Discussion
In d-TGA, failed spiraling during great artery septation causes the pulmonary and systemic circulations to exist in parallel. Newborns with physiologically elevated pulmonary vascular resistance have pulmonary to systemic flow through the PDA resulting in reverse differential cyanosis. Our patient does not show reverse differential cyanosis. Instead, she demonstrates persistently elevated right arm saturations as compared to the right ear and other extremities. This, along with cyanosis that very typically spared the right arm and hemithorax, strongly raised the suspicion for an anomaly with the aortic arch branching, specifically that the right subclavian could be originating from the pulmonary circuit and carrying oxygenated blood.
AORSA from the RPA can also be understood embryologically. In normal embryonic development, the right 7th intersegmental artery travels cranially and becomes the right subclavian artery, eventually coming off of the brachiocephalic artery which itself arises from the 4th aortic arch.7,8 The extrapericardial portion of the pulmonary arteries arises from the 6th aortic arch.9 Regression of the distal right 4th and 6th arches allows the left 4th arch to become the aortic arch and the left 6th arch to become the PDA. When the right 6th arch persists, it forms the right ductus arteriosus and pulmonary artery, potentially leading to an AORSA from the RPA.3
Our report emphasizes the difficulty in visualizing AORSA from the RPA, something that was missed on the initial TTE and also which most other reports have described to be diagnosed using a CT or invasive angiogram. TTE interrogation has classically depended on the suprasternal axial plane sweeping superiorly to define the aortic arch sidedness and branching. This traditional imaging plane, though adequate in raising the suspicion of AORSA, fails to itself evaluate for AORSA from the PA. We recommend a modified imaging plane to evaluate for this lesion, where the suprasternal axial arch view is started with the transducer aiming more inferiorly such that the distal main and proximal right PAs are visualized. Careful slow sweep of the transducer to an axial view and then oblique coronal view should be performed with color comparison (Figure 2). This imaging plane can clearly demonstrate the origin, proximal and distal course of the AORSA, including its relationship to the aorta and presence or absence of the right sided PDA (Figure 3). Once this is obtained, an orthogonal sagittal plane can be obtained to visualize the AORSA in a long axis (Figure 4).
Effective diagnosis of AORSA from the RPA is crucial because, if missed, may result in steal of blood away from the subclavian artery and to the pulmonary system. Some reports have described surgical correction with ASO with subclavian artery ligation and division from the RPA (our patient),2,4 but this carries the risk of subclavian steal syndrome where the subclavian artery is supplied by the vertebral artery. Alternatively, reports have described ASO with subclavian artery re-implantation to the systemic circulation, usually the aorta or common carotid.1,3 Although this is more physiologic, the mobilization of the subclavian artery is technically challenging and there is an increased risk of restenosis.