There are three broad categories of congenital lung lesions: bronchogenic cysts, bronchopulmonary sequestrations (or sequestrations, for short), cystic adenomatoid malformations (also called CCAM), and congenital lobar emphysema. All three types represent the abnormal development of lung tissue, which may occur during normal fetal development. Why these lesions form is not really known; how they differ from each other may have to do with the timing and location of their development.
In the embryo, the lungs develop as an outpouching of what will become the esophagus. A "bud" develops, which elongates to become the trachea, then divides to become the left and right main stem bronchi. Further division of each bronchus will, like the branches of a growing tree, form the "tracheobronchial" tree of airways. At the same time, the lung parenchyma (the actual lung tissue responsible for exchange of oxygen and gases into and out of the bloodstream) develops around each of those airway branches.
Congenital lung lesions are seen in approximately in 1 of every 3,000 births. They are most often isolated findings, not associated with chromosomal or genetic disorders. CCAMs and sequestrations are sometimes seen in fetuses with a congenital diaphragmatic hernia, and can be an incidental finding (i.e., discovered by chance) in otherwise healthy children or children with unrelated anomalies.
A bronchogenic cyst is nothing more than an airway branch that buds off, loses all connections with the rest of the tracheobronchial tree, and does not connect with actual tissue. As a result, a cyst forms, which is lined by the same cells one finds in the trachea or the bronchi.
A sequestration represents a portion of lung (with a bronchus and some lung tissue) that has completely separated from the rest of the lung. If this separation occurs relatively early in embryonic life (before the 6th week of pregnancy), this "mini-lung" is usually fully developed and separate from the rest of the organ. It also contains its own blood supply: an artery that is directly connected to the aorta, for example. However, because it is separate from the rest of the lung, no air goes in or out of this sequestration.
If the separation from the rest of the lung occurs later (between the 8th and the 12th week of pregnancy), it usually remains within the normal lung, but may still have separate blood vessels. Even though it is not connected to the rest of the tracheobronchial tree by a normal bronchus, there may be microscopic communications with the rest of the lung. In this type of sequestration, some air (and bacteria) may get trapped in the lesion after birth, which may cause recurrent infections.
Congenital cystic adenomatoid malformations (CCAM)
The congenital cystic adenomatoid malformation (CCAM) consists of some or all types of lung and bronchial cells and structures, arranged in a disorderly fashion. It is not a tumor; the cells themselves are not malignant and do not grow, invade other organs or spread to other parts of the body. Rather, it is a "clumsily put together" part of the lung that doesn't function properly. Because a CCAM tends to develop within the normal lung, microscopic communications with that lung also place the child at risk for infections from trapped air and bacteria.
It is important to note that there are many "hybrid" forms: lesions that have characteristics of more than one type. In addition, in up to 50% of sequestrations, part of the otherwise normal lung tissue is replaced by a CCAM.
Congenital lobar emphysema
Congenital lobar emphysema is much less common than those mentioned above. In congenital lobar emphysema, or CLE, one of the bronchi of a lung may be partially and/or intermittently blocked, causing accumulation of fluid and distension of the lung tissue connected to that bronchus. While the cause of this condition is different, its course and treatment is remarkably similar to that of the lesions described above.
The Fetal Treatment Program is a partnership of Hasbro Children's Hospital, Women & Infants' Hospital and The Warren Alpert Medical School of Brown University.