Right Aortic Arch

Right aortic arch (RAA) is an arch sidedness anomaly. Arch sidedness is defined by which side of the trachea the aortic arch (AoA) courses to. A left aortic arch (LAA) courses to the left of the trachea, over the left mainstem bronchus while RAA courses to the right of the trachea, over the right mainstem bronchus (1-3). Normally the left-sided aortic arch joins the left -sided arterial duct. Two main variants of RAA can be differentiated: right aortic arch with right ductus arteriosus (DA) and right aortic arch with left ductus arteriosus.

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Incidence is estimated to be 1 in 1000 in the general population.⁴

Variations of aortic arch laterality and branching pattern result from an abnormal regression of the primordial paired aortic arches during embryonic development. Normally, the aortic arch is formed by the left fourth aortic arch and the left dorsal aorta. Normal regression leads to an LAA, left-sided DA and the usual branching pattern: right brachiocephalic artery (or innominate artery), left common carotid (LCC) and left subclavian (LS) artery.^1-3 RAA is formed if the right dorsal aorta persists and the distal left aorta disappears.

The exact cause of RAA is as yet unknown. However, it can be associated with 22q11.2 deletion even in the absence of intra-cardiac disease. 

The different variants of RAA can be divided into those that cause a vascular ring around the trachea and esophagus, often resulting in compression of both structures, and those that do not. In fact the presence or absence of a vascular ring in the setting of a RAA depends on the branching of the brachiocephalic vessels and the location of the DA.² Rarely, it can  occur in case of circumflex retro-esophageal right aortic arch.

• A vascular ring is present if the DA courses to the left of the trachea. Commonly, this condition is associated with an aberrant course of the LSA. In this case an aberrant left subclavian artery (ALSA) originates from the aortic arch, distal to the takeoff of the right subclavian artery, and courses behind the esophagus, instead of arising from the left brachiocephalic (or innominate) artery as expected. Less frequently, the ALSA arises from the descending aorta through a remnant of the primordial aortic arch, called Kommerell’s diverticulum.^2-3
• If the DA is located on the right of the trachea there is no vascular ring. In this case the brachiocephalic vessels arise in mirror image branching to the normal LAA. The first branch of the aortic arch is a left brachiocephalic artery, followed by a right common artery and a right subclavian artery. This less common variant of RAA may be associated with major congenital heart disease (CHD), most commonly conotruncal defects.2

RAA can be associated with chromosomal abnormalities, intra-cardiac and extra-cardiac malformations.  The most frequent chromosomal anomaly is microdeletion of chromosome 22q11.2. or trisomy 21. Associated intra-cardiac malformations can be present and the most common structural cardiac malformations are Tetralogy of Fallot (TOF) and Common Arterial Trunk. This is observed especially in the case of RAA with mirror image branching. Extra-cardiac malformations can be found in 15-20% of cases including renal and gastrointestinal malformations.^4-5

The risk of recurrence is related to the recurrence of associated chromosomal anomalies.

The detection of RAA is achieved in the three-vessel and trachea view (3VTV). In the most common variant, the aortic arch is right sided and the DA is to the left to the trachea, which can be visualised as an echogenic structure between the arches. These vessels encircling the trachea form the so-called “U-sign”. Colour Doppler can easily demonstrate the U-sign and the course of the ALSA, when present. Three- dimensional rendering, achieved by colour Doppler, power Doppler and B-flow mode, may show the spatial relationship of the RAA.^{1-3, 6}
In the less frequent variant, the DA is right sided and both arches are on the right of the trachea in a V-configuration.                                                                                                                    
 

Double aortic arch (DAA): differential diagnosis between left common carotid artery in U-sign RAA and narrow left aortic arch in DAA is challenging as both have a similar anatomic course.

Following a prenatal detection of RAA, a detailed fetal cardiac assessment is suggested, in order to assess the position of the DA and the course of the LSA and to exclude associated cardiac anomalies. Furthermore, extra-cardiac defects should be excluded by a careful fetal anatomy survey,in order to identify high risk cases for 22q11.1 deletion by thymus hypo/agenesis (sensitivity 96%), isolated cleft palate and renal defects.^4,5,7

ISUOG guidelines recommend that the 3VTV should be included in routine pregnancy screening in order to improve the prenatal detection of RAA and its related findings.8

If completely isolated, in most cases RAA is an asymptomatic vascular variant. The prognosis of RAA depends on the presence of a chromosomal abnormality, particularly 22q11.2 deletion and on the complexity of the associated CHD and/or extracardiac anomalies. 
Most children with isolated RAA and ALSA are asymptomatic, however compression symptoms, such as dysphagia, asthma, stridor, wheeze and recurrent upper respiratory tract infection can occur in approximately 25% of cases. Other signs can include cyanosis and obstruction of the ALSA. In majority of cases isolated RAA does not require surgery.^2-4
 

As previously reported, a detailed fetal anatomy investigation is recommended. The first-trimester combined risk for aneuploidies should be reviewed to evaluate the individual risk. An invasive procedure should be offered to assess fetal karyotype and the presence of 22q11.1 deletion through CGH microarray.⁴ Commonly RAA and ALSA (vascular ring) does not represent a neonatal emergency. For this reason delivery in a tertiary referral center is not mandatory and intervention should be planned based only on symptoms occurrence.

1. Yoo SJ, Min JY, Lee YH, Roman K, Jaeggi E, Smallhorn J. Fetal sonographic diagnosis of aortic arch anomalies. Ultrasound Obstet Gynecol. 2003 Nov;22(5):535-46.

2. Paladini D, Volpe P. Ultrasound of Congenital Fetal Anomalies. 2nd Edition. Taylor & Francis, 2014.

3. Abuhamad A, Chaoui R. A Practical guide to fetal echocardiography: normal and abnormal hearts. Edition, 3. Lippincott Williams & Wilkins, 2015. 

4. D'Antonio F, Khalil A, Zidere V, Carvalho JS. Fetuses with right aortic arch: a multicenter cohort study and meta-analysis. Ultrasound Obstet Gynecol. 2016 Apr;47(4):423-32. doi: 10.1002/uog.15805. Epub 2016 Mar 16.

5. Perolo A, De Robertis V, Cataneo I, Volpe N, Campobasso G, Frusca T, Ghi T, Prandstraller D, Pilu G, Volpe P. Risk of 22q11.2 deletion in fetuses with right aortic arch and without intracardiac anomalies. Ultrasound Obstet Gynecol. 2016 Aug;48(2):200-3. doi: 10.1002/uog.15766.

6. Chaoui R, Schneider MB, Kalache KD. Right aortic arch with vascular ring and aberrant left subclavian artery: prenatal diagnosis assisted by three-dimensional power Doppler ultrasound. Ultrasound Obstet Gynecol. 2003 Dec;22(6):661-3.

7. Mogra R, Kesby G, Sholler G, Hyett J. Identification and management of fetal isolated right-sided aortic arch in an unselected population. Ultrasound Obstet Gynecol. 2016 Dec;48(6):739-743. doi: 10.1002/uog.15892. Epub 2016 Nov 8.

8. International Society of Ultrasound in Obstetrics and Gynecology, Carvalho JS, Allan LD, Chaoui R, Copel JA, DeVore GR, Hecher K, Lee W, Munoz H, Paladini D, Tutschek B, Yagel S. ISUOG Practice Guidelines (updated): sonographic screening examination of the fetal heart. Ultrasound Obstet Gynecol. 2013 Mar;41(3):348-59. doi: 10.1002/uog.12403.
 

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