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 cm2 and 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.