Pulmonary Hypoplasia
Jennifer M. Boland, M.D.
Marie-Christine Aubry, M.D.
Definition
Pulmonary hypoplasia (PH) is an incomplete development of the lung, primarily due to a decreased amount of alveolated parenchyma, although the number of airways is also decreased.1 This leads to abnormally low lung weight and small size, and usually a decreased alveolar radial count, which is a measure used to estimate number of alveoli per unit area.1,2 The reported incidence of PH in the general population ranges between 9 and 14/10,000 births.3,4 The estimated prevalence of some degree of PH from neonatal autopsy series ranges between 7% and 26%,3,5 and involvement may vary from an isolated lobe to complete involvement of bilateral lungs.5
Etiology and Pathogenesis
PH can be primary or secondary.1,6 Primary PH is characterized by isolated PH in the absence of other congenital anatomic abnormalities. It is thought to be extremely rare at 0.8 to 1.6 per 10,000 births3,4 and can be caused by a catastrophic congenital developmental abnormality such as acinar dysplasia.7 Other embryologic defects of the lung or vascular tissue may cause PH, or it may be secondary to an in utero vascular accident. Some examples of primary PH may show no obvious histopathologic abnormalities but have an abnormally low lung weight to body weight ratio and/or radial alveolar count (see below).6 Most cases of primary PH are sporadic, although familial cases have been reported.8,9 Primary PH may involve a unilateral lung.5
TABLE 12.1 Causes of Pulmonary Hypoplasia | ||||
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The vast majority of PH is secondary to another fetal abnormality (Table 12.1). Normal fetal lung growth is dependent on a number of physical factors including adequate intrathoracic space, sufficient volume of amniotic fluid, normal fetal breathing movements, and balance of fluid volume/pressure. Thus, any fetal, maternal, or placental abnormalities that interfere with these factors may potentially lead to PH. These abnormalities classically cause either intrathoracic or extrathoracic compression of the lungs with subsequent impairment of lung development (congenital diaphragmatic hernia, thoracic cysts and masses, etc.) or decreased amount of amniotic fluid reaching the alveolated parenchyma in utero (urogenital abnormalities, upper airway abnormalities, anencephaly, premature rupture of membranes, etc.).1,10,11
Clinical Presentation, Radiology, and Outcome
The clinical presentation and outcome of PH vary with its severity. Most cases are rapidly lethal,2,6 with a mortality rate of 50% to 80%.4 However, there are some sublethal forms,6 and the clinical manifestations of PH depend on the severity of the respiratory underdevelopment, which in turn depends on the underlying etiology and the gestational age of occurrence. For example, as would be expected, survival of infants with PH due to premature rupture of membranes depends on the gestational age at which rupture occurred.12 One study of fetuses and infants experiencing sudden unexplained death found a high incidence of PH and brainstem hypodevelopment, suggesting that subclinical cases of PH occur, possibly due to developmental issues involving the brainstem, and this may put infants at risk for sudden unexplained death.13
The clinical presentation of PH may vary from the classic lethal form with severe respiratory failure and hypoxemia leading to early neonatal death6 to less severe forms characterized by respiratory insufficiency with hemorrhage, bronchopulmonary dysplasia, or even mild respiratory symptoms in some cases. In most secondary cases of PH, the etiology is clear and the diagnosis of PH is easily made. However, some cases of primary PH may present with hypoxia and clear small lung fields, and it may be unclear that PH is the underlying cause, with other entities such as congenital heart disease and persistent fetal circulation entering the differential diagnosis.6 At birth, PH due to a space-occupying lesion is suspected in infants with respiratory distress who have decreased breath sounds on one side and contralateral shift of the heart sounds. Primary treatment is surgical excision/repair of the mass lesion (if present) with subsequent supportive care to provide oxygenation while pulmonary growth occurs.6,14 Outcome will depend on the severity of the hypoplasia and ability for the lung to maintain adequate gas exchange.6 If PH is the result of decreased amniotic fluid or other etiologies, the infant will have small clear lung fields on chest radiograph.6 Treatment is supportive with mechanical ventilation as needed, which is commonly complicated by air leaks with pneumothorax and pulmonary interstitial emphysema.6 Extracorporeal membrane oxygenation (ECMO) may be successfully used for patients with PH, with survival observed in infants having lung volumes at least 45% predicted for age-matched controls.15
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