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Congenital Cystic Adenomatoid Malformation
Updated: Oct 16, 2007
Introduction
Background
Ch'in and Tang first described cystic adenomatoid malformation (CAM) as a distinct entity in 1949.1 CAM is a developmental hamartomatous abnormality of the lung, with adenomatoid proliferation of cysts resembling bronchioles. CAM represents approximately 25% of all congenital lung lesions.
Pathophysiology
Pathogenesis and pathophysiologic features
CAM is believed to result from focal arrest in fetal lung development before the seventh week of gestation secondary to a variety of pulmonary insults. Depending on the time and type of insult, 4-26% of cases can be associated with other congenital abnormalities. However, arrest of pulmonary development with distortion of architectural differentiation may take place at any stage of embryonic development.
CAM differs from normal lung tissue because of a combination of increased cell proliferation and decreased apoptosis. A well-defined intrapulmonary bronchial system is lacking, and normally formed bronchi supplying the mass are absent.
Types of CAM
CAM is subdivided into 3 major types2 :
- Type I lesions, the most common, are composed of 1 or more cysts measuring 2-10 cm in diameter. Larger cysts are often accompanied by smaller cysts, and their walls contain muscle, elastic, or fibrous tissue. Cysts are frequently lined by pseudostratified columnar epithelial cells, which occasionally produce mucin. Mucinogenic differentiation is unique to this subtype of CAM.
- Type II lesions are characterized by small, relatively uniform cysts resembling bronchioles. These cysts are lined by cuboid-to-columnar epithelium and have a thin fibromuscular wall. The cysts generally measure 0.5-2 cm in diameter.
- Type III lesions consist of microscopic, adenomatoid cysts and are grossly a solid mass without obvious cyst formation. Microscopic adenomatoid cysts are present.
CAM receives its blood supply from the pulmonary circulation and is not sequestered from the tracheobronchial tree. However, type II and III lesions can occasionally coexist with extralobar sequestration, and in such cases, they may receive systemic arterial supply. CAM may also occur in combination with a polyalveolar lobe. A polyalveolar lobe is a form of congenital emphysema with increased number of alveoli with normal bronchi and pulmonary vasculature. CAM usually occurs early in fetal life, whereas polyalveolar lobe occurs late.
Differential diagnosis
CAM is differentiated from other congenital cystic disease by 5 characteristics3 :
- Absence of bronchial cartilage (unless it is trapped within the lesion)
- Absence of bronchial tubular glands
- Presence of tall columnar mucinous epithelium
- Overproduction of terminal bronchiolar structures without alveolar differentiation, except in the subpleural areas
- Massive enlargement of the affected lobe that displaces other thoracic structures
Frequency
International
In Canada, the estimated incidence of CAM is 1 case per 25,000-35,000 pregnancies4 .
Mortality/Morbidity
The prognosis primarily depends on the size of the lesion. In a Canadian series of 48 patients, the incidence of postnatal demise was 10% (10 of 40 patients) with 8 spontaneous and voluntary abortions.4 Larger lesions have a higher incidence of mediastinal shift, vascular compromise, polyhydramnios, pulmonary hypoplasia, and hydrops, which may lead to intrauterine fetal demise or neonatal death (see Polyhydramnios, Pulmonary Hypoplasia, and Hydrops Fetalis).
Type III CAM tends to be extensive and therefore tends to have a poor prognosis. The prognosis is also poor with bilateral lung involvement, prematurity, and severe associated malformations. The most commonly associated anomalies occur in the type II form. The anomalies affect the renal (cystic disease, agenesis, dysgenesis), intestinal (atresias), cardiac, and osseous systems.
- When CAM is identified in utero, as many as 56% of the lesions detected can regress spontaneously, although they may initially progress. As the lesion decreases in size, mediastinal shift is corrected. Persistent lesions may only be discovered later in life, and some may be asymptomatic. Eventual removal of even asymptomatic masses is recommended because of potential risk of secondary infection and hemorrhage and because of reports of carcinomas arising in CAM.
- In fetuses with life-threatening lesions, such as the development of hydrops, the anticipated mortality is nearly 100%.
Race
No clear racial predilection for CAM exists.
Sex
Sex-related incidences are equal for CAM.
Age
Most cases of CAM are diagnosed in the patient's first 6 months of life, with 70% of patients presenting in the first month of life. As many as 90% of cases are reported within the patient's first 2 years of life. Occasionally, CAM is discovered later in life, usually as a result of chronic or recurrent pulmonary infection.
When CAM was diagnosed prenatally in one study, the mean gestational age at diagnosis was 22.6 weeks ± 3.3.
Anatomy
Communication with the tracheobronchial tree usually is retained, and the vascular supply and venous drainage are to the pulmonary circulation, unless CAM is associated with sequestration, as discussed above. Lesions occur with equal frequency in the lungs, but the lesions have a slight predilection for the upper lobes. Lobar involvement is seen most often, but multiple lobes, the entire lung, or segments of both lungs may be involved.
Presentation
In the newborn, 80% of cases of CAM present with some degree of respiratory distress secondary to mass effect and pulmonary compression or hypoplasia. Severe symptoms may be related to air trapping within the lesions. The affected region is dull to percussion, and air entry is poor. Older patients may present with persistent or recurrent pneumonia.
Preferred Examination
CAM may be initially detected during prenatal ultrasonography. After birth, chest radiography should be performed first. Although lesions remain filled with fluid, postnatal sonography can be used for a more detailed assessment, particularly in type III lesions. Once lesions are air-filled, CT scanning is necessary for determination of the type and extent of the lesions.
Limitations of Techniques
Prenatal ultrasonography is accurate in diagnosing CAM. Prenatally diagnosed lesions may be asymptomatic at birth (71%), and they have normal radiographic findings (57%). A concurrent sequestration may not be identified. Usually, radiographic findings are apparent in a symptomatic individual, but they may not be as apparent in an asymptomatic child.
Most often, the diagnosis can be made by using plain radiographs. CT scans may be used to diagnose confusing cases. Overlapping CT features exist among cases of CAM, pulmonary sequestration, bronchogenic cyst, and other foregut malformations. CT is more accurate than radiography or ultrasonography in classifying the type of CAM.
Differential Diagnoses
Bronchogenic Cyst
Congenital Diaphragmatic Hernia
Congenital Lobar Emphysema
Pulmonary Interstitial Emphysema
Pulmonary Sequestration
Other Problems to Be Considered
Postinfectious pneumatoceles
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References
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Further Reading
Keywords
CAM, congenital adenomatoid malformation, congenital cystic adenomatoid malformation, developmental hamartomatous abnormality, adenomatoid cysts, fetal lung development
