In nearly half of the cases, unilateral lung agenesis is accompanied by other anomalies of the cardiovascular, gastrointestinal, musculoskeletal, or genitourinary systems. The commonest associated cardiac defects are tricuspid insufficiency, atrial and ventricular septal defects, patent ductus arteriosus, tetralogy of Fallot, aortic coarctation, and anomalous pulmonary venous return [5].
The lungs start developing toward the late first month of intrauterine life when a ventral bud (respiratory diverticulum) arises from the foregut which elongates and divides to form two lungs. The failure of division of this primitive bud into two leads to failure in the development of one lung whereas the other lung develops normally [6]. Depending on the stage of arrest of growth process, there can be complete or partial absence of lung tissue. It has been divided into 3 types, initially by Schneider and later refined by Boyden [7].
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1.
Pulmonary agenesis (type 1): There is complete absence of bronchus, lung tissue, and pulmonary vessels.
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2.
Pulmonary aplasia (type 2): There is a short, rudimentary, blind ending main bronchus with absent lung tissue and pulmonary vessels.
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3.
Pulmonary hypoplasia (type 3): There is presence of variable amounts of lung tissue, bronchial tree, and pulmonary vasculature.
However, from the functional point of view, unilateral lung agenesis and lung aplasia represent the same entity.
The exact etiology of pulmonary agenesis is not known. It has been postulated to occur secondary to a complex interplay of environmental insults, genetic, and mechanical factors [5].
The condition presents usually in the first few months or years of life and nearly half of the cases die in the first 5 years of life [5]. These children usually present with respiratory distress or recurrent chest infections. They are at an increased risk of pulmonary infections possibly due to the altered mechanics of trachea owing to its stretching and compressing due to the mediastinal shift. Late presentation is very uncommon. A total of 66 cases of unilateral lung agenesis presenting in adulthood have been reported till date. The oldest patient reported was 72 years old [8]. In some instances, the diagnosis is made accidentally from a radiograph or on autopsy. The timing of clinical presentation and the further clinical course is greatly modified by the presence of associated anomalies affecting the cardiovascular, musculoskeletal, gastrointestinal, or genitourinary systems. The condition is also associated with tracheal abnormalities like narrowing and stenosis [9, 10].
On clinical examination, the patient may have deformity of chest wall on the affected side or scoliosis. There may be reduced respiratory movements on inspection. On auscultation, breath sounds will be absent particularly at the base and axilla as this is occupied by the heart and other mediastinal structures. However, breath sound may be heard in the upper zone owing to the presence of herniated opposite lung into the upper part of affected hemithorax.
Before the availability of cross sectional imaging, the diagnosis was usually clinched at autopsy However, due to the advancements in diagnostic armamentarium, ante-mortem diagnosis of this anomaly has become easy.
Chest radiography reveals large basal homogenous opacity corresponding to the shifted heart and great mediastinal vascular structures with signs of volume loss reflected by the raised hemidiaphragm, and mediastinal shift. There is herniation of opposite hypertrophied lung with associated lucent opacity in the affected hemithorax in the upper zone. On radiograph, the differential diagnosis includes total lung collapse, diaphragmatic hernia, cystic adenomatoid lung malformations, pulmonary sequestration, scimitar syndrome, and prior pneumonectomy [11].
CT is an excellent modality as it provides exquisite details about the anatomy of tracheo-bronchial tree, lungs, heart, and major mediastinal vessels. CT shows direct evidence of continuation of trachea into a single bronchus with contralateral absent bronchus and lung. The main pulmonary artery also does not bifurcate and continues into a single pulmonary artery. The pulmonary veins on the affected side are absent. Heart and great mediastinal vessels are shifted into the affected hemithorax. The heart usually lies flush with the chest wall. There is variable degree of proliferation of fat on the affected side [12].
There is enlargement of the solitary lung. This enlargement represents true hypertrophy of the lung to meet the physiological demands of the body. It is essential to demonstrate and report any tracheal or bronchial narrowing or kinking due to the mediastinal shift. Any compression of mediastinal vessels is essential to report as these have therapeutic implications [4].
Due to the refinements in ultrasound and magnetic resonance imaging, the condition can be diagnosed antenatally [13].
Asymptomatic cases do not require any specific treatment apart from regular follow-up. However, earnest treatment is required for those presenting with lower respiratory tract infections. Some patients may be the candidates for corrective surgery to alleviate symptoms of trachea-bronchial narrowing or kinking or vascular compression. The procedures aimed at mediastinal stabilization include slide tracheoplasty, aortopexy, aortic resection and grafting, expansion prosthesis, and detour of the left pulmonary artery [14]. The prognosis of this condition is impacted by two factors of associated congenital malformations and the status of the normal lung. Any acquired affliction of the normal lung particularly infections can be life threatening and must be treated aggressively for a favorable outcome.