Congenital Cystic Adenomatoid Malformation of the Lung

Definition

CCAM is a developmental, nonhereditary, usually mixed (solid and cystic) lung mass consisting of abnormal hamartomatous or dysplastic lung tissue and bronchoalveolar structures thought to result from abnormal branching of the immature bronchioles during early stages of lung morphogenesis.

Prevalence and Epidemiology

CCAM is the most common fetal hyperechogenic lung lesion and accounts for 50% to 75% of detected fetal lung abnormalities. These lesions may display dramatic changes during pregnancy, with spontaneous regression and total resolution in more than half of cases. Thus, postnatal studies probably underestimate the real incidence of lung lesions, with a commonly quoted incidence of 1 : 25,000 to 1 : 35,000. In prenatal studies performed in nonreferred populations, an incidence of 1 : 4000 to 1 : 6000 has been reported. Prenatal diagnosis has dramatically increased as a result of improvement of US equipment over time, so the precise prevalence of prenatal CCAM is still unknown and might change in the future.

Etiology and Pathophysiology

CCAM is characterized by lack of normal alveoli and originates from a dysplastic overgrowth and cystic dilatation of terminal bronchioles with various types of epithelial lining.

CCAM may have a mainly solid, mainly cystic, or mixed appearance. Stocker et al. described three types of lung lesions based on cyst diameter, which probably represent differences in the origin of the dysplastic lesion in the bronchial tree levels.

• Type I macrocystic CCAMs are characterized by single or multiple cysts greater than 2 cm in diameter, lined by ciliated pseudostratified columnar epithelium. These represent nearly 50% of CCAM cases in postnatal series. They frequently cause mediastinal compression, are rarely associated with other anomalies, and generally have a good prognosis.

• Type II lesions account for up to 40% of CCAMs, are single or multiple cysts less than 2 cm in diameter, and are lined with mixed ciliary, columnar, and cuboidal epithelium, with a thin underlying fibromuscular layer.

• Type III lesions are microcystic, predominantly solid lesions, with small cysts (<0.5 cm). These lesions represent 10% of CCAM lesions and are histologically composed of alveoluslike structures lined by ciliated cuboidal epithelium separated by microscopic masses lined with nonciliated cuboidal epithelium.

Thus, type I represents an anomaly of the more proximal part of bronchial tree, the principal bronchioles, whereas type III represents changes in the distal most peripheral alveolar tissues. Types II and III are more frequently associated with other anomalies and have graver prognosis.

Stocker et al. later expanded his classification to include another two types. Type 0, acinar dysplasia or agenesis, is very rare, involves all the lung lobes, and is incompatible with life. Type IV CCAM lesions are represented by large, thin-walled peripheral cysts caused by hamartomatous malformation of the distal acinus.

Another, more clinically appropriate classification, based on US appearance, was introduced by Adzick et al., who divided prenatally diagnosed lung lesions into two groups: macrocystic (type I), with cysts greater than 5 mm, which is seen on US as a cystic mass, and microcystic (type II) with cysts smaller than 5 mm, which appear as solid echogenic lesions on US. The presence or absence of cysts is important because this determines options for therapy in cases of fetal hydrops.