Successful development of a functional lung involves multiple biochemical and physical processes. The process of lung growth starts to occur at the 4th-5th week of gestation and reaches maturity by the 36th week. Any pathologic insult to any level of lung development may cause disruption of pulmonary growth. The exact etiology of pulmonary agenesis has yet to be understood, however studies show that genetic factors, vitamin A deficiency in pregnancy, viruses and chromosomal abnormalities may play a roles in the pathogenesis of the disease. Schneider and Schwatbe 5 have classified lung agenesis into 3 groups (Table 1), with our patient classified as Type 1 due to the complete absence of right lung parenchyma, vessels and bronchus.
- Type I (Agenesis): Complete absence of lung and bronchus and absence of blood vessels to the affected side.
- Type II (Aplasia): Rudimentary bronchus with complete absence of lung tissue.
- Type III (Hypoplasia): Presence of variable amounts of lung parenchyma,bronchial tree and supporting vasculature.
Almost half of the cases diagnosed present with multiple congenital anomalies such as cardiovascular, musculoskeletal, genitourinary and gastrointestinal defects. The most common being cardiac congenital defects. A study done by Booth and Berry 6 (1976), tabulated 9 living patients with pulmonary agenesis with their corresponding cardiac anomalies, the most common being patent ductus arteriosus. The cardiac shunt is said to be possibly protective by its ability to act as a bidirectional shunt during attacks of severe hypoxia. Another incidental and rare finding upon CT scan was tracheal stenosis. Tracheal stenosis can be congenital in nature or be caused by extrinsic compression from the overstretched aorta due to displacement and narrowing of the mediastinal compartment.
Patients with lung agenesis are at increased risk for respiratory tract infections due to maldevelopment of cilia and tracheobronchial tree cartilages which may hinder proper drainage of the functioning lung and may act as a reservoir for infection.
With difficulty, this disease entity may be diagnosed early during the prenatal period by ultrasound and is visualized as a hyperechoic hemithorax with associated mediastinal shift. Radiographically, and similarly on CT imaging, patients with right pulmonary agenesis will show an opacified hemithorax with resultant displacement of the heart and mediastinal structures towards the affected side (Figure 1 ). There will also be dextroposed heart due to the inequality of pressures intrathoracically and resultant pulling of the mediastinal structures towards the affected side. Often herniation of the left lung across the mediastinum is seen as a compensatory mechanism of hyperaeration. On the side of agenetic lung, the posterior ribs may be narrowed in appearance due volume loss because of the absence lung parenchyma. Given the clinical history of respiratory distress, a radiographic differential for a white out lung with mediastinal shift would include complete atelectasis, diaphragmatic hernia and pulmonary agenesis/hypoplasia. Although agenesis of the lung is a very rare entity, it must always be entertained because it is life threatening and may cause mortality especially in the setting of infection.
Conclusion
In patients that present with repeated respiratory infections with a persistently opacified hemithorax with accompanying ipsilateral mediastinal shift on serial radiographs, computed tomography is recommended in order obtain an accurate diagnosis of infection and rule out possible congenital malformations such as pulmonary agenesis. Despite being a rare congenital anomaly, the diagnosis must always be kept in mind to reduce mortality and improve patient prognosis.