Differential diagnosis of vascular structures in relation to upper ascending aorta: The retro-aortic innominate vein.

The retroaortic course of left innominate vein is a rare entity which can be misinterpreted during echocardiography for other abnormal vascular structures under the arch of aorta. We report the case of a 2 month old infant where the suprasternal window showed 2 vascular structures beneath the aortic arch, one of which was traced to be a retroaortic innominate vein.

Echocardiography from suprasternal long axis view of a 2 month old female child with pulmonary atresia and ventricular septal defect demonstrated two vessels in relation with the upper ascending aorta and aortic arch, the upper one being small in size [Figure 1]. Usually there is only one vessel in relation with ascending aorta, that is the right pulmonary artery. The differential diagnosis of this additional superior vessel include aorto-pulmonary collateral, persistent ductus arteriosus (PDA) arising from aortic arch, persistent fifth arch, vertical vein in azygos type of total anomalous pulmonary venous connection (TAPVC), and venous structures such as retro-aortic innominate vein. In this patient Doppler demonstrated continuous flow, suggestive of venous channel. The probe was tilted leftward and anteriorly to trace the vessel which revealed venous flow from the left side of the neck passing below the aorta to the right side to drain into lower superior vena cava [Figure 2], confirming the diagnosis of retro-aortic left innominate vein.

Figure 1

Suprasternal window shows an additional vessel in relation with the upper ascending aorta and aortic arch (marked by arrow)

Figure 2

Suprasternal window shows venous flow from the left side of the neck passing below the aorta to the right side (marked by arrow)

DISCUSSION

The left innominate vein normally courses obliquely downward to the right passing superoanterior to the aortic arch and in front of its branches to drain into superior vena cava. Retro-aortic innominate vein is a rare entity where the innominate vein takes an anomalous course below the aortic arch with an incidence of 0.2-1% among congenital cardiac anomalies.[1] More than 80% of the patients with anomalous left innominate vein have obstruction of the right ventricular outflow tract.[2] Right aortic arch is a common association.[1]

Embryologically, as most of the left cardinal veins disappear, the venous drainage from the left side of the head and neck and the left arm is directed into the right anterior cardinal vein by 2 transverse capillary plexi above and below the fourth aortic arch. Normally, the aortic arch shortens during the embryological development and occupies the space of the inferior transverse capillary plexus, thus causing its regression. The rest of venous blood shunts into the superior transverse capillary plexus and forms the normal supra aortic course of the left innominate vein.[2] In contrast, reduced shortening of the aortic arch (right-sided or high aortic arch) may cause regression of the superior capillary plexus and results in preservation of the inferior capillary plexus and formation of retroaortic vein [Figure 3]. Abnormal development of the pulmonary arteries (pulmonary atresia or pulmonary stenosis) encourages the sparing of the lower dorsal plexus, leading to formation of an anomalous innominate vein.[1]

Figure 3

Schematic diagrams illustrating the development of retroaortic left innominate vein (RLIV) (a) In the sixth week of gestation: 1 - right anterior cardinal vein; 2 - left anterior cardinal vein; 3 - primitive aorta; 4 - superior transverse capillary plexus; 5 - inferior transverse capillary plexus; 6 - common cardinal veins; 7 - right posterior cardinal vein; 8 - left posterior cardinal vein; 9 - sinus venosus; 10 - vitelline vein; 11 - umbilical vein; 12 -developing inferior vena cava. (b) Retroaortic left innominate vein: 1 - innominate vein; 2 - azygos vein; 3 - superior vena cava; 4 - right atrium; 5 - inferior vena cava; 6 - coronary sinus; 7 - ascending aorta

The retroaortic innominate vein can usually be traced by echocardiography in the suprasternal long axis view to the left side of neck when the transducer is tilted leftwards and anteriorly. The color flow and Doppler spectrum shows low velocity venous flow towards the heart.[3] An aorto-pulmonary collateral and PDA arising from aortic arch will show high velocity continuous flow.

Usually, the retroaortic innominate vein in isolation has no clinical importance. The anomalous innominate vein may cause technical difficulties during pacemaker insertion or central venous line placement through the left arm approach. For patients undergoing cardiac surgery, the superior vena caval cannulation for cardiopulmonary bypass has to be done more caudally than usual to avoid obstruction of the retroaortic innominate vein.[1] The anomaly may complicate exposure of the pulmonary arteries while creating systemic vein to pulmonary artery anastomosis during Glenn shunt. Also, it may obscure the surgical field in the construction of a subclavian to pulmonary artery shunt and the ligation of a patent ductus arteriosus.[4] Retroaortic innominate vein has been used in pulmonary artery reconstruction by creating a wide side-to-side anastomosis[5] and for right atrial pulmonary anastomosis.[6]