Pulmonary hypoplasia

Last revised by Yaïr Glick on 25 Feb 2024

Pulmonary hypoplasia refers to underdevelopment of one or both lungs. This can be rapidly fatal at birth or mild, escaping detection for decades. It is most often secondary to congenital abnormalities that either restrict intrathoracic space or alter pulmonary fluid dynamics.

Pulmonary hypoplasia has been reported to affect 1.4% of all live births 13; however, the true prevalence is difficult to establish, as some die in utero and mild cases may go undetected. In cases of premature rupture of membranes at 15-28 weeks gestation, the reported prevalence of pulmonary hypoplasia is 9-28%, the variation being due to the lack of strict diagnostic criteria 2.

Pulmonary hypoplasia is characterized by disproportionately low lung volume and weight with consequent reduced gas-exchange. However lung weight can be increased by edema and congestion and so histological features such as low alveolar count, reduced branching of airways, fewer, smaller vessels and low DNA content are helpful findings. Unilateral pulmonary hypoplasia can be associated with compensatory hypertrophy of the contralateral lung.

The lungs are derived from the primitive foregut and vascular development follows airway development. Disease that affects the earlier stages of fetal lung development are more severe. The stages of lung development are:

  • embryonic: day 22-6 weeks: formation of the larynx, trachea, lung primordia, lobes, and bronchopulmonary segments

  • pseudoglandular: 5-17 weeks: terminal bronchioles, arterial system, cartilage, smooth muscle, cilia, lung fluid production and movement

  • canalicular: 16-25 weeks: respiratory units and capillary network

  • saccular: 24 weeks - birth: increasing surface-area for efficient gas-exchange and increasing surfactant production

  • alveolar: 36 weeks' gestation to 8 years: increasing alveolar maturity, number, and size

Primary pulmonary hypoplasia is rare and congenital acinar dysplasia is typically lethal. Milder forms of hypoplasia accompany trisomy 21. The disease can be multifactorial:

  • genetic mutations may affect any of the various growth factors 1616. which promote cell proliferation and differentiation, bronchial branching, angiogenesis, alveolar cell proliferation, and alveolarisation

  • environmental

  • maternal

  • nutritional

Secondary pulmonary hypoplasia has two main underlying causes: lack of space within the pleural cavity or abnormal fluid dynamics.

During the pseudoglandular phase of development, airway contractions and fetal breathing move fluid in and out of the lungs. Positive pressure (about 2.5 mm water) maintains patency of the distal airways, stimulating branching and development of terminal sacs. Excessive fluid drainage and oligohydramnios interfere with this development.

50-85% of cases of pulmonary hypoplasia are associated with other congenital anomalies. Causes include:

Extrathoracic causes include:

Other associations include:

Pulmonary hypoplasia is commonly lethal in the neonatal period. Milder forms present with neonatal respiratory insufficiency, pulmonary hypertension and persistent fetal circulation, bronchopulmonary dysplasia, pulmonary hemorrhage, recurrent infections, bronchiectasis, and hypoxemia exacerbated by exercise. Associated congenital abnormalities are often apparent. Occasionally, the first presentation maybe an abnormal chest radiograph in an adult.

Pulmonary function tests can demonstrate an obstructive or restrictive pattern and reduced diffusion capacity.

Possible findings include

  • chest wall deformity

  • small lung with mediastinal shift and herniation of contralateral lung

  • clear lung unless respiratory distress syndrome

  • features of underlying cause

Antenatal ultrasound may show ancillary features such as the presence of oligohydramnios and/or also show any of the causative anomalies.

Several sonographic parameters may give indirect clues as to the presence and extent of pulmonary hypoplasia. Such include:

In the case of intrathoracic causes, both these parameters can be normal.

Peripheral pulmonary arterial resistance is often increased with pulmonary hypoplasia.

Fetal MRI may be useful to more accurately calculate lung volumes.

  • T1: normal fetal lung is usually low signal on T1

  • T2: normal fetal lung is usually high signal on T2; fetuses with pulmonary hypoplasia are thought to have low signal 1

Pulmonary hypoplasia is often a critical determinant of fetal survival, however underlying causes affect both prognosis and treatment. Perinatal mortality rate is typically between 70 - 95%. Right-sided lesions and genetic abnormalities carry a poor prognosis.

PPROM: corticosteroids promote lung maturation in fetuses over 24 weeks gestation. Amnioinfusions decrease the incidence of pulmonary hypoplasia.

Congenital diaphragmatic hernia: fetal tracheal occlusion by balloon causes distal fluid distension which promotes lung maturation.

Congenital pulmonary airway malformation and congenital diaphragmatic hernia: fetal surgery may be attempted.

Complications such as tracheomalacia, persistent pulmonary hypertension, and pneumothorax may require treatment. Growth retardation is common.

In adult cases of unilateral pulmonary hypoplasia, consider conditions associated with a small hemithorax such as:

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