Discussion
UPAD is rare congenital malformation and usually demonstrated as an absence of proximal unilateral pulmonary artery on prenatal echocardiography; therefore, it has also been referred to as unilateral absence of the pulmonary artery.1 As the distal segment of the affected pulmonary artery actually exists, UPAD is a more accurate term to describe this malformation.1 It also should be distinguished from the other rare form of CHD, hemitruncus arteriosus, which is defined as an anomalous origin of one of the branch pulmonary arteries (PAs) from the aorta and a normal origin of other PAs from the RVOT.2 If untreated, it results in a large left-to-right shunt, with the whole cardiac output from the right ventricle directed to the unaffected lung while the other lung receives blood at a systemic pressure from the systemic aorta; its 1-year survival rate has been reported to be <30%.2,3
Unlike patients with hemitruncus arteriosus, those with UPAD receive blood supply to one lung from ductus-like collateral vessels, not the systemic aorta, and it is often associated with other CHDs, especially tetralogy of Fallot (TOF).2 Although, its pathophysiology is not fully understood most studies on UPAD have described that a failure in the connection of the sixth aortic arch with the pulmonary trunk results in this developmental anomaly.1,4 Clinical presentations vary depending on the affected site.1,4,5 In case of LPAD, 75% of patients have an associated congenital heart disease, including TOF, right-side aortic arch, septal defect, or persistent DA.1,5,6 By contrast, most patients with RPA discontinuity have the isolated form without other intracardiac anatomies. The affected pulmonary artery is on the side opposite the aortic arch.6 As known previously, in this case, the LPA was affected, and the neonate showed a right aortic arch. This implies that the confirmation of the aortic arch location has an important clinical significance.
As previous mentioned, the initial diagnosis was PAVSD only. In the case of PAVSD, the initial maintenance of DA is also important because the pulmonary circulation is dependent on DA. However, the atretic MPA was not dependent on DA and was responsible for the right lung circulation in the present case. If PGE1 was administered for a certain period to maintain DA without prenatal detection of UPAD in our case, it would be associated with a decrease in the size of the right PA, resulting in PA size discrepancy. Meanwhile, an isolated form of UPAD (usually affecting the right PA) could be missed during prenatal care. Diagnosis is often delayed in patients with pulmonary hypertension, recurrent pulmonary infections, congestive heart failure, and haemoptysis.4 Prenatal detection is important because it aids in the prompt initiation of PGE1 administration to ensure early rehabilitation of the affected lung.7
No consensus has been reached regarding the treatment for UPAD. In cases complicated by other CHDs, treatment would depend on major cardiac abnormalities. In our case, the prenatal detection of UPAD in addition to PAVSD allowed the paediatric cardiologist to make a precise operative plan. Our paediatric cardiologist knew the size difference between the LPA and the MPA, which made it possible to prepare the Gore-Tex graft patch for pulmonary angioplasty. In cases of the isolated form of UPAD, early intervention for UPAD has been supported owing to the concern for regression of the affected PA after DA closure. However, the operation timing should be determined on the basis of many other clinical situations, including the neonatal condition or birthweight.7 If early intervention is unavailable, administration of PGE1 is usually required. In cases of delayed diagnosis made in the adolescent or adult period, lobectomy and ligation of the affected PA are required.5 In conclusion, UPAD is relatively rare, but when undetected, it could affect neonatal prognosis. Therefore, clinicians should examine the route of the both pulmonary arteries regardless of the existence of other intracardiac abnormalities.