Failure of the lung to develop to its normal size. Primary isolated pulmonary hypoplasia is rare. Secondary pulmonary hypoplasia is more common and occurs in association with congenital diaphragmatic hernia, oligohydramnios (mostly related to renal dysfunction), skeletal dysplasias, fetal hydrops, malformations of the central nervous system, and neuromuscular diseases.
Isolated primary pulmonary hypoplasia is very rare. However, conditions such as congenital diaphragmatic hernia are more common and inevitably associated with pulmonary hypoplasia. There is no sex or race predilection.
Possibility of autosomal recessive transmission in the isolated form has been described.
There is a decrease in lung volume and weight and there is a commensurate decrease in pulmonary function. Microscopic examination may reveal absent or reduced development of the lungs with a deficit in any combination of tubular bronchioles, acini, or alveoli, incapable of significant gas exchange or reduced airway generation, with fewer and smaller alveoli.
Based on measurement of lung volumes (eg, whole body plethysmography or inert gas dilution method) and the exclusion of other pathologies that may cause tachypnea. Oligohydramnios is a well-known risk factor. Chest CT scan or ante- and postnatal MRI are presently the diagnostic tools of choice.
The symptoms depend on the severity of the hypoplasia. Pulmonary hypoplasia is a (milder) form of pulmonary agenesis and can occur unilaterally or bilaterally. Whereas unilateral pulmonary hypoplasia has the same frequency for each side, left sided lesions have a much better prognosis. Secondary pulmonary hypoplasia describes a decrease in intrathoracic volume as a result of extrathoracic compression (eg, oligohydramnios secondary to renal agenesis or dysplasia, congenital thoracic or skeletal dysplasia causing thoracic constriction and leading to limited lung development with bilateral pulmonary hypoplasia) or intrathoracic compression (eg, congenital diaphragmatic hernia, polycystic kidney disease [secondary to the large abdominal mass], tumors, congenital cystic adenomatoid malformation, or large pleural effusions). Furthermore, there is an association with malformations of the central nervous system (eg, anencephaly) and neuromuscular diseases (affecting respiratory muscles/diaphragm). In a few cases, abnormalities of the face (hemifacial microsomia, dysplastic ears, torticollis) or jaw (unilateral mandibulofacial dysostosis) ipsilateral to the side of the pulmonary lesion (if unilateral) have been described. Anomalies of the extremities (in the majority of cases, the arm ipsilateral to the lesion) are not uncommon and include ulnar, radial, and thumb anomalies. Thoracic asymmetry may be associated with vertebral anomalies. The diaphragm may be high riding on the affected side, but in contrast to hemidiaphragmatic paralysis, it is not paralyzed and functions normally. Unilateral pulmonary hypoplasia must be differentiated from other conditions potentially resulting in mediastinal shift, such as cystic adenomatoid malformation, diaphragmatic hernia, or bronchopulmonary sequestration. Inhaled nitric oxide does not improve survival in preterm neonates with pulmonary hypoplasia.