What is the diagnosis for cystic spaces seen on the fetal lung at 32 weeks gestation in a pregnant woman?

Differential Diagnosis for Cystic Spaces in Fetal Lung at 32 Weeks

The most likely diagnosis for cystic spaces seen in the fetal lung at 32 weeks gestation is congenital pulmonary airway malformation (CPAM, formerly called congenital cystic adenomatoid malformation or CCAM), though bronchopulmonary sequestration must also be considered in the differential.

Primary Diagnostic Considerations

Congenital Pulmonary Airway Malformation (CPAM)

• CPAM is the most common congenital cystic lung lesion detected on prenatal ultrasound, characterized by cystic or adenomatous lesions in the terminal bronchioles caused by airway dysgenesis 1, 2.
• Ultrasound findings typically show cystic spaces of varying sizes within the lung parenchyma, which may appear as small cystic spaces that become larger and more numerous as the pregnancy advances 3, 1.
• The lesions can be predominantly cystic, predominantly solid, or mixed, and cyst size varies widely even within a single lesion 4.
• At 32 weeks gestation, serial ultrasound monitoring should have already been established to assess for mediastinal shift, contralateral lung compression, and development of hydrops fetalis 1, 5.

Bronchopulmonary Sequestration

• This represents the second most common fetal lung malformation and can present with cystic changes, particularly when associated with tension hydrothorax 2, 5.
• Approximately 68% of extralobar sequestrations (28 of 41 cases) dramatically regress on serial prenatal sonography and may become asymptomatic after birth 5.
• Color Doppler ultrasound is critical to identify the systemic arterial feeding vessel (typically from the descending aorta), which is pathognomonic for sequestration 6, 2.

Essential Diagnostic Workup at 32 Weeks

Immediate Ultrasound Assessment

• Perform detailed transabdominal ultrasound to fully characterize the lesion, assess for mediastinal shift, evaluate contralateral lung compression, and measure amniotic fluid volume 1.
• Calculate the CPAM volume ratio (CVR) using the formula for a prolate ellipse divided by head circumference, as CVR >1.6 predicts high risk for hydrops development 1, 5.
• Evaluate for signs of hydrops fetalis (scalp edema, pleural effusion, ascites, pericardial effusion), which dramatically worsens prognosis 5, 7.
• Use color Doppler to identify any systemic arterial feeding vessel to distinguish sequestration from CPAM 6, 2.

Consider Fetal MRI

• MRI should be performed at 32 weeks if ultrasound findings are incomplete or additional anatomic detail is needed for delivery planning, particularly for airway management 1.
• MRI is especially helpful for thoracic anomalies and provides superior soft tissue characterization 1.

Establish Serial Surveillance

• Perform serial ultrasounds weekly or biweekly to monitor for development of hydrops, changes in mass size, mediastinal shift, and amniotic fluid volume 1, 5.
• Complete a detailed anatomic survey to identify any additional structural anomalies, though associated anomalies are rare with isolated lung lesions 5.

Prognostic Stratification and Management Planning

Lesions WITHOUT Hydrops (Excellent Prognosis)

• All babies with CPAM not associated with hydrops survived in a large series of 134 cases 5.
• Management consists of maternal transport to a tertiary center, planned term delivery, and postnatal resection 5.
• Many lesions, particularly sequestrations, dramatically decrease in size or disappear completely before birth (9 of 50 cases in one series) 8.

Lesions WITH Hydrops (Poor Prognosis Without Intervention)

• All 25 large CPAMs with associated hydrops that were followed expectantly resulted in fetal or early neonatal death 5.
• Fetal intervention is indicated for large unilateral lesions causing hydrops fetalis, with ultrasound-guided laser coagulation of feeding artery (for sequestration) or fetal lobectomy (for CPAM) 6, 5.
• Fetal surgical resection (lobectomy) performed at 21-29 weeks resulted in 8 of 13 hydropic fetuses surviving with hydrops resolution and impressive in utero lung growth 5.
• For very large solitary cysts with hydrops, thoracoamniotic shunting achieved survival in 5 of 6 cases 5.

Critical Management Decisions at 32 Weeks

Delivery Planning

• Delivery should occur at a tertiary center with neonatal intensive care and pediatric surgical capabilities 6.
• Administer antenatal corticosteroids between 24-34 weeks if preterm delivery is anticipated for fetal lung maturation 6.
• Close monitoring in the first 24-48 hours after birth is critical, as pleural effusion can recur rapidly even after complete prenatal regression 6.

Referral Considerations

• All cases requiring fetal intervention should be referred immediately to a tertiary fetal treatment center with expertise in fetal therapy 6.
• At 32 weeks with hydrops, the window for fetal intervention may be closing, as most procedures are performed before 29 weeks 5.

Important Caveats

• The natural history of prenatally diagnosed lung masses is highly variable, and initial impressions may not accurately predict outcome since marked improvement can occur during fetal life 8.
• Chromosomal anomalies are rarely identified with isolated CPAM, though amniocentesis and karyotype may be considered 7.
• Do not assume a small or regressing lesion requires no postnatal follow-up—imaging and potential resection may still be needed due to risk of infection or malignant transformation 5, 7.