Congenital lung malformations represent a diverse spectrum of developmental defects. A high index of suspicion for these relatively rare malformations is essential in evaluating a thoracic lesion in a child. The clinical presentation is variable, presenting as respiratory distress in a newborn or remaining asymptomatic until adolescence or even adulthood. The revolution in prenatal imaging, with the ubiquity of antenatal ultrasound, has enabled earlier and more frequent detection of pulmonary anomalies: The most commonly identified prenatal pulmonary anomaly remains the cystic lung lesion with an estimated incidence of 1 in 8300 to 35,000 births.^1,2 Prenatal characteristics such as the presence and size of cysts, lung echogenicity, anatomic location, vascular supply, and associated pathophysiology including hydrops and mediastinal shift can characterize a lung lesion (see Table 4-1). Since congenital lung lesions vary in behavior from regression to causing fetal demise, the prognosis of a prenatally detected lesion is not always evident. Advances in prenatal imaging have been quintessential in defining the criteria for early therapeutic intervention in lesions adversely affecting fetal development as opposed to expectant management with postnatal resection of symptomatic cases.

A brief highlight of normal lung development is useful in conceptualizing the development of bronchopulmonary foregut malformations. The primitive foregut is the anlage of both pulmonary and gastrointestinal systems, which explains the persistent connection in some anomalies.³ At 3 weeks, in the embryonic stage of gestation, lung development is initiated by the divergence of the tracheal diverticulum from the primitive foregut. Maturation of the conducting airways from the trachea to the terminal bronchioles is completed by the end of the pseudoglandular stage (16th week). The development of the highly differentiated respiratory unit for gas exchange occurs in the remaining 24 weeks in utero during the cannalicular and terminal sac stages wherein the distal respiratory bronchioles and alveoli are added to the pulmonary matrix. In the final 4 weeks in utero up until 8 years of age, the lung’s surface area undergoes an exponential increase in alveolar growth.

Bronchogenic cysts arise from abnormal budding of the tracheobronchial tree as true foregut anomalies to form a nonfunctioning cystic mass.⁴ Two-thirds of these cysts separate early in development and thus are centrally located in the mediastinum, firmly adherent to the carina. These solitary cysts are well-encapsulated spherical lesions, lined with bronchial epithelium and mucous glands. They remain asymptomatic until superimposing infection of the fluid-filled cyst occurs: Less frequently they can be seen in a newborn due to the size of a large lesion compressing surrounding normal lung. Bronchogenic cysts should be excised to avoid further complications of infection, air trapping, and hemoptysis. Otherwise, they present as the most common benign mediastinal cyst in adulthood.

Those that appear later in development as peripheral cysts are less common and are located in the periphery of lower lobes. Since they occur during rapid bronchial division, a bronchial communication is more common. These are lined by ciliated respiratory epithelium resulting from the pseudoglandular stage of development. Respiratory distress is the most common presentation, causing cyanosis, retractions, and grunting from rapid enlargement of the cyst. Air trapping occurs during expiration due to the cyst’s lack of cartilaginous support: A ball valve mechanism results in continued cyst expansion. The mass effect of the cyst causes further airway compression and distal pulmonary hyperinflation, worsening the respiratory compromise. The cyst can be misdiagnosed as a pneumothorax, especially in the setting of a large coexistent bronchial communication; similarly a pneumothorax can be an anticipated complication.

Many of the asymptomatic lesions are discovered incidentally, or when infectious complications intervene with an air fluid level or with recurrent pneumonias localized to the same lobe, pneumothorax, hemoptysis, and upper gastrointestinal bleeding.^5–7 In congenital cystic lung lesions, 3 percent asymptomatic lesions become symptomatic within 10 months⁷ and up to 10 percent can become symptomatic in 3 years.⁸ Although a chest radiograph is diagnostic in over three-fourth of the cases, thin section computed tomography (CT) scanning with contrast is highly accurate in demonstrating these lesions, particularly in the setting of recurrent respiratory exacerbations.

Regardless of location and presence of symptoms, surgical resection is mandated. Resection of even asymptomatic bronchogenic cysts is advised to precede symptoms to avoid complications. Mediastinal lesions can be enucleated thoroscopically or via a thoracotomy. Segmental or lobar resection is curative for peripheral lesions.

