Ductus arteriosus (DA) from the descending aorta (AoDesc) to the left pulmonary artery (LPA)
three separate equal aortic arch branches
two branches: a trunk formed by the common carotid arteries and a separate left subclavian artery
Ductus arteriosus (DA) from the anomalous right subclavian artery (ARSA) to the right pulmonary artery (RPA)
Left aortic arch, right descending aorta
Typical branches (mirror image of normal left aortic arch): the left brachiocephalic trunk (LBCT), the right common carotid artery (RCCA), the right subclavian artery (RSA)
Ductus arteriosus (DA) from LBCT to LPA
Ductus arteriosus (DA) from the descending aorta to LPA
An aberrant left subclavian artery (ALSA)
DA from ALSA to LPA (Kommerell’s diverticulum)
DA from AoDesc to LPA
DA from AoDesc to RPA
Right aortic arch, left descending aorta
Normal transverse aortic arch
Hypoplastic transverse aortic arch
RAA and complex vascular anomalies
Two patent aortic arches
Both arches with a similar lumen diameter (symmetric flow)
One of the arches significantly narrowed (hypoplastic)
One of the arches obstructed (or atretic)
Cervical aortic arch (CAA)
Double lumen aortic arch (DLAA)
A thorough discussion on the imaging of proper ultrasonographic aortic arch anatomy seems to be crucial for understanding the differences found in other types of aortic arches.
In this case, the ascending aorta runs upward from the aortic valve, which is located centrally in the heart, along the left sternal edge; then it curves posteriorly and left-wards, thereby forming the transverse part of the arch, after which it passes downward towards the abdominal cavity, along the left edge of the thoracic spine. The ascending part anteriorly crosses the right pulmonary artery. The superior vena cava runs parallel to this vessel, from the posterior and on the right side. The transverse part of the aortic arch usually gives rise to the right brachiocephalic trunk (RBCT), followed by the left common carotid artery (LCCA) and the left subclavian artery (LSA). The transverse part of aortic arch is usually crossed from above by the left brachiocephalic vein. The initial segment of the descending aorta, known as the isthmus, is a short segment between LSA and the aortic orifice of the ductus arteriosus (DA), which is already closed at the time of examination in a vast majority of patients and remains in the form of ligamentum arteriosum for the rest of life, which is of great importance in the case of a different course of the aortic arch than the one described above. Just below the origin of the DA, the descending aorta is anteriorly crossed by the left pulmonary artery (LPA) and the left main bronchus, which generates characteristic acoustic shadowing due to its location between the transducer and the aorta. In its further downward course, the aorta runs behind the wall of the left atrium and posteriorly crosses the left pulmonary veins.
Echocardiography is a two-dimensional imaging by its nature, which is the reason of the first difficulty in aortic arch imaging: the arch and its branches along with other mediastinal vessels are three-dimensional structures that are not arranged in one plain in the chest; therefore, it is not possible to capture them in a single image. A full assessment of aortic and pulmonary artery anatomy requires gradual, systematic following of the course of these vessels(2,4,5,9,11). The examination should begin with a suprasternal frontal view (Figs. 1 A–D), by positioning the ultrasound transducer in such a way so as to visualize the ascending aorta, optimally with the valve; then the ultrasonic beam should be moved upward, tilting the transducer down without changing its position. When moving the ultrasound beam, the course of aorta, which initially runs upwards with a slight rightward curve, then is directed leftwards and posteriorly, forming a pronounced convexity, gives off the left subclavian artery and runs downwards along the left edge of the thoracic spine, should be followed. Normally, aortic arch branches originate in its upper surface, from the major curvature. The first branch is the right brachiocephalic trunk, which is clearly wider than the other two, i.e. the left common carotid artery and the left subclavian artery. Further upward movement of the ultrasound beam makes it possible to follow the course of aortic arch branches, RBCT in particular, up to the division into the right subclavian artery and the right common carotid artery. In the initial phase of this scan (when analyzing the course of the ascending aorta), the relationships between the aorta and the right pulmonary artery and the pulmonary trunk, as well as the superior vena cava and right pulmonary veins should be evaluated. When analyzing the transverse part, the arrangement of branches and the relationships with neighboring veins (the brachiocephalic vein in particular) should be assessed, while the aortic isthmus, spatial relations with the ductus arteriosus, the left pulmonary artery and left pulmonary veins should be assessed as part of analysis of the descending portion.
