Pulmonary underdevelopment has been classified into three categories:
- Pulmonary agenesis
Complete absence of the lung parenchyma, bronchus and pulmonary vasculature
- Pulmonary aplasia
Blind-ending rudimentary bronchus is present, without lung parenchyma or pulmonary vasculature
- Pulmonary hypoplasia
Bronchus and rudimentary lung are present, however, the airways, alveoli and pulmonary vessels are decreased in size and number
It has been hypothesized that abnormal blood flow in the dorsal aortic arch during the 4th week of gestation (embryonic phase) causes pulmonary agenesis. Unilateral pulmonary agenesis is difficult to diagnose with prenatal ultrasonography;, however, it can be suspected on the basis of mediastinal shift. More than 50% of affected fetuses have other abnormalities involving the cardiovascular (patent ductus arteriosus, patent foramen ovale), gastrointestinal (tracheoesophageal fistula, imperforate anus), genitourinary, or skeletal (limb anomalies, vertebral segmentation anomalies) system. Imaging findings in pulmonary aplasia and agenesis are similar, except for the presence of short blind ending bronchus in aplasia. Postnatal radiography demonstrates diffuse opacification of the involved hemythorax with ipsilateral mediastinal shift and computed tomography helps confirm the absence of the lung parenchyma, bronchus and pulmonary artery on the involved side.
Pulmonary hypoplasia can be primary or secondary. Primary pulmonary hypoplasia, in wich a cause cannot be elucidated , is much less common than secondary hypoplasia. The majority of cases of pulmonary hypoplasia are secondary to a process limiting the thoracic space for lung development, which can be either intrathoracis or extrathoracic. The most common intrathoracic cause is congenital diaphragmatic hernia, which is left sided in 75-90% of cases, right sided in 10%, and bilateral in 5%. Left-sided congenital diaphragmatic hernia is relatively easier to detect due to the presence of an identifiable fluid-filled stomach in the thorax. In right-sided congenital diaphragmatic hernia, the liver herniates into the chest, which may be difficult to detect due to the solid echotexture of the liver. The herniated liver can be confused with a mass originating in the lung. Color Doppler imaging maybe helpful in identifying the portal and hepatic veins. Magnetic resonance imaging provides greater soft tissue contrast, which is useful in assessing the size of the hernia and the location of other abdominal viscera. Other intrathoracic causes of the pulmonary hypoplasia include congenital pulmonary airway malformations, broncho-pulmonary sequestrations, a cardiac or mediastinal mass, lymphatic malformations, and agenesis of the diaphragm.
The most common extrathoracic cause of pulmonary hypoplasia is severe oligohydramnions, occuring secondary to either fetal genitourinary anomalies such as renal agenesis, cystic renal dysplasia and urinary tract obstruction or prolonged rupture of membranes. A hypoplastic thorax, occurs in skeletal dysplasias, such as thanatophoric dysplasia or Jeune syndrome, in wich a small and rigid thoracic cage causes pulmonary hypoplasia. Antenatally, thoracic circumference measurements are obtained in an axial plane at the level of the four-chamber view of the heart.