Isolated Right Common Carotid Artery Arising from the Right Pulmonary Artery

Abstract

Isolated aortic arch vessels arising anomalously from the pulmonary arterial system are rare congenital anomalies. Case reports of isolated arch vessels are often associated with 22q11 deletion, CHARGE syndrome, or right aortic arch. Isolation of the carotid artery may lead to cerebral steal phenomenon and ischemia or to pulmonary overcirculation. The authors report what is, to their knowledge, the first case of isolated right common carotid artery arising from the right pulmonary artery, associated with 22q11 deletion, and describe the challenging multimodality image evaluation.

Keywords: Congenital, Anatomy, Carotid Arteries

© RSNA, 2022

Introduction

Aortic arch anomalies, including right aortic arch and anomalous origin of the arch vessels, are relatively common, particularly in the setting of 22q11 deletion or CHARGE syndrome (1–4). However, isolation of an aortic arch vessel from the remainder of the aortic system, with the vessel instead arising from the pulmonary arterial tree, is quite rare and often involves the brachiocephalic or subclavian arteries. A search of the literature disclosed only 14 cases of isolated left common carotid artery (CCA) (5–9), with a single case report of isolation of the right internal carotid artery (10). To our knowledge, we report the first case of isolated right CCA.

Case Report

We present a 14-month-old boy with 22q11 deletion syndrome, with isolation of the right CCA arising from the right pulmonary artery.

The patient was born at term to a 27-year-old mother (gravida 3, para 2) who had diet-controlled gestational diabetes. Newborn screening results were positive for severe combined immune deficiency, with single-nucleotide polymorphism microarray confirming 22q11 deletion. Other findings related to 22q11 deletion included T-cell lymphopenia, hypocalcemia, and hypothyroidism. A screening echocardiogram revealed a small posterior muscular ventricular septal defect, and flow signal was observed entering the pulmonary artery, thought to be a small patent ductus arteriosus. Recommended follow-up evaluation at 6 months was delayed because of the COVID-19 pandemic.

At 1 year of age, an echocardiogram showed interval closure of the ventral septal defect, but the abnormal flow signal was now prominent and low velocity, clearly entering the right branch pulmonary artery and directed toward the main pulmonary artery (Fig 1). Differential diagnosis at the time included a coronary artery fistula or aberrant patent ductus arteriosus.

Figure 1:

Parasternal short-axis echocardiogram in a 14-month-old boy with 22q11 deletion syndrome demonstrates abnormal continuous incoming flow (red flow at arrow) entering the right pulmonary artery (RPA) directed toward the main pulmonary artery (MPA).

Given the possibility of a coronary artery fistula, a prospectively gated coronary CT angiogram was obtained, with field of view limited to the heart to reduce radiation dose. The coronary arteries were normal, without evidence of fistula. However, an anomalous vessel was observed arising from the right pulmonary artery, extending cranially toward the right neck out of the scan field of view (Fig 2). The aortic arch was left-sided with three branches; the first two vessels extended toward the left neck, presumed to be the left CCA and left subclavian artery, while the third vessel arising off the descending arch extended rightward posterior to the esophagus, consistent with an aberrant retroesophageal right subclavian artery without a diverticulum of Kommerell. A ductal ampulla was also noted at the aortic isthmus directed toward the left pulmonary artery, suggesting a left-sided ligamentum arteriosum.

Figure 2:

Frontal view three-dimensional rendered image from contrast-enhanced chest CT angiography demonstrates an anomalous vessel arising from the right pulmonary artery coursing cranially out of the field of view. LPA = left pulmonary artery, RCC = right common carotid artery, RPA = right pulmonary artery.

Given this arch configuration and lack of an aortic arch vessel consistent with a right CCA, the anomalous vessel arising off the pulmonary artery was hypothesized to represent an isolated right CCA. Potential clinical ramifications of this anomaly included the risk of cranial steal phenomenon and ischemia. To confirm the identity of the vessel and understand the hemodynamics and complete intracranial vascular anatomy, further evaluation was performed with MR angiography and imaging.

Time-of-flight (TOF) MR angiogram of the neck helped confirm a left-sided aortic arch with branches, including the left CCA, followed by the left subclavian artery and then an aberrant retroesophageal right subclavian artery. The right CCA arose from the right pulmonary artery without cranially directed flow-related enhancement, suggesting reversed cranial-to-caudal flow within the isolated right CCA. This was confirmed by flow-related enhancement within the right CCA on a TOF MR venogram of the head and neck (Fig 3). TOF MR angiogram of the brain revealed intact circle of Willis, with normal flow-related enhancement in the terminal right internal carotid artery and distal intracranial arteries and no evidence of intracranial ischemia.