Fig. 1.
Ultrasound sections showing the thoracic aorta with left aortic arch in frontal and transverse planes. The images were obtained by gradually tilting the ultrasound beam (scan) from the frontal plane posteriorly and upwards, until reaching a horizontal plane:
The precise analysis of left aortic arch anatomy, the measurements of the diameters of its individual segments in particular, should be performed in a suprasternal view showing the entire aortic arch (Figs. 2 A, B). However, it should be emphasized that this view rarely allows to clearly differentiate between left and right aortic arch or even double aortic arch variants.
Fig. 2.
A typical left aortic arch en face.
Following the course of aorta in sagittal-like planes (Figs. 3 A–D) may provide additional data. The scanning begins in high left parasternal view with the transducer tilted in such a way so as to visualize the ascending aorta along with the valve and the bulb; then the scan is gradually moved leftwards, which allows to investigate the course of aorta and its spatial relations with pulmonary arteries, trachea, pulmonary veins and the main bronchi.
Fig. 3.
Thoracic aorta in a patient with left aortic arch in sagittal-like parasternal views. The transducer located in the left parasternal line, slightly lower than in Fig. 1A and 1B (right-to-left scan).
The characteristic course of the individual aortic segments, which may be followed by performing maneuvers described in Fig. 2 A, B and Fig. 3 A–D, is decisive for the assessment of the location and course of the aortic arch. In the case of a left aortic arch, the aorta gradually travels on the screen from the right anterior aspect to the left posterior aspect, posteriorly crossing the left pulmonary artery. The characteristic convexity of the aortic arch is directed upward and to the left. The first branch – usually a wide brachiocephalic trunk, which divides after a short course, runs to the right. In the case of a right aortic arch, the convexity of the arch is directed to the right, with the descending aorta remaining on the right side of the chest and posteriorly crossing the right pulmonary artery, while the first branch always runs to the left.
An aberrant right subclavian artery (ARSA) is an anomaly in which the right subclavian artery arises directly from the aortic arch, following its branching into the left subclavian artery(1,2). It has a retroesophageal course and may potentially cause dysphagia and respiratory disorders(11–15); however, formation of a complete vascular ring is very rare (only when the DA connects ARSA with the right pulmonary artery)(2,11); therefore, clinical manifestations, usually in childhood, are very rare. Symptoms and complications are significantly more common in adults, mainly in elderly patients, which is due to atherosclerotic complications (aneurysm and/or ARSA dissection)(11–15). Patients with ARSA present with the course of the aortic arch which does not seem to differ from the one under normal conditions. Since the aberrant artery arises in the posteromedial aortic surface, its imaging in a typical suprasternal view is very difficult. Our experience indicates that the visualization of ARSA is easier in the high left parasternal view (Fig. 4 A, B), crossing the chest in a transverse-like plane (2D and color imaging). Since this is not a standard view, it is very easy to overlook ARSA, even when aortic arch anatomy is carefully assessed, particularly when examining a restless infant. A different branching pattern is a suggestive symptom(2,6): absence of a typical wide RBCT, instead there are three successive vessels with nearly identical diameters (Fig. 4 C), of which the first vessel is not divided; or, alternatively, a short trunk divided into two common carotid arteries is the first branch, while the left subclavian artery is the second branch.
Fig. 4.
Imaging of the aortic arch in the case of the left aortic arch with an aberrant right subclavian artery.
