Cystic lesions of the fetal chest include a broad list of anomalies that differ in origin, prenatal natural history, pathophysiology, and implications for management of the fetus. The most common lesions of the fetal chest include congenital pulmonary airway malformation (CPAM) (see Chapter 35) and bronchopulmonary sequestration (BPS) (see Chapter 34). This chapter focuses on other less common causes of fetal thoracic cysts including bronchogenic cysts, congenital lobar emphysema, pericardial cyst, thymic cyst, esophageal duplication cysts, and neurenteric cysts.

Bronchogenic cysts result from premature foregut remnants that originate in embryonic bud tissue prior to the formation of the bronchi. While bronchogenic cysts can share common features with esophageal duplication cysts, they are histologically characterized by the presence of cartilage, smooth muscle, and glands in the cyst wall (McAdams et al., 2000; Stocker, 2002; Langston, 2003). Most bronchogenic cysts occur in the mediastinum adjacent to the distal trachea or mainstem bronchi (McAdams et al., 2000; Stocker, 2002). Bronchogenic cysts can also be found within the lung parenchyma (McAdams et al., 2000; Langston, 2003). However, this is a controversial point, as some have considered these lesions a form of type I CPAM (Stocker, 2002). Bronchogenic cysts are unilocular and do not communicate with the tracheobronchial tree and are usually filled with mucus. Bronchogenic cysts can enlarge to produce airway compression and may ulcerate the cyst wall lining due to ectopic gastric mucosa (Eber, 2007). Histologically, bronchogenic cysts are lined with ciliated columnar epithelium and contain bronchial mucous glands, elastic tissue, and hyaline cartilage (Maier, 1948) (Figure 36-1).

Congenital lobar emphysema is characterized by over-inflation, either by retained fluid produced in the lobe or segment prenatally or by air trapped in the lobe or segment postnatally (Clements, 1999). Congenital lobar emphysema is thought to be a consequence of bronchial valve mechanism from localized malformations or deficiencies of bronchial cartilage, mucosal folds, or extrinsic bronchial compression (Clements, 1999). This same appearance may be due to an increase in the number of normally expanded alveoli, called a polyalveolar lobe (Hislop and Reid, 1970; Mani et al., 2004). Almost half of all cases involve the left upper lobe (Eber, 2007), and the right middle lobe is the next most often affected. These lesions rarely contain cysts and appear homogeneous by ultrasound and MRI.

Some authors have suggested that both congenital lobar emphysema and polyalveolar lobe may be two outcomes of a similar inciting lesion during lung development (Mani et al., 2004). The timing of the lesion during lung development may account for different anatomical and functional bronchial abnormalities (Mani et al., 2004). Prenatally, lobar emphysema appears as a homogeneously enlarged lobe or segment that may be impossible to distinguish from a type III CPAM. Lobar emphysema tends to follow a different prenatal history from type III CPAM and can continue to enlarge slowly throughout gestation with resulting mediastinal shift and compression of adjacent normal lungs. Congenital lobar emphysema may be associated with other anomalies including cardiac anomalies in 12% to 14% of patients (Stocker et al., 1977; Kuga et al., 2001).

Pericardial cysts are rare lesions that are typically located at the cardiophrenic angle. Approximately 50% to 70% of pericardial cysts occurring in the costophrenic angle occur on the right side, whereas 28% to 38% occur on the left. More rarely, pericardial cysts may occur in other mediastinal locations not adjacent to the diaphragm (8–11%) (Stoller et al., 1986; Patel et al., 2004). Pericardial cysts are mesothelial lined with clear watery fluid. Pericardial cysts are usually asymptomatic but may cause compression of adjacent structures. Bernasconi et al. (2007) reported a small case series of prenatally diagnosed pericardial cysts that all spontaneously resolved without intervention by 28 weeks’ gestation. However, there have been larger pericardial cysts that have been associated with fetal hydrops or with cardiac or respiratory compromise at birth due to impaired cardiac filling or airway compression (Fernandes et al., 1991; Muraskas et al., 1993; Rizalar et al., 1995; Daher et al., 1996; Macaulay et al., 1997; Wilkinson et al., 1999; Jung et al., 2000). Aspiration of cyst fluid has been associated with resolution of hydrops caused by a pericardial cyst (Gulrajani et al., 1993; Muraskas et al., 1993; Macaulay et al., 1997).

