Fetal Lung Lesions: CPAM, BPS & Hybrid Lesions

What Is a Fetal Lung Lesion?

There are three types of fetal lung lesions:

  • Congenital pulmonary airway malformation (CPAM)
  • Bronchopulmonary sequestration (BPS)
  • Hybrid lesions

CPAM previously referred to as congenital cystic adenomatoid malformation (CCAM) arises from the abnormal growth of lung tissue during fetal development. 

Frequently, the CPAM has small bubbles of fluid inside the lesion called cysts. They result from an abnormal growth of lung tissue that is benign and not a cancer.  

However, CPAMs can grow rapidly inside the fetal chest during development and can take up valuable space in the chest, restricting normal lung growth. Large lesions can also compress the esophagus and the heart. If the compression is severe enough, the esophagus collapses, which impairs normal fetal swallowing leading to accumulation of excess amniotic fluid around the fetus. This situation is called polyhydramnios and can lead to preterm labor.

Occasionally, massive growth of the CPAM significantly compresses the heart and results in fetal heart failure, which appears as body swelling called hydrops. Over time, fetal hydrops can cause “maternal mirror syndrome,” which is potentially a life-threatening situation for the mother.  In these rare cases, the mother can develop high blood pressure and fluid retention in her lungs.  

Bronchopulmonary sequestration (BPS) is another type of fetal lung lesion that share some similarities to CPAM, but also some important differences. BPS lesions do not contain cysts and are formed from normal lung tissue that does not connect to the airway. As a result, a BPS does not contribute to gas exchange after birth like the normal lung. Since BPS lesions are made up of normal lung tissue, they do not typically grow faster than the rest of the lung and rarely cause the compression problems seen with CPAM. BPS lesions get their blood supply from a feeding vessel arising directly from the aorta as opposed to the blood supply to the normal lung. The identification of a feeding vessel helps to distinguish a BPS from a CPAM.

While similar in many ways, CPAM and BPS are separate diagnoses. However, some CPAMs have an abnormal feeding vessel like a BPS. These “hybrid” lesions can growth rapidly and otherwise look and behave similar to other CPAMs.

How Are Fetal Lung Lesions Diagnosed?

Fetal lung lesions are easily detected by routine mid-gestational ultrasound at 18 – 20 weeks gestation. Depending on the size and location of the lesion, additional testing may be recommended, including a level II ultrasound, fetal echocardiogram and fetal MRI. In 90% of cases, CPAMs are located on one side of the chest within a single lobe of the lung. Rarely, they are found in more than one lobe (5%) or on both sides of the chest (bilateral, 5%). 

CPAMs are classified into three types:

  • Type I (macrocystic): characterized by existence of large cysts > 2 cm within the lesion. This type comprises approximately 40% of the cases.
  • Type II (mixed): characterized by many small cysts each less than 2 cm within the lesion.
  • Type III (microcystic): characterized by the absence of cysts visible within the lesion on ultrasound.

Once diagnosed, it is important to note the size of the lesion and look for signs of compression of the normal lung, heart, esophagus and windpipe. Size calculations require ultrasound measurements in three perpendicular directions (A, B, C). Using the mathematical equation for the volume of an ellipsoid (0.52 × A × B × C), the absolute volume of the CPAM is estimated (absolute CAM volume). 

To gauge the size of the CPAM relative to the size of the fetus, the volume is expressed as a ratio to the fetal head circumference known as the CAM-volume-ratio or CVR. The CVR measurement is the best measurement to track the growth of the CPAM relative to the growth of the fetus during pregnancy.  An increasing CVR suggests that the lesion is growing faster than the fetus while a falling CVR suggests the opposite. 

From the 22nd to the 28th week of pregnancy, CPAMs frequently grow faster than the overall growth of the baby and take up progressively more space within the chest causing worsening problems with compression of the other structures in the chest. In some cases, the CVR becomes greater than 1.6 cmand carries significantly increased risk for the development of fetal hydrops and potentially even in utero fetal demise. Frequent ultrasounds permit the tracking of growth and may occur as often as every day in some cases.

BPS lesions are also detected during mid-gestational ultrasound screening. These lesions do not contain cysts but have a feeding blood vessel arising directly from the aorta, distinguishing them from CPAMs. The size is also tracked using the CVR and growth of a BPS lesion is typically more moderate than a CPAM, rarely exceeding 1.6 cm2, except in the case of hybrid lesions.   

How Are Fetal Lung Lesions Treated Before Birth?

For most babies with a prenatal finding of CPAM or BPS, the diagnosis is not life threatening. Most often, lung growth recovers in the third trimester as growth of the lung lesion decreases. Lesions that remains small will simply be monitored throughout pregnancy. However, CPAMs with a CVR greater than 1.6cm2 put the baby at higher risk for developing hydrops and treatment is warranted. Treatment options depend on the nature of the lesion and the gestation age of the baby. Prenatal treatment options include:

  • Medication: steroids given to mom slow the growth of the CPAM.
  • Thoracentesis: draining fluid from a large cyst reduces the size of the lesion.
  • Fetal surgery: in the setting of severe hydrops that does not respond to other measures, in utero surgery to remove the CPAM may be necessary.
  • Early delivery: if maternal mirror syndrome has occurred, early delivery may be needed to protect the life of the mother.
  • EXIT procedure: (ex-utero intrapartum treatment procedure) a technique similar to a Cesarean delivery to partially deliver the baby and surgically resect the lesion while the fetus is still connected to the umbilical cord and placenta. Removing the lesion before the baby takes its first breath allows the lungs more room to expand while securing a fetal breathing tube and ensuring stable oxygen supply as the baby transitions to newborn life. 

How Are Fetal Lung Lesions Treated After Birth?

If the growth of the lesion reaches a plateau prenatally, the baby can be delivered vaginally or by cesarean if indicated, at a hospital that has neonatologists and pediatric surgeons available. Once delivered, a neonatologist will assess the baby for potential breathing difficulties. If the baby is breathing easily, the baby can be fed and discharged to home with the mother when ready. However, if there are any breathing problems, the baby will be stabilized and transported to the neonatal intensive care unit (NICU) for further evaluation and surgery if indicated.  

A follow-up appointment with a pediatric surgeon is required in order to best care for your baby. At around 2-3 months of age, the baby will be seen in the surgery outpatient clinic where a chest CT scan will be performed. Once the anatomy of the lung lesion is well defined, surgical intervention will be discussed.  

No matter the diagnosis (CPAM, BPS or hybrid lesion) or the size, all fetal lung lesions should be surgically removed after birth. The surgical procedure is called a lobectomy. During this procedure the lobe of the lung that contains the mass will be removed. The surgery typically takes place between 3-4 months of life. Nearly all of these operations are performed using minimally invasive surgical techniques through three very small incisions. The babies rebound quickly after the operation and are typically discharged on the first or second day after the procedure.

What Are the Outcomes After Surgery?

Usually by 12 months of age, the remaining normal lung will fill the entire chest with equal number of air sacs compared to the normal children. However, this compensatory lung growth will only happen with removal of the lesion. If the mass is left in place, there will be an increased risk of pneumonia and even a small risk of developing a rare form of pediatric lung cancer later in life. Thus, we recommend removal of the lung lesion between 3-4 months of age. After full recovery, children typically do not have any limitations.