The lack of a clear disproportion between the first branch and subsequent branches of the aortic arch is relatively easy to detect and requires further, more detailed analysis. First of all, the course of the first arterial branch, which runs high, up to the neck without giving rise to the subclavian artery, but which only divides to the internal and external carotid artery. If an early division is visualized, it is almost certainly possible to exclude an aberrant artery; however it may happen that the common carotid artery untypically gives rise to the vertebral artery(2,13) (Fig. 4 D–G), which may mask ARSA. Detection of abnormalities is difficult due to the second variant of branching pattern, i.e. with the trunk composed of common carotid arteries. Since this is a relatively wide vessel, the image may appear normal, especially in a restless patient (Fig. 4 H).
If direct visualization of ARSA origin from the aorta is impossible, following the course of the right subclavian artery (RSA) from the periphery to the aorta may be of help. Under normal conditions, the RSA passes horizontally and arises relatively high from the RBCT. The ARSA runs from the level of the clavicle to the left and downward; therefore it is usually possible to visualize two parallel vessels – the upper located common carotid artery and the lower subclavian artery, which eventually disappears behind the lung apex and trachea (Fig. 4 J). Since an isolated ARSA is rarely the cause of significant clinical manifestations, the lack of diagnosis does not expose patients to complications; however, in the case of coexistence with some other heart defects, such as aortic coarctation (Fig. 4 I–K) or hypoplastic left heart syndrome (HLHS), it may significantly complicate the course of surgery, as in the case of defects requiring other thoracic surgeries. Therefore, a precise assessment of the aortic arch and its branches is necessary, especially when planning this type of surgery.
There was no case of a complete vascular ring in the course of ARSA (LAA, ARSA, DA from ARSA to RPA) in our material. If such a pathology occurs, increased flow in the para-aortic RSA segment causing its dilation is observed in fetal life, and compression on the esophagus and trachea, which may cause clinical manifestations, may occur after birth, especially after DA closure(2,11).
This is an extremely rare anomaly(2,16,17), in which the initial segment of the aortic arch shows a typical course (with a leftward convexity of the arch, RBCT or RCCA arising first); however, after giving rise to the left subclavian artery, the arch turns right and passes downwards after another crossing of the esophagus and the trachea. In addition to this crossing, the aorta may also give rise to the right subclavian artery (Fig. 5 A–H); isolated cases of a completely normal arrangement of aortic branches have also been described(2), however such branching patterns were not observed in our study.
Fig. 5.
Imaging of the left aortic arch with the right descending artery. The images were obtained by gradually tilting the ultrasound beam posteriorly and upwards from the frontal plane until reaching a horizontal plane.
The main characteristic is that the ascending and descending aorta are on the same side of the chest: the aortic arch encircles the right pulmonary artery and the right bronchus, while the descending aorta initially runs at the right spinal edge and is directed to the left side of the chest only after reaching a lower level(1). The first branch of the RAA is an artery running towards the left part of the chest; the left brachiocephalic trunk or the left common carotid artery. The occurrence of symptoms of tracheal and esophageal compression depends on the branching pattern and the course of the ductus arteriosus. The echocardiographic analysis of the direction of RAA should be analogous to the right aortic arch. Gradual movement of the ultrasound beam in the suprasternal view or high parasternal view showing the ascending aorta in the frontal plane visualizes the rightward aortic course, with the convexity of the arch directed upwards and to the right. The image of the first branch, which runs much more horizontally than in the case of the right trunk in the left aortic arch, is very characteristic. If the brachiocephalic artery (TBC) is the first branch, it is wide and its division into common carotid artery and subclavian artery may be easily visualized. In the case of an aberrant LSA, the diameter of the first aortic arch branch is very small; after a short horizontal course it is arched upwards without forming any branches. The aortic arch first gives rise to RCCA followed by RSA. The RAA is crossed from above by the left brachiocephalic vein (in extremely rare cases it may run under the arch). The ascending aorta anteriorly crosses the right pulmonary artery (RPA); the superior vena cava runs typically, parallel to this vessel. The descending aorta also crosses thorough the RPA (posteriorly) and the right pulmonary veins (also posteriorly) (Fig. 6 A–F).
Fig. 6.