Thymic cysts are rare, accounting for less than 4% of all anterior mediastinal masses postnatally (Kelley et al., 1997; Tollefson et al., 2001). Thymic cysts arise from remnants of the thymopharyngeal duct that can occur anywhere from the hyoid bone to mediastinum (McEwing and Chaoui, 2005). Thymic cysts are usually asymptomatic and are usually an incidental finding but respiratory complications, vocal cord paralysis, and dysphagia have been reported (Rudick and Wood, 1980; Dunne and Weksberg, 1983; Graeber et al., 1984; Wernicke and Diederich, 1994; Tollefson et al., 2001). Prenatal diagnosis of asymptomatic thymic cysts has been reported (deMiguel Campos et al., 1997; McEwing and Chaoui, 2005).

Enteric duplication cysts of the mediastinum may or may not communicate with the gastrointestinal lumen, either in the chest or below the diaphragm (Jaggers and Balsara, 2004). There can be associated vertebral anomalies, but there is no communication with the spinal cord or meninges. Enteric duplication cysts may have ectopic gastric mucosa that can cause pain or bleeding due to peptic ulceration. In contrast, a neurenteric cyst that contains both ectodermal and endodermal or neurogenic elements is characterized by a connection to the meninges and spinal cord by a narrow stalk. Neurenteric cysts are associated with spinal anomalies such as congenital scoliosis, hemivertebrae, or spina bifida.

All of the cystic lesions described above are rare conditions for which only anecdotal reports of prenatal diagnosis exist. There are no reliable estimates of their incidence.

Bronchogenic cysts are rarely diagnosed prenatally but may be suspected from the sonographic appearance of a unilocular fluid-filled cyst in the middle or posterior mediastinum (Avni et al., 1986). Alternatively, a bronchogenic cyst may be suggested by mass effect on adjacent structures (Figure 36-1). Patel et al. (2004) diagnosed a cystic mass at the time of fetal echocardiogram compressing the left atrium. Bronchogenic cysts can also present in atypical locations such as in the neck or subdiaphragmatic but retain the unilocular fluid-filled cystic appearance (Bagolan et al., 1999).

Esophageal duplication cysts are second only to those of the ileum comprising 15% to 20% of all reported duplications (Benson et al., 1985; Berrocal et al., 1999). Esophageal duplications may be tubular or sausage-shaped but most often are spherical cysts that have a thick wall of smooth muscle and an inner lining of gastrointestinal mucosa that may include gastric mucosa (Tam et al., 1987). These duplications are intimately associated with the esophagus but may appear as an isolated posterior mediastinal unilocular cyst on ultrasound. On T₁-weighted fetal MRI images, esophageal duplications may appear as a discrete, sharply defined low signal intensity mass and high signal intensity on T₂-weighted images (Berrocal et al., 2003). Esophageal duplications may enlarge sufficiently to obstruct the esophageal lumen and cause polyhydramnios (Gul et al., 2004).

Only a few thymic cysts have been diagnosed prenatally, usually recognized as an intrathoracic cyst and confirmed to be thymic postnatally (de Miguel Campos et al., 1997; McEwing and Chaoui, 2005). The normal fetal thymus, lying immediately posterior to the sternum, can be either hyperechoic or hypoechoic with a tendency to become more hypoechoic as the gestation progresses (Felker et al., 1989). Thymic cysts arise from remnants of the thymopharangeal duct and may appear from the level of the hyoid bone to the carina (Han et al., 2001). These cysts tend to present in the anterior mediastinum and are usually small. If completely surrounded by thymic tissue, a diagnosis of thymic cyst can be made. However, in a large mediastinal cyst, it may be difficult to differentiate a thymic cyst from other etiologies such as lymphangioma, thyroid cyst, parathyroid cyst, bronchogenic cyst, or pericardial cyst (Tollefson et al., 2001).