Aberrant Right Subclavian Artery

Abnormalities of the aortic arch branching are fairly common and concerns the order and pattern of origin of major branches from the aortic arch.¹  Under normal conditions, the left aortic arch gives rise to 3 branches:

1. The brachiocephalic (or innominate) artery which, after a short course, splits into the right subclavian artery (RSA) and the right common carotid artery,
2. The left common carotid artery and
3. The left subclavian artery.

The aberrant right subclavian artery (ARSA) arises directly from the distal portion of the aortic arch, or from the upper portion of the descending aorta, as the lowest fourth branch.^1-2 The aortic branches, in this case, present the following sequence from proximal to distal :

1. Right common carotid artery,
2. Left common carotid artery,
3. Left subclavian artery and
4. Right subclavian artery

Furthermore, ARSA presents an anomalous course as well. While the RSA directs to the right arm, anteriorly to the trachea, presenting an “S-like” course, the ARSA has a retrotracheal/esophageal course, without the typical “S” shape.³ 

The incidence of ARSA is estimated around 1 - 2% in normal population.

As stated by the hypothetical double aortic arch theory suggested by Edwards⁴, in the embryo a double aortic arch first develops with a left and a right arch surrounding the trachea and the esophagus. The left and right aortic arches give rise to two vessels each; the left and right common carotid and subclavian arteries. Normally, the right arch regresses distally to right common carotid and subclavian artery which merge to form the brachiocephalic artery. Any segment of the embryonic aortic arch system can regress or persist abnormally, resulting in extensive array of aortic arch anomalies.

ARSA occurs when there is a break in the primitive right arch between the right common carotid and subclavian arteries. This results in a left aortic arch with four branches, where the right subclavian artery arises as the most distal one coursing from the left side of the spine behind the esophagus and trachea to the right upper arm, known as aberrant right subclavian artery or arteria lusoria.

A higher incidence of ARSA has been observed in cardiac catheterization studies in Down’s syndrome series, during surgery for congenital cardiac defects (CHD)5 or in fetal echocardiography, underlining its association with chromosomal abnormalities and cardiac defects.^6-7

The first observation of ARSA in fetuses with trisomy 21 was reported by Chaoui et al.⁸ In one of the few prenatal studies on routine population, conduced at first and second trimester screening, there was found a 20% association with other fetal abnormalities, the majority of which were genetic and cardiac anomalies.³

The most frequent genetic abnormalities associated to ARSA are chromosomal aberrations, especially trisomy 21 and trisomy 18. Although in trisomy 21 fetuses ARSA is usually associated to other typical 1st or 2nd trimester ultrasonographic markers of the syndrome^8-12  however it has been also reported as isolated US marker for Down’s syndrome.³.

The combination of ARSA with a cardiac defect, especially conotruncal anomaly, increases the risk for 22q11 microdeletion.^{6, 7, 13}

The feasibility of RSA assessment in the first and second trimester depends mainly on the skills of the operator. The prenatal ultrasonographic technique, during either first or second trimester examination, requires an axial view of the upper fetal thorax, at the level of the three vessels and trachea view, to assess the aortic arch. Subsequently, the fetal clavicles should be positioned at 12 and 6 o’clock by rocking the probe, so that the course of the subclavian vessels would be parallel to the beam. In both first or second trimester assessment, color Doppler should be used to visualize the course of the RSA by reducing the pulse repetition frequencies (PRF).

Normally, the subclavian arteries present an ‘S’ shape, with the proximal part concave posteriorly and the distal part concave anteriorly.³ The proximal concavity of a normal RSA surrounds anteriorly the trachea. Using low pulse repetition frequencies, often both subclavian arteries and veins are simultaneously visualized overlapping in their entire course, which is another typical feature of a normal subclavian artery.^{3, 9}

ARSA is diagnosed when a retrotracheal vessel is depicted arising from the posterior part of the aortic arch, directed towards the right arm.^{1-3, 8-14} The entire course of the RSA should be visualized (from its aortic origin to the right fetal arm). The course of ARSA is straight, without the typical ‘S’ shape and in its proximal part it is distant from the homolateral brachiocephalic vein (BCV), which remains anterior to the trachea, and they overlap only at the distal part.

When isolated, ARSA has no pathologic implications and it is considered a normal variant. In fact, despite its anomalous course, ARSA is usually asymptomatic and does not need surgical correction. On rare occasions, it has been reported esophageal compression resulting in dysphagia (lusoria.)¹⁵ 

The course of the RSA or the presence of ARSA is not readily detectable at routine postnatal echocardiography. Although transesophageal echocardiography can be used to detect ARSA, the best method of postnatal identification of the course of the RSA is represented by magnetic resonance imaging (MRI) which is indicated only in symptomatic cases of suspected ARSA.

Differential diagnosis should be done with the left brachiocephalic vein and the azygos vein.

The left brachiocephalic vein reaches the superior vena cava crossing anteriorly the trachea. Due to the low PRF, in presence of ARSA, the left brachiocephalic vein can mimic a normal RSA, presenting the same flow direction and coursing both anteriorly to the fetal trachea. However, the latter reaches the right clavicle while the left brachiocephalic vein ends in the superior vena cava.

The azygos vein, ends as well in the superior vena cava and does not arise from the aorta; however some Authors have described a possible false interpretation of this vessel as an ARSA due to its similar course and the same flow direction.⁹ 

To avoid both false positive or false negative diagnosis of ARSA, it is evident that visualization of the origin and the entire course of the RSA is mandatory. In case of any doubt, the use of pulsed Doppler can distinguish between arterial and venous waveforms avoiding any confusion.