Imaging of the right aortic arch.
In this case, no vascular ring is formed – the ductus runs forward and to the left from the trachea(2,18–24). In our material, this type was observed in a vast majority of patients with tetralogy of Fallot or pulmonary atresia with ventricular septal defect (VSD), which corresponds to literature data(18–20). The ECHO shows a first wide horizontal branch of the aortic arch (the left brachiocephalic trunk, LBCT), which, after a short course, divides into LCCA and LSA. In the case of patent DA, an increased continuous flow may be seen in the LBCT; it is not uncommon to visualize the downward flow (from the pulmonary trunk) in the ductus arteriosus (Fig. 7 A–G). In such cases, the visualization of RAA and its branching pattern are important when planning Blalock-Taussig shunt (location of choice – opposite to the descending aorta), while the assessment of DA patency and course is of importance for both neonatal treatment in patients with duct-dependent pulmonary circulation, as well as for the course of total correction.
Fig. 7.
RAA with the brachiocephalic trunk forming the ductus arteriosus to LPA (most often the tetralogy of Fallot and pulmonary valve atresia with VSD).
The paraaortic portion of the duct forms a wide Kommerell’s diverticulum, which, together with a tight ligamentum arteriosum, completes the vascular ring and usually causes significant compression of the trachea(10,19,21–24) (Fig. 8 A–C).
Fig. 8.
RAA with brachiocephalic trunk, the ductus arteriosus arising from the descending aorta and passing to LPA.
In this form, no vascular ring is formed as the DA remains on the same side of the spine and is practically impossible to diagnose after DA closure. The isolated form of the anomaly is asymptomatic (Fig. 9 A–C). This type may also coexist with aortic coarctation and tetralogy of Fallot with pulmonary valve atresia(2).
Fig. 9.
RAA – the ductus arteriosus from the descending aorta to the right pulmonary artery.
This type of RRA also has several variants, with the course of the ductus arteriosus being the primary differentiating factor, which also determines the clinical presentation.
The ALSA segment between the aorta and the origin of the ductus is wide as a result of increased fetal flow, and is referred to as the Kommerell’s diverticulum(1,2,10,11,14,16,18,19,24–27). Along with the ductus (ligamentum) arteriosus, which connects to the pulmonary trunk near the origin of the left pulmonary artery, it completes the vascular ring, causing compressive symptoms. During the period of DA patency, the initial retrotracheal segment of ALSA is easier to visualize as it carries a large blood volume. Once the DA is closed, LSA perfusion drops; therefore it is difficult to visualize, especially in the proximal segment, which is obscured by the trachea. Since ALSA is very common in the right aortic arch, this anomaly should be always sought in RAA. The presence of a first narrow branch which runs in an arch directed upwards and forms no branches makes the diagnosis easier (Fig. 10 A–D).
Fig. 10.
RAA ALSA Kommerell’s diverticulum.
In this case, the LSA does not carry an increased blood volume and is not dilated during fetal life (Fig. 11 A–C). Other elements do not differ from the above described variant. Furthermore, the ductus (ligamentum) arteriosus completes the vascular ring and may cause compressive symptoms.
Fig. 11.
RAA with an aberrant subclavian artery, the ductus arteriosus arising directly from the descending aorta and connecting to the LPA.
In this case, the transverse part of the aortic arch first forms the left common carotid artery (the left brachiocephalic trunk in exceptional cases) and both right-sided branches, then it runs retroesophageally and retrotracheally and passes horizontally towards the left side of the chest, where it forms the left subclavian artery from a usually wide Kommerell’s diverticulum, after which it is directed vertically downwards, along the left spinal border(2,17,28). The Kommerell’s diverticulum also gives rise to the ligamentum arteriosus closing the vascular ring, which is usually loose and causes no evident compression (Fig. 12 A–C). It is worth noting that although the left subclavian artery meets the diagnostic criteria for an aberrant artery (it is the last branch of the right aortic arch), it has no retroesphageal course; therefore, it should not be referred to as retroesophageal (the term is often used for aberrant subclavian arteries). In this case, the transverse part of the right aortic arch has a retroesophageal course; cases of hypoplasia and even atresia of this segment have been described in literature(28).
