Congenital Pulmonary Airway Malformation (CPAM) is a rare fetal lung condition involving the abnormal development of lung tissue during gestation. This malformation results in a mass of non-functioning tissue, typically located in one lung lobe, which can range from solid to fluid-filled cysts. While the majority of cases have excellent outcomes, this abnormal tissue can occasionally affect the growth of the remaining healthy lung.
Understanding Congenital Pulmonary Airway Malformation
CPAM is the most common type of congenital lung lesion, previously known as Congenital Cystic Adenomatoid Malformation (CCAM). It is characterized by a mass of tissue. This lesion is essentially an overgrowth of immature bronchial structures that do not connect properly to the normal airway system.
The condition is considered rare, with an estimated incidence of about 1 in 10,000 to 35,000 live births, though improved prenatal screening suggests this number may be higher. CPAM lesions are classified into five types (Type 0 through Type 4) based on the size of the abnormal airway structures and their location within the lung. Type 1 is the most common, involving large cysts, while Type 2 involves multiple medium-sized cysts.
The lesions often vary in appearance, being either cystic (fluid-filled) or solid. A significant number of these masses may remain stable, or even decrease in size and become less apparent as the pregnancy progresses. The presence of this mass can still compress adjacent normal lung tissue, which can impede its development.
The Etiology of CPAM
The cause of Congenital Pulmonary Airway Malformation is not fully understood, but it is regarded as a sporadic developmental error. The malformation is believed to arise from aberrant branching of the fetal bronchial tree, the system of airways that forms the lung structure. This error occurs very early in gestation, typically around the eighth or ninth week of development.
The current hypothesis suggests that a localized failure in the normal maturation of the developing airways leads to an overgrowth of immature tissue. This results in the formation of the CPAM mass, which is essentially a disorganized collection of terminal bronchioles and associated structures. Because this process is an isolated anomaly of organ development, CPAM is not considered to be a hereditary or inherited condition.
CPAM is not typically caused by anything the mother did or did not do during the pregnancy. The condition is not strongly linked to environmental factors, drug use, or known genetic patterns in most cases, making it a random event in fetal development. While some rare subtypes can be associated with other congenital anomalies, the majority of cases are isolated findings.
Fetal Diagnosis and Tracking
CPAM is most often detected during a routine prenatal ultrasound around the 18- to 20-week anatomy scan. The lesion typically appears as an echogenic, or bright, mass within the fetal chest. Additional imaging, such as a fetal Magnetic Resonance Imaging (MRI), may be used for a more detailed anatomical assessment of the lesion’s size, location, and its relationship to the heart and other chest structures.
A crucial tool for tracking the condition is the CPAM Volume Ratio (CVR), which assesses the lesion’s size relative to the fetus. The CVR is determined by measuring the lesion’s volume and dividing it by the fetal head circumference, which serves as a proxy for gestational age. This measurement provides a standardized way to monitor growth and predict potential complications.
A CVR value of 1.6 or greater is a significant threshold, indicating an increased risk for the development of hydrops fetalis. Hydrops fetalis is a serious complication characterized by an abnormal accumulation of fluid in at least two areas of the fetal body, signaling fetal heart failure. This occurs because a large CPAM mass can shift the structures in the chest, compressing the heart and major blood vessels, which puts an overwhelming strain on the fetal circulatory system. Fetal surveillance with ultrasound is often performed weekly or bi-weekly to monitor the CVR and watch for early signs of hydrops.
Management and Long-Term Outlook
The management of a prenatally diagnosed CPAM is determined by the lesion’s size and whether it is causing any strain on the developing fetus. In the majority of cases, the lesion is small or remains stable, allowing for a management strategy of close monitoring throughout the pregnancy. Many lesions will spontaneously decrease in size or volume by the third trimester, leading to a very favorable outcome.
For the minority of high-risk cases, typically those with a CVR greater than 1.6 or the presence of hydrops, in-utero interventions may be considered. The administration of maternal corticosteroids, such as betamethasone, is often the first-line therapy for microcystic lesions to help reduce the lesion size and potentially resolve hydrops. In rare instances of large cystic lesions with impending hydrops, fetal surgery, such as the placement of a thoraco-amniotic shunt, may be performed to drain the dominant cyst and relieve pressure on the heart.
After birth, infants without symptoms are typically monitored, as many CPAMs do not cause breathing problems. For lesions that persist, surgical removal, usually a lobectomy, is the standard treatment to prevent future complications like infection or, rarely, malignancy. This elective postnatal surgery is often performed between three and six months of age, allowing the healthy lung to compensate and grow. The long-term prognosis is excellent, with most children leading completely normal lives.