Congenital malformations of the airways and lungs make up approximately 10-15% of all malformations and are often found with other congenital anomalies (18-20%). The following review includes a description of two of the more common lung malformations: bronchogenic cysts and congenital cystic adenomatoid malformations (CCAM). The vast majority of foregut cysts found in infancy are bronchogenic cysts (1).
Bronchogenic cysts are one type of a foregut cyst (a closed epithelial-lined sac developing abnormally from both the upper gut and respiratory tract). A bronchogenic cyst is thought to develop as a diverticulum of the primitive foregut. Since most form very early, usually 4-8 weeks gestation and before the development of distal airways, they rarely connect to a normal bronchus. Most are right sided, midline and in close proximity to the tracheobronchial tree. On rare occasions they can separate the connection to the airway and migrate to the periphery, parahilar area or even below the diaphragm. Five categories have been described by location: 1) paratracheal 2) carinal 3) para-esophageal 4) hilar and 5) other. They may contain normal tracheal tissue including mucus glands, elastic tissue, smooth muscle and cartilage. They are lined with ciliated epithelium. They range from 2-10 cm in diameter. The cyst may contain serous (with the consistency of water) or proteinaceous fluid (2,3).
Congenital cystic adenomatoid malformation (CCAM) is a congenital bronchopulmonary anomaly resulting from a maldevelopment of the lung bud in the fetus (1). CCAMs are a defect of non-cartilage containing terminal respiratory structures, resulting from an abnormality occurring in the mid to late stages of lung development. Although these lesions are frequently described as hamartomatoid, they are not true hamartomas because skeletal muscle can be found in the wall of the cyst. The following is a list of distinguishing features that define the group: 1) absence of cartilage, 2) absence of bronchial tubular glands, 3) presence of tall columnar mucinous epithelium 4) increased production of terminal bronchiolar structures without alveolar differentiation 5) increased enlargement of the affected lobe (4).
There are at least 4 subtypes described, although type 0 is not compatible with life. The different subtypes are primarily described by their gross physical appearance, but they also differ by their variations in microscopic findings and embryologic origin. Each subtype has differing prognostic indications. Type 0 (most rare) is tracheobronchial in origin, with small, firm and granular lungs. Microscopically there are bronchial-like structures separated by mesenchymal tissue.
Type I (macrocystic subtype) is the most frequent variant (60-70%). It has bronchial-bronchiolar origins and at least one prominent cystic structure, although several smaller cysts may also be present. Type I malformations have little adenomatoid component and are mainly lined by ciliated pseudostratified epithelium. They contain cysts interspersed with bronchiolar and alveolar tissue.
Type II (microcystic subtype) is the next most frequent variant (15-20%). Smaller cysts with ciliated cuboidal or columnar epithelium are the dominant feature. It has a mix of cystic and adenomatoid components and is bronchial in origin. Between the cysts are distended respiratory bronchioles and alveolar tissue. These may also contain skeletal muscle. This subtype is associated with a higher incidence of other anomalies.
Type III (solid subtype) is rare (8-10%). It is a bulky lesion, with thin walled cysts. Type III is almost entirely adenomatoid in make-up. It is an airless mass of bronchiolar elements, lined by patchy ciliated cuboidal epithelium mixed with alveolar elements.
Some describe Type IV (10-15%) as a large cystic lesion in the periphery of the lung, believed to be of acinar origin. Others do not describe this subtype and incorporate it into the others. These cysts are lined by flattened pneumocytes (5-6).
The clinical manifestations of a bronchogenic cyst depend on size, location and whether there is a communication with the airway or esophagus. They can present with fever, dyspnea, stridor, chronic cough, chest pain, dysphagia, cyanosis, crackles, wheezing, pulmonary sepsis or suppuration of the cyst, respiratory distress or swelling. Bronchogenic cysts can present as a draining sinus, typically located in the suprasternal notch or supraclavicular area. Superior vena cava syndrome has been seen. They are asymptomatic in up to 30% (7).
CCAMs present early in the newborn period with respiratory distress (dyspnea, tachypnea, grunting, retractions or cyanosis) in approximately 75% of cases. The mass lesion comprised of growing cysts can compress the surrounding structures. Compression during development of the surrounding lung can cause pulmonary hypoplasia, maldevelopment of the heart and great vessels (may cause fetal hydrops), or hypoplasia of the airways (can lead to respiratory distress). For those who do not present in the newborn period, they may present at any point in life. The lesions can develop infections, as they do not have normal clearance mechanisms, leading to recurrent pulmonary sepsis. A higher percentage of these lesions are being diagnosed or suspected prenatally by ultrasound.
On chest radiographs, bronchogenic cysts usually appear as a spherical or ovoid mass close to the carina or mainstem bronchus. CCAMs appear as obvious solid or cystic masses with or without pleural effusion. Diagnosis is suspected by CXR, CT, MRI, endoscopy or fluoroscopy, but is confirmed by pathologic evaluation of tissue.
Bronchogenic cysts are most commonly confused with the other main type of foregut cysts, esophageal duplication cysts. The rest of the differential diagnosis includes cystic hygroma, thymoma, thyroid tumors, dermoid cyst, congenital lung emphysema, pulmonary abscess, pneumatocele, thyroglossal duct cyst, bronchial duct cyst, teratomas, necrotic cervical lymphadenopathy, neurogenic tumors, primary malignancy, lipoma and leiomyoma.
CCAMs are most frequently mistaken for congenital diaphragmatic hernia (since the cysts can resemble bowel gas in the chest on CXR) but the differential includes simple parenchymal cysts, infections, sequestration, mesenchymal cystic hamartomas, mesothelial cysts or cystic lymphangiectasis (8-10).
The treatment of choice in all forms of bronchogenic cysts and CCAMs is surgical excision, which also provides confirmation of the diagnosis. Bronchogenic cysts may rupture into a bronchus or pleura, bleed profusely or become infected. These complications can cause problems at the time of surgical excision or produce sudden death. If they have already been secondarily infected, the excision may have to be delayed until antibiotic treatment can clear the area of infection. If resection is not complete, recurrence is possible. For CCAM, early resection will allow for compensation of lung growth from the remaining sections, and prevents secondary infections, that otherwise commonly occur.
Left untreated, bronchogenic cysts may develop malignancy including rhabdomyosarcoma, leiomyosarcoma, or anaplastic carcinoma. In one study of symptomatic infants, there was 100% mortality without surgery. Of those infants undergoing surgery, mortality rates were reported to be 0-14%. The prognosis for those surviving surgery was good. In some, residual tracheomalacia or bronchomalacia may be present.
CCAM Type 0 is not compatible with life and these infants are usually stillborn, or spontaneously aborted. Type I lesions have the best prognosis. For those surviving surgical resection, the prognosis is excellent with compensatory lung growth of the remaining segments. Type II has a worse overall outcome compared to Type I, largely because of the other associated anomalies. Type III has a poor prognosis, due to the degree of hypoplasia frequently seen in the other lung segments. If untreated, there is also a potential for malignant transformation in CCAM.
Another important consideration for those patients with either type of lesion is air travel, when transport to a tertiary care center is needed for further management. The cystic lesions have been known to expand 30% in size during flight, which may cause a significant mass effect and further compression of vital structures. Care must be taken to avoid significant pressure changes by flying at low altitudes, or in special aircraft capable of pressurization to sea level.