Figure 3:

(A) Axial proton density MR image demonstrates normal configuration of the bilateral common carotid (arrows) and vertebral arteries (arrowheads). (B) Axial time-of-flight MR angiogram demonstrates normal cranial flow-related enhancement in the left common carotid artery (thick arrow) and bilateral vertebral arteries (arrowheads), with complete lack of cranial flow-related enhancement in the right common carotid artery (thin arrow). (C) Coronal maximum intensity projection time-of-flight MR venogram demonstrates normal caudal flow-related enhancement in the bilateral jugular veins (arrowheads), as well as confirming reversed caudal flow-related enhancement in the right common carotid artery (arrow).

The decision was made to proceed with surgical repair of the anomalous anatomy given the risks for pulmonary overcirculation and resultant pulmonary hypertension, as well as brain ischemia over time. The patient underwent reimplantation of the right CCA from the right pulmonary artery to the ascending aorta (Fig 4) at 21 months of age. Postoperative imaging showed proximal tortuous flow of the reimplanted right CCA, though there was no stenosis and distal arterial flow appeared normal. Follow-up monitoring with serial echocardiography is ongoing.

Figure 4:

Intraoperative frontal image following sternotomy demonstrates the isolated right common carotid artery (arrow, looped by yellow string) arising off the right pulmonary artery (arrowhead). * = ascending aorta.

Discussion

The aortic arch and arch vessels arise from the embryonic dorsal and ventral aortic arches, as well as portions of the third and fourth primitive aortic arches. The pulmonary arterial trunk, meanwhile, arises from the sixth embryonic arch. Aortic arch anomalies are thought to occur secondary to anomalous regression or persistence of various segments of the embryonic aortic arches; however, full understanding of this embryologic process is lacking.

The hypothetical double aortic arch model proposed by Edwards (11) (Fig 5) explains the development of the aortic arch via selective regression of various portions of the double arch. A typical left-sided aortic arch develops due to regression of the right dorsal aortic root and right ductus, while a right aortic arch with an aberrant left subclavian artery is explained by regression of the left mid arch between the left subclavian artery and left CCA.

Figure 5:

Edwards hypothetical arch model, assuming that the ductus arteriosus connects to the respective arch proximal to the subclavian arteries. Formation of the typical left aortic arch (top), a right arch with aberrant left subclavian artery (middle), and our patient’s arch configuration (bottom) with left arch, isolated right common carotid artery, and aberrant right subclavian artery. Red indicates systemic arterial structures, light blue indicates pulmonary arterial structures, dark blue indicates ductus arteriosus, and black lines indicate the absorption points. AAo = ascending aorta, DAo = descending aorta, L CC = left common carotid artery, L Sc = left subclavian artery, PA = pulmonary artery, R Bc = right brachiocephalic artery, R CC = right common carotid artery, R Sc = right subclavian artery.

In this model, isolation of an arch vessel from the pulmonary tree results from the resorption of the two arch segments proximal and distal to that vessel. The connection of this vessel to the pulmonary arterial system is thought to occur due to persistence of the ductus arteriosus supplying the now isolated arterial branch (12). Isolation of the left CCA would result from regression of the proximal and mid left aortic arches with persistence of the left ductus supplying the left CCA, also resulting in a right aortic arch and aberrant left subclavian artery. Consistent with this, all 14 previously reported cases of isolation of the left CCA also demonstrate right aortic arch with aberrant left subclavian artery (5–9).

Isolation of the right CCA, as in our case, would require regression of the proximal right arch and the mid right arch between the right carotid and right subclavian arteries, with persistence of both a right ductus supplying the right CCA and a left ductus for pulmonary-to-systemic fetal flow. This results in a left aortic arch with aberrant right subclavian artery, as observed in our case. Interestingly, although a left-sided aortic arch with aberrant right subclavian artery is the most common arch anomaly (11), isolation of the right CCA with left arch and aberrant right subclavian appears much less common than isolation of the left CCA with right arch and aberrant left subclavian.

Of note, while the Edwards model provides a theoretical explanation for the formation of these anomalies, this theory has been challenged by some who report a separate identifiable ductus on the same side as the isolated arch vessel (13). Others have postulated that malseptation of the aorticopulmonary trunk may have a role in the isolation of arch vessels (14).

Conclusion

Aberrant connection of a systemic arterial vessel to the pulmonary system can lead to reversal of normal directional flow toward the lower-pressure pulmonary system, resulting in a “steal” phenomenon with decreased perfusion of the end organ involved. In this case, TOF MRI sequences demonstrating the direction of flow helped confirm reversed flow in the isolated right CCA, and there was no evidence of brain ischemia at brain MRI. Proper treatment is necessary, as chronic increased blood flow to the pulmonary circulation may trigger pulmonary vascular disease and pulmonary hypertension.