Isolation of the left subclavian artery in an infant with tetralogy of Fallot, right aortic arch and DiGeorge syndrome. Echocardiographic diagnostic case study

Isolated left subclavian artery (iLSA) coexisting with right aortic arch is a rarely described vascular anomaly. It is a consequence of impaired aortic arch formation, when the arterial duct connects the left subclavian artery with the pulmonary trunk (Fig. 1). Other congenital defects that coexist with iLSA (60% of cases) and are usually found in the right side of the heart include tetralogy of Fallot (ToF)^(1–6), double outlet right ventricle (DORV)⁽⁷⁾, tricuspid atresia⁽¹⁾, proximal stenosis of left pulmonary artery⁽⁸⁾, and pulmonary sling⁽⁹⁾. The presence of iLSA was also reported in simple defects, such as ventricular septal defect⁽¹⁰⁾, and complete atrial septal defect (Department’s own data). We present a case of iLSA coexisting with tetralogy of Fallot, with a focus on important elements of non-invasive preoperative diagnosis.

Fig. 1.

A 6-month-old girl (second pregnancy, second delivery, birth weight 2,630 g) with DiGeorge syndrome and a congenital heart defect in the form of tetralogy of Fallot was admitted to the Department of Cardiac Surgery for surgical correction.

Echocardiographic diagnosis revealed situs solitus, compatible venoatrial and atrioventricular junctions; aortic dextroposition (40–50%) over a large (approx. 12 mm) perimembranous ventricular septal defect (VSD). There was a bidirectional ventricular septal defect shunt, with left-to-right predominance. Right-sided aortic arch. Due to difficult anatomical conditions, no conclusion was made regarding the morphology of arterial branches of the arch (Fig. 2, Fig. 3, Fig. 4. Fig. 5, Fig. 6, Fig. 7 and Fig. 8). Figure 2, Fig. 3, Fig. 4 and Fig. 5 show a series of upper mediastinal vessel sections in a plane similar to transverse plane – horizontal.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

Fig. 8.

Subsequent images were recorded by gradually moving the ultrasound beam bottom-to-top. Figure 6, Fig. 7 and Fig. 8 show a series of upper mediastinal vessel sections in a plane similar to sagittal plane. Subsequent images were acquired by moving the ultrasound beam from the median plane toward the left side of the patient). The pulmonary trunk originated from a hypoplastic, relatively short outflow tract. The diameter of the ostium of the outflow tract was about 4 mm (stenosis due to hypertrophied septomural trabeculae and displacement of the infundibular septum) in the midportion – about 8 mm. A tortuous patent ductus arteriosus with increased flow was detected, most probably originating from the left subclavian artery or left brachiocephalic trunk. The examination was hampered by very poor echogenicity of this region due to the absence of thymus.

The diagnosis was extended by CT angiography (Fig. 9) – the presence of isolated left subclavian artery giving rise to a tortuous patent ductus arteriosus extending vertically (from the anterior aspect) to the pulmonary trunk was confirmed. It was shown that the major portion of left vertebral flow is directed to the ductus arteriosus and pulmonary circulation. We assumed that closure of the ductus arteriosus will reduce steal from the left vertebral artery, which supplies both the ductus and the normal left subclavian artery; therefore, it will not increase cerebral circulation disorders or reduce blood supply in the left upper extremity^(10,11).

Fig. 9.

We attempted to experimentally clamp the posterior descending artery (PDA) during echocardiography-guided cardiac correction and found no deficit in blood supply in the left upper limb. PDA was ligated and divided. A total correction of tetralogy of Fallot was performed. The post-operative period was uneventful. Postoperative ultrasonography still showed reversed flow in the left vertebral artery (LVA), representing the only supply of the left subclavian artery (LSA) (Fig. 10 and Fig. 11 show altered flow in the left common carotid artery and left vertebral artery after defect correction and ductus arteriosus closure). The flow in both arteries was systolic and high-resistance, which confirmed the decreased volume of cerebral steal. It may be therefore concluded that the closure of ductus arteriosus reduced the risk of cerebral perfusion deficit, with maintained blood supply to the left upper extremity.

Fig. 10.

Fig. 11.

The development of the final form of a single aortic arch and its main branches is a complex process of transition from a symmetrical structure of pharyngeal arch arteries (III and IV) and two dorsal aortas to a single left- or right-sided arch (Fig. 3). This transition involves both involution of one of dorsal aortas, incorporation of fourth pharyngeal arch artery in the aortic arch, as well as cranial migration of the seventh intersegmental artery, from which the subclavian artery will eventually form⁽¹²⁾. It seems that some significant genetic disorders, such as 22q11.2 microdeletion, may promote intracardiac defects of the outflow tract (ToF), as well as impair normal aortic arch formation⁽⁹⁾. The presence of DiGeorge syndrome in our patient was also important⁽³⁾.

Therefore, we assumed that isolated left subclavian artery (iLSA) results from the lack of involvement of the fourth left pharyngeal arch artery in the formation of aortic arch segment between the left common carotid artery and the left subclavian artery, with a simultaneous involution of the left dorsal aorta distally from LSA origin from the arch. Consequently, a right-sided aortic arch is formed, and the remainder of the fourth pharyngeal arch artery, i.e. ductus arteriosus, becomes the only source of supply for LSA. Reduced pressure in the pulmonary artery after birth or obstructed right ventricular outflow, which is observed in e.g. tetralogy of Fallot, increases the symptoms of basilar-vertebral steal. The described iLSA is a very rare defect, found in only 0.8% of right-sided aortic arch cases⁽⁹⁾.

Patients with right-sided aortic arch usually present with one of the following two types of arterial distribution:

In the discussed case, the image of aortic arch branches deviated from typical systems described above. The first branch, i.e. the left common carotid artery, was relatively narrow, corresponding to an image typical of ALSA, but a long artery, which could be easily considered as a continuation of the brachiocephalic trunk – the left subclavian artery, was present in its immediate vicinity. However, despite a very close proximity of both vessels, their continuity was not shown. Furthermore, Doppler ultrasound revealed an opposite direction of flow in each of the evaluated arteries: systolic upward (cranial) flow in the common carotid artery, and full-cardiac-cycle caudal flow in the other vessel. A more detailed evaluation showed a vessel linking the LSA with the pulmonary trunk. The nature and direction of flow suggested that the vessel was a ductus arteriosus. Due to very difficult imaging conditions in the mediastinum as a result of absent thymus (DiGeorge syndrome) and the growing anxiety of the child (intracardiac anatomical anomalies associated with tetralogy of Fallot were assessed in the initial phase of diagnostic imaging), we did not obtain the image of the peripheral segment of this vessel, particularly its junction with systemic arteries; therefore, a decision was made to extend the diagnosis with 3-dimensional CT angiography, which resolved the doubts.

The above reasoning allows for a thesis that echocardiography may be an important diagnostic tool to help make a correct diagnosis. The presence of tetralogy of Fallot with a right-sided aorta should prompt the diagnostician to analyze the direction and the nature of flow in the left subclavian artery. Medical history and clinical examination indicative of reduced pulse in the left radial artery, headache, and impaired vision (due to the steal syndrome) help make a correct diagnosis^(10,11). It should be emphasized that the topographic proximity between LCCA and LSA (Fig. 2) accounts for the difficulties in ultrasonographic detection of vascular anomaly and relying mainly on CT findings.