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.