CPAM, formerly known as congenital cystic adenomatoid malformations (CCAM), are hamartomatous, multicystic, or mixed adenomatoid intrapulmonary masses, arising from the overgrowth of terminal respiratory bronchioles with suppression of alveolar growth. They are usually localized to one lobe, although they can affect all lobes with equal frequency. These cystic structures may interconnect and communicate with the tracheobronchial tree.

The current classification of CPAMs⁹ is a modification of the initial Stocker classification of CCAMs based on cyst size¹⁰ to include the site of origin in the tracheobronchial tree, assuming each type correlates to a perturbation increasingly distal along the airway from trachea/bronchus to acinar/alveolar sac (see Table 4-1). Type 0 malformations are the least common, are composed of multiple, small (<0.5 cm) cysts, and are uniformly fatal at birth (see Table 4-1). Type I malformations, the “large cyst lesion,” are the most common (>65%) with a typically dominant large cyst(s) (~10 cm). Type II malformations, “the small cyst lesion,” occur 20 to 25 percent of the time, consist of multiple intermediate size (0.5–2.0 cm) cysts, and are associated with the highest incidence of congenital anomalies (20%), including renal dysgenesis, cardiac anomalies, congenital diaphragmatic hernia (CDH), and congenital heart disease. Type III malformations, the only one known as “the adenomatoid lesion,” represent 8 percent of CPAMs and are solid appearing with tiny cysts (<1.5 cm) involving typically the entire lobe or lung. Poor prognostic indicators are often associated with Type III cysts since these microcystic lesions tend to be capable of inducing fetal hydrops and pulmonary hypoplasia. Type IV malformations are newly identified large cysts ~7 cm in size usually in one lobe that consists of nonciliated alveolar cells (Type I pneumocytes) in contrast to the ciliated cells in all other types. They often appear in newborns to 6 years of age and have an association with respiratory distress with pneumothorax or infection as well as an association with a risk of malignancy.^11–13

Prenatally the presentation of a CPAM is highly variable with detection as early as 12 to 14 weeks. For an attempt at a more clinically relevant classification, the ultrasonographic appearance was used as the basis by Adzick et al. using either macrocystic with cysts >5 cm (previously Type I) or microcystic with cysts <5 cm (Type II or III depending on the underlying histology).¹⁴ Macrocystic has a more favorable prognosis and microcystic is more solid with an unfavorable prognosis associated with more anomalies. What has more recently become evident is the recognition of “hybrid” lesions containing features of both CPAMs and bronchopulmonary sequestrations (BPS) indistinguishable until histologic review. Thus, the differential diagnosis of a prenatal cyst can include a variety of lesions, such as CPAM, bronchopulmonary sequestrations, bronchogenic cyst, congenital lobar emphysema (CLE), esophageal duplication cyst, and a CDH (see Fig. 4-1). Thus, further imaging with Doppler ultrasonography can detect the anomalous blood supply to the aorta found with sequestrations. CT imaging can correlate with a pathologic diagnosis of a CPAM with 100 percent concordance.¹⁵ Magnetic resonance imaging (MRI)/fetal MRI can assist in distinguishing whether an echogeneic lung mass is a Type III CPAM or a CDH given the heterogeneity of the mass.

The natural history of these lesions is unpredictable and can range from complete regression in 25 percent to 20 percent that are symptomatic prenatally, and then the ~5 percent category of those with such a rapid growth that mediastinal shift, caval obstruction, impaired cardiac filling, and subsequent nonimmune hydrops and polyhydramnios result.¹⁶ CPAMs without hydrops have a generally favorable survival rate, whereas those 5 percent of CPAMs that develop hydrops and anasarca are associated with a higher mortality rate of up to 100 percent without intervention.^17,18 Large masses causing fetal hydrops in a fetus <32 weeks require in utero intervention to reverse the hydrops and potential fetal demise.¹⁹ In macrocystic lesions, cyst aspiration and thoracoamniotic shunting have been used to alleviate mediastinal compression and hydrops. In contrast, in microcystic lesions, open fetal surgery has been a surgical option to treat those lesions causing hydrops fetalis. CPAMs may grow progressively or regress by unclear mechanisms. Maternal administration of betamethasone has been associated with resolution of hydrops and an increased survival in fetuses with high-risk CPAMs compared with historical controls^20,21: These findings may be related to a reduction in CPAM growth or improved maturation and need to be confirmed in a controlled trial as opposed to being compared to historical controls.²²