History: The term broncho-pulmonary foregut malformation was coined by Gerle et al in 1968 to describe pulmonary sequestration having patent congenital communications with the upper gastrointestinal tract. The primitive foregut gives rise to the pharynx and lower respiratory tract as well as upper gastrointestinal tract. The most common foregut cysts, bronchogenic cyst and the esophageal duplication cyst represent abnormal budding of the ventral and dorsal primitive foregut respectively, indicating their common origin from the primitive foregut.
Prevalence: Foregut cysts represent 15% of mediastinal masses in infants and children. Most of these cysts are in the perihilar region. The real incidence of bronchogenic cyst is unknown, since a large number of them are asymptomatic. They are extremely rare in the neonatal period. No sex predilection is apparent.
Etiology: Bronchogenic cysts result from an abnormal budding of the foregut. They may remain attached to the primitive tracheo-bronchial tree, in which case they are found along the trachea, in the mediastinum or within the pulmonary sequestration. If the outpouching separates from its site of origin, the cyst may migrate into the mediastinum, neck, pericardium, vertebrae, subpleural site and other sites (Jeanty).
Pathogenesis: Congenital cystic adenomatoid malformation, congenital lobar emphysema, pulmonary sequestrations and bronchogenic cysts are anomalies of foregut duplications which can present as cystic masses in the chest. They usually present as separate lesions but rarely, they can occur in combination.
The foregut cysts are lined by ciliated epithelium which lines both the early tracheo-bronchial tree and the esophagus. Bronchogenic cysts and extralobar sequestrations may originate from the embryonal foregut and thus may have a close embryologic relationship. Bronchogenic cyst is lined by a ciliated columnar epithelium similar to the kind lining the normal bronchi. It may contain cartilage, muscle and mucous glands. The size and location of the cysts are extremely variable. Bronchogenic cysts though rare represent the most common cystic lesion of the mediastinum.
Bronchogenic cysts are the result of anomalous development of the ventral foregut. They are usually single but may be rarely multiple. They have been found all along the tracheo-esophageal course, in perihilar or intraparenchymal sites, with a predilection for the area around the carina. Nevertheless these cysts have also been described in more remote locations, including the intra-atrial septum, neck, abdomen and retroperitoneal space.
Lesions arising before or during the separation of the embryonic foregut are situated in the mediastinum (30%). Lesions arising after separations are frequently situated within the lungs (70%). Intraparenchymal cysts have been consistently reported to be lined by respiratory epithelium and cartilage. This supports the hypothesis of a developmental error of the tracheo-bronchial tree rather than of he embryonic foregut.
Sonographic findings: A diagnosis can be made based on antenatal sonography, routine chest radiography and during evaluations for cardiac symptoms. Definitive diagnosis may be achieved using chest CT and MRI studies.
Conventional 2-view chest radiography : This typically shows a sharply demarcated spherical mass of variable size, most commonly located in the middle mediastinum around the carina. When the cyst is infected or contains secretions, it may appear as a solid tumor or may demonstrate an air fluid level.
Chest CT: Bronchogenic cysts appear as lesions with smooth borders and thin walls and may contain secretions, pus or blood. Calcification may be observed.
MRI: A homogeneous mass of moderate –to- bright intensity is observed on T2-weighted MRI. On T1-weighted images, lesions may vary in their intensity because of their protein content.
The finding on CT or MRI of a cystic lesion at the level of the carina is most frequently associated with a bronchogenic cyst.
Antenatal diagnosis of bronchogenic cysts are rare. The appearance of bronchogenic cyst has been described in-utero as an anechoic unilocular intrathoracic cyst. Romero described one case, in which case bronchogenic cyst was identified as a simple hypoechoic mass in the upper part of the right lung and measured 15 X 19 mm at 36 weeks of gestation. No associated hydrops was seen. The diagnosis was confirmed at a thoracotomy performed eight days postpartum. Levine described the antenatal diagnosis of a mediastinal bronchogenic cyst leading initially to unilateral and then bilateral bronchial obstruction. Bagolan et al described for the first time the prenatal ultrasound observation of an intra-abdominal bronchogenic cyst.
The site and the sonographic characteristics of the lesion made us consider the differential diagnosis of cystic neuroblastoma, adrenal hemorrhage, duplication of the inferior third of the esophagus and retroperitoneal cystic lymphangioma. The same ultrasound features are common to all these diagnostic options.
Implications for targeted examinations: In infants, these cysts may produce symptoms of recurrent cough, wheezing, pneumonia, atelectasis or obstructive emphysema as a result of airway or esophageal compression. Therefore, the in-utero visualization of the obstructing form of these anomalies is important because neonates can have severe respiratory distress at birth.
Differential diagnosis: The sonographic finding of an intra-abdominal cystic lesion renders the differential diagnosis difficult and cystic malformation of mesenteric, omental, choledochal, hepatic, ovarian and intestines have been reported in the literature (Bagolan).
The prenatal differential diagnosis of cysts of the fetal chest is complex and includes cystic adenomatoid malformation, diaphragmatic hernia, lung sequestration, bronchogenic cyst, laryngeal atresia, neuroenteric cyst, duplication of the oesophagus, congenital lobar emphysema, cystic adenomatoid malformation, Swyer-James syndrome and other mediastinal tumors.
The fetal prognosis depends mainly upon the etiology of the cyst, which is often impossible to establish antenatally, and also on the presence of associated anomalies. (Jauniaux).
Associated anomalies: The other broncho-pulmonary foregut malformations include tracheoesophageal fistula, oesophageal diverticulum, oesophageal cyst and lung sequestration. Vertebral abnormalities (hemivertebrae) are often associated with bronchogenic cyst of mediastinal origin.
Prognosis: Long term cyst related complications such as infection, rupture, bleeding and compression are common. A risk of malignant degeneration also exists.
Management: A prenatal diagnosis of thoracic or abdominal cyst does not indicate any variation in the management of either the pregnancy or its delivery. Furthermore with the exception of adrenal hemorrhage which is the only condition that does not require surgery, prenatal diagnosis does not change the indication for neonatal surgery. In the case of any of the other hypothesized lesions, surgery is indicated to enable a precise diagnosis to be made and to prevent or eliminate any possible symptoms caused by compression. The long term prognosis of asymptomatic bronchogenic cyst is unpredictable and the potential risks are life threatening. Many surgeons support surgical resection in these patients. The case of symptomatic patients is much clearer now and lobectomy is the standard treatment. No recurrence has been reported after complete resection of the cyst. Palliative procedures such as a transparietal, transbronchial or mediastinal puncture and aspiration may be considered in cases where patients are asymptomatic.
References:
1- Romero R., Jeanty P.
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