A CPAM, or congenital pulmonary airway malformation, is a rare lung abnormality that forms before birth when part of the lung develops abnormally, creating fluid-filled or air-filled cysts instead of normal lung tissue. It occurs in roughly 1 in 10,000 births. Most CPAMs are detected on a routine prenatal ultrasound, and while the diagnosis can be alarming, the majority of affected babies do well with appropriate monitoring and, when needed, surgery.
How a CPAM Forms
During normal fetal development, the lungs go through several stages of branching and growth. A CPAM happens when that process stalls at some point during early pregnancy. Instead of forming the tiny air sacs that allow normal breathing, a section of lung tissue develops into a mass of abnormal cysts. The affected area connects to the rest of the lung and receives blood supply from normal pulmonary arteries, which distinguishes it from some other congenital lung conditions.
The malformation typically affects only one lobe of one lung, leaving the rest of the lung tissue healthy. Several genes involved in cell growth and tissue formation play a role, but CPAMs are not inherited in a predictable pattern. They appear to arise spontaneously during embryonic development.
Types of CPAM
CPAMs are classified by where in the airway tree the abnormal growth originates and how the cysts look under a microscope. In practical terms, what matters most is the size of the cysts and how much lung tissue is involved.
- Type 1 (large cyst): The most common form, accounting for about 70% of all CPAMs. It consists of one or more large cysts lined with airway-type tissue. This type generally carries a good prognosis.
- Type 2 (small cyst): The second most common type. It contains many smaller, more uniform cysts. Type 2 is sometimes associated with other congenital abnormalities.
- Type 3 (solid): Less common. The abnormal tissue appears more solid because the cysts are very small. This type tends to involve a larger portion of the lung and can be more clinically significant.
Older classification systems included two additional categories (types 0 and 4), but these are now recognized as separate conditions. What was once called type 4 CPAM is now diagnosed as a type of childhood lung tumor called pleuropulmonary blastoma, which is an important distinction for treatment planning.
How CPAM Is Detected
Most CPAMs are discovered during a second-trimester ultrasound, typically between 18 and 22 weeks of pregnancy. On imaging, the mass appears as a bright area in the fetal chest, sometimes with visible cysts. Doctors measure the size of the mass relative to the baby’s head circumference to calculate a ratio that helps predict how the pregnancy will progress. A larger ratio signals higher risk for complications like fluid buildup around the fetus.
Many CPAMs actually shrink on their own as the pregnancy continues. Some become nearly invisible on ultrasound by the third trimester. This does not mean the malformation has disappeared, just that it has become less prominent relative to the growing lung.
After birth, even if the baby appears completely healthy, a CT scan is typically performed to get a detailed picture of the malformation. This imaging is most accurate after 4 weeks of age, because residual fetal fluid in the lungs can obscure the findings in the first few days of life. Most centers schedule the scan somewhere between 4 weeks and 12 months.
Symptoms at Birth and Beyond
The majority of babies with a prenatally diagnosed CPAM are asymptomatic at birth. A large review of over 1,000 cases found that only about 3.2% of babies diagnosed before birth and asymptomatic at delivery went on to develop symptoms, at a median age of about 7 months. Other studies have placed that number closer to 5% within the first five years of life.
When symptoms do appear, they typically involve recurrent lung infections in the same area of the chest. Babies and young children may develop pneumonia that keeps coming back in the same lobe, persistent coughing, or breathing difficulty. In rare cases, a very large CPAM can cause respiratory distress immediately after birth, requiring urgent treatment.
Surgery vs. Observation
Whether to operate on an asymptomatic CPAM is one of the most debated questions in pediatric surgery. There are two main concerns that push most specialists toward recommending elective removal. First, when a child does eventually develop symptoms (usually a lung infection), emergency or semi-urgent surgery carries roughly twice the complication risk compared to a planned elective procedure. Second, there is a small but real association between CPAM and lung malignancies.
The link to cancer deserves some context. About 4% of childhood lung tumors have been found in association with congenital cystic lung malformations. The International Pleuropulmonary Blastoma Registry has reported that 66% of pleuropulmonary blastomas were associated with lung cysts that either existed at diagnosis or had been identified beforehand. A separate analysis found a 2% association between pleuropulmonary blastoma and congenital lung malformations. These numbers are small in absolute terms, but because pleuropulmonary blastoma is aggressive and difficult to treat once advanced, many surgeons view elective removal as a reasonable preventive step.
The standard surgery is a lobectomy, which removes the entire lobe of the lung containing the malformation. This is increasingly performed using minimally invasive (thoracoscopic) techniques. Surgical outcomes are generally excellent, and research suggests that timing of the procedure (whether performed at a few months or closer to a year of age) does not significantly affect complication rates.
Long-Term Outlook After Surgery
Children who undergo a lobectomy in infancy have a remarkable capacity for compensatory lung growth. Studies tracking children for up to 11 years after surgery have found that total lung volume reaches approximately 90% of normal predicted values, even when 8% to 45% of lung tissue was removed. The remaining healthy lung expands to fill the space left behind.
There is a nuance to this recovery. While the overall volume of the lungs returns close to normal, some of that growth appears to be expansion of existing tissue rather than the creation of truly new functional lung. Follow-up studies have found slightly elevated residual air volume and localized ventilation differences in the operated side. In practical terms, though, most children who have surgery in infancy grow up with normal exercise tolerance and no meaningful breathing limitations in daily life.
For children managed with observation rather than surgery, ongoing monitoring with periodic imaging is standard. The goal is to detect any growth of the lesion or early signs of infection before they become serious problems.