Fig. 12.
RAA – the left descending aorta. A series of images from scanning in the suprasternal, frontal view.
The right aortic arch may occur in many configurations of vascular anomalies, e.g. in a child with an aortopulmonary window, pulmonary sling, persistent left superior vena cava and ventricular septal defect (Fig. 13 A–D).
Fig. 13.
RAA, complex forms.
Double aortic arch is a congenital vascular defect characterized by the persistence of both aortic arches (left and right). Both aortic arches connect to the ascending aorta from the anterior aspect, and with the descending aorta from the posterior aspect, thereby completely encircling the esophagus and trachea(1–10). Among all types of vascular rings, compression symptoms are most common in the double aortic arch(1–3,6–10,25–27). DAA variants with full patency of both arches, or where one arch (usually the left one) is narrower or completely atretic with a fibrous tissue band connecting the neighboring arch segments and completing the ring have been encountered(17–19).
An optimal visualization of the double aortic arch is achieved in suprasternal views and high parasternal views, which enable a simultaneous visualization of flow in both arches, especially when their diameters are similar (Fig. 14 A–F).
Fig. 14.
A double aortic arch with preserved flow in both branches:
Flow visualization with color Doppler may be very difficult if not impossible in the case of significant hypoplasia (Fig. 15 A–D). In such cases, CT angiography or MRI may be of pivotal importance. It may be even much more difficult differentiating between double aortic arch with atretic left or right arch and brachiocephalic trunk as none of the currently available imaging modalities enables visualization of narrow tissue bands with no blood flow. A much more arched, posteriorly directed course of the patent part of the double aortic arch may be helpful in the differentiation(7,8,28–30). In most cases, only direct visualization by a cardiac surgeon makes it possible to establish a reliable diagnosis(2,10).
Fig. 15.
A double aortic arch with one of the segments being hypoplastic or atretic. The images were obtained by frontal-to-horizontal plane scanning:
In our opinion, systematic ultrasonographic evaluation of the aortic arch and its branches allows for the detection of virtually all types of anomalies of the thoracic segment of the main artery. Innominate (brachiocephalic) artery syndrome, in which anterior tracheal compression due to untypical course of this vessel is observed, is an exception. Since the RBCT forms normally, i.e. as the first branch in the aortic arch, in such cases, there are no echocardiographic markers for the conflict between RBCT and the trachea. In the case of patients with a clinical picture indicative of mechanical obstruction located in the central respiratory tract, the diagnosis may be based on CT or NMRI and tracheoscopy. It should be noted that since the abnormally coursing artery in this syndrome is not adjacent to the esophagus, it does not cause its typical deformity seen in vascular rings. Normal appearance of the aortic arch and its branches makes it possible to exclude a classical vascular ring and confirm normal morphology of the pulmonary trunk – pulmonary sling. Depending on the patient’s symptoms, the obtained echocardiographic data may be sufficient for making therapeutic decisions. First of all, it is possible to exclude most anatomical anomalies of the aortic arch (as well as pulmonary arteries) and redirect the diagnostic process to other areas in symptomatic children. Secondly, in the case of aortic arch abnormalities detected in children with mild clinical symptoms or in whom the diagnosis was not associated with compressive symptoms and no surgical intervention is planned (and also for other reasons), there is no need for extended diagnosis. In the case of patients planned for surgical treatment, whether to divide the vascular ring and relieve the esophageal and tracheal compression or due to other thoracic abnormalities, CT or NMRI is needed for precise visualization of all vascular elements and their relationships with adjacent organs. These techniques are not limited by the impact of non-conductive tissues and allow for an excellent three-dimensional reconstruction of all thoracic organs. It is advisable to plan these diagnostic evaluations based on possibly accurate knowledge of the anomaly, which in turn may be acquired from a carefully performed ultrasonography.