ARSA presents an important association with fetal abnormalities, including aneuploidies, cardiac defects and genetic anomalies. The combination of ARSA with a cardiac defect, especially conotruncal anomaly, increases the risk for 22q11 microdeletion.¹⁶  For these reasons, should ARSA be detected in a fetus a detailed anatomic scan, including fetal echocardiography, should be performed by an expert to exclude the presence of cardiac and extracardiac anomalies, such as thymus hypoplasia.

Counselling regarding invasive testing, after detecting an ARSA, remains a dilemma, if this is an isolated finding or if a low risk for aneuploidies was detected at the first trimester screening.

An appropriate counseling should consider the background risk after first trimester screening and the presence or absence of other sonographic markers of aneuploidies. Some authors offer an invasive procedure, in cases with ARSA and intermediate risk result after NT and biochemistry calculation, even if all other additional markers were normal.³ 

In case of fetal karyotyping because of increased risk for aneuploidies, FISH for 22q11 microdeletion or CGH array could be additionally offered even if ARSA is not associated to apparent cardiac defects because some cases of isolated ARSA associated to 22q11 microdeletion have been reported in postnatal studies.¹⁷

1.Yoo SJ, Min JY, Lee YH, Roman K, Jaeggi E, Smallhorn J. Fetal sonographic diagnosis of aortic arch anomalies. Ultrasound Obstet Gynecol 2003;22:535–546.

2.Chaoui R, Rake A, Heling KS. Aortic arch with four vessels: aberrant right subclavian artery. Ultrasound Obstet Gynecol 2008;31:115–117

3.Rembouskos G, Passamonti U, De Robertis V, Tempesta A, Volpe G, Gentile M, Volpe P. Aberrant right subclavian artery (ARSA) in unselected population at first and second trimester ultrasonography. Prenat Diagn. 2012 Oct;32(10):968-75.

4.Edwards JE. Vascular rings and slings. In: Moller JH, Neal WA, eds. Fetal, Neonatal, and Infant Cardiac Disease. Norwalk, CT: Appleton &Lange, 1990:745–754.

5.Zapata H, Edwards JE, Titus JL. Aberrant right subclavian artery with left aortic arch: associated cardiac anomalies. Pediatr Cardiol 1993;14:159–61

6.Boudjemline Y, Fermont L, Le Bidois J, Lyonnet S, Sidi D, Bonnet D. Prevalence of 22q11 deletion in fetuses with conotruncal cardiac defects: a 6-year prospective study. J Pediatr 2001; 138: 520–524.

7. Volpe P, Marasini M, Caruso G, Marzullo A, Buonadonna AL, Arciprete P, Di Paolo S, Volpe G, Gentile M. 22q11 deletions in fetuses with malformations of the outflow tracts or interruption of the aortic arch: impact of additional ultrasound signs. Prenat Diagn 2003; 23: 752–757.

8.Chaoui R, Heling KS, Sarioglu N, et al. Aberrant right subclavian artery as a new cardiac sign in second- and third-trimester fetuses with Down syndrome. Am J Obstet Gynecol 2005;192:257-63.

9.Borenstein M, Cavoretto P, Allan L, et al. Aberrant right subclavian artery at 11 + 0 to 13 + 6 weeks of gestation in chromosomally normal and abnormal fetuses. Ultrasound Obstet Gynecol 2008;31:20-4.

10. Borenstein M, Minekawa R, Zidere V, et al. Aberrant right subclavian artery at 16 to 23 + 6 weeks of gestation: a marker for chromosomal abnormality. Ultrasound Obstet Gynecol 2010;36:548-52

11.Chaoui R, Thiel G, Heling KS. Prevalence of an aberrant right subclavian artery (ARSA) in normal fetuses: a new soft marker for trisomy 21 risk assessment. Ultrasound Obstet Gynecol 2005;26:356.

12.Paladini D, Sglavo G, Pastore G, et al. Aberrant Right Subclavian Artery (ARSA). Incidence and correlation with other markers of Down syndrome in 2(nd) trimester fetuses. Ultrasound Obstet Gynecol 2012;39(2):191-5.

13. Zalel Y, Achiron R, Yagel S, Kivilevitch Z. Fetal aberrant right subclavian artery in normal and Down syndrome fetuses. Ultrasound Obstet Gynecol 2008;31:25-9.

14.Tuo G, Volpe P, Bava GL, et al. Prenatal diagnosis and outcome of isolated vascular rings. Am J Cardiology 103:416-19: 2009

15.Kent PD, Poterucha TH. Images in clinical medicine. Aberrant right subclavian artery and dysphagia lusoria. N Engl J Med 2002; 347: 1636

16.Rauch R, Rauch A, Koch A, et al. Laterality of the aortic arch and anomalies of the subclavian artery-reliable indicators for 22q11.2 deletion syndromes? Eur J Pediatr 2004;163:642-5.

17.McElhinney DB, Clark BJ 3rd, Weinberg PM, et al. Association of chromosome 22q11 deletion with isolated anomalies of aortic arch laterality and branching. J Am Coll Cardiol 2001;37:2114-9.

This article should be cited as: Rembouskos G, Volpe P. Aberrant Right Subclavian Artery VISUOG, July 2014

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