Pulmonary artery agenesis refers to a rare congenital absence of pulmonary artery due to a malformation in the sixth aortic arch. It can occur bilaterally, with both left and right pulmonary arteries being absent, or unilaterally, the absence of either left or right pulmonary artery (UAPA). About 67% of UAPA occurs isolated in the right lung. The absence of pulmonary artery can be an isolated disorder, or accompanied by other related lesions, most commonly Tetralogy of Fallot. Back in 1868, Fraentzel was the first to report isolated unilateral absence of pulmonary artery (IUAPA) in literature. Subsequently, literature has documented a total of 420 cases. The estimated prevalence of IUAPA is 1 in 200,000 adults. No sex preference is observed. Patients with severe complications are usually diagnosed early in age while adult patients are mainly asymptomatic. The overall mortality rate reaches 7%. Individuals may exhibit a variety of symptoms, or they may not exhibit any symptoms at all. Recurrent lung infections and exercise intolerance are some of the most common symptoms. Serious complications include hemoptysis and pulmonary hypertension. These non-specific symptoms make UAPA challenging to diagnose. Multiple medical imaging techniques are often employed in combination in order to obtain a comprehensive diagnosis.
Cause Pulmonary artery agenesis refers to the absence or maldevelopment of one or both pulmonary arteries in foetal development. This rare congenital condition arises from abnormal maturation of the sixth aortic arch during embryogenesis. In embryonic growth, the primordial truncus arteriosus is divided into aorta and pulmonary trunk by septation. Any disruptions in septation may cause the failure of the development of the branch pulmonary artery from the sixth aortic arch. The exact pathogenesis is not fully understood.
Mortality rate and cause of death The overall mortality rate of UAPA in all patients is about 7%. Newborns with respiratory distress and severe pulmonary hypertension have poor outcomes. 30% of patients remain asymptomatic throughout adult life. Delayed diagnosis and follow-up medical interventions may lead to lung hypoplasia. Pulmonary haemorrhage, recurrent infections and pulmonary hypertension may hinder the possibility of long-term survival. Survival is probable until patients reach their sixties. Common causes of death include right heart failure, respiratory failure, massive pulmonary hemorrhage and high-altitude pulmonary edema.
Signs and symptoms 40% of UAPA patients exhibit symptoms of exercise intolerance or dyspnea during exertion. Other common symptoms include hemoptysis in 20% of patients, chest pain, pleural effusion or recurrent pulmonary infections. UAPA may rarely cause the development of severe, life-threatening hemoptysis. Pulmonary hypertension is another potentially fatal condition that affects 20% of patients. IUAPA patients with no associated cardiac anomalies might remain largely asymptomatic into adulthood. It has been noted that individuals who have a unilateral absence of the right pulmonary artery in particular are more vulnerable to high-altitude pulmonary edema.
Hemoptysis In IUAPA patients, collateral arteries and shunts are developed in the affected lung from the systemic network. The collateral network supplies the pulmonary blood from the heart to the affected lung in place of the absent pulmonary artery. Hemoptysis occurs when the thin walls of the extensive systemic collateral network rupture. The occurrence of hemoptysis may resolve on its own and persist without intervention for many years. However, it might also lead to severe pulmonary haemorrhage and fatal outcomes.
Lung hypoplasia on affected side UAPA can cause hypoplasia in the affected lung due to the disruption of blood flow to the lung. The decreased blood flow can interrupt normal lung development, resulting in the lung being small and hypoplastic.
Pulmonary hypertension (PHT) PHT can be caused by excess blood flow diverted to the remaining pulmonary artery from the absent pulmonary artery. Vasoconstrictive substances, like endothelin, are released as a result of shear pressure caused by increased blood flow in the unaffected pulmonary artery. Persistent constriction due to the substances can cause remodelling in the pulmonary arterioles, which raises the resistance of the pulmonary vasculature and causes PHT. Additional explanations for PHT include inadequate elasticity of the pulmonary vascular bed on the unaffected side to withstand the full cardiac output and abnormal response to vasoconstrictive substances.
Diagnosis The non-specific symptoms and the lack of awareness of such an uncommon condition make UAPA difficult to diagnose. Abnormalities found in imaging tests are more subtle and can be overlooked in infants. Patients with uncomplicated isolated UAPA typically have normal electrocardiograms. Making a diagnosis requires a complete medical history, physical assessment, and laboratory examination, coupled with a high index of suspicion. To obtain a comprehensive evaluation of the conditions, multiple imaging modalities are often used in conjunction.
Chest Radiography Chest X-rays are often the initial imaging modality used to evaluate patients with symptoms of cardiovascular diseases. While they may not provide detailed visualization of the pulmonary arteries, certain findings that raise suspicion for UAPA could be incidentally detected. Patients with UAPA typically have asymmetric lung fields on their chest radiographs, with a hyperlucent lung held in an ipsilateral small hemithorax. The mediastinum and trachea are shifted to the affected side of the lung, and the hilar vasculature is absent or significantly reduced on that side. Ipsilateral diminished pulmonary vascular markings, ipsilateral elevated hemidiaphragm, hyperinflated contralateral lung and enlarged pulmonary artery may also be observed in chest X-rays.
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