Congenital pulmonary airway malformations (CPAMs) are lung lesions that result from disordered development of the lower respiratory tract. These malformations have a wide spectrum of severity and vary substantially in size and composition. 
The expanded use of prenatal ultrasonography has increased the recognition of these lesions and has helped characterize their natural history. [2, 3, 4] Prenatal therapy for CPAM is considered if the affected fetus begins to show signs of hydrops fetalis. 
CPAMs are characterized by airway cysts of varying size that are connected to the tracheobronchial tree.  Although an elaborate postnatal staging system exists, prenatal diagnosis focuses on the size of the lesion (see image below)—whether the cysts are smaller than 5 mm (microcystic) or larger than 5 mm (macrocystic). [6, 7] These lesions are almost always unilateral and can be associated with other lung lesions such as bronchopulmonary sequestrations and congenital lobar emphysema. [2, 8, 9] The vascular supply and drainage from CPAMs is almost always to the pulmonary circulation; however, bronchopulmonary sequestrations may be connected to the systemic circulation.
Although the natural history of CPAMs varies, several trends have been described.
Microcystic lesions typically have a rapid growth phase between 20 and 26 weeks’ gestation that usually peaks at about 25 weeks.  Subsequently, growth of these lesions plateaus. In many cases, the lesions actually regress, [11, 12] and some disappear completely. [13, 14] Macrocystic lesions may grow rapidly throughout gestation and do not have the characteristic pattern of growth through the late second trimester with subsequent plateau or decrease in size.
Large CPAM lesions that are either microcystic or macrocystic can compress other thoracic structures such as the esophagus, mediastinum, and inferior vena cava (see image below). This compression can result in impaired venous return, polyhydramnios, and hydrops fetalis. The small percentage of CPAMs with this behavior that lead to hydrops are associated with high mortality rates, and fetal intervention may be considered in these patients. 
Determining which fetuses will develop hydrops is critical to formulating appropriate surveillance and therapeutic strategies. Investigators have used a ratio that compares the calculated volume of the lesion to the fetal head circumference (CPAM volume ratio [CVR]).  A ratio of greater than 1.6 suggests hydrops development and warrants close prenatal surveillance. [1, 15]
CPAMs are the most common congenital lung lesion. [3, 4] Registry data suggest that CPAM affects 1 in 8000-35,000 live births.  This may be an underestimate of the true incidence of the disease because of nonregistered in utero mortality such as the “hidden mortality” that was seen with congenital diaphragmatic hernia. 
Therapy for fetal CPAM depends on the size of the lesion, its physiologic consequences, and its cystic features.  Fetal abnormalities in addition to CPAM have traditionally been a contraindication to intervention, but this may be changing, since therapy has become less invasive. 
Until recently, the only therapies for microcystic lesions resulting in hydrops was either fetal resection if hydrops developed prior to 32 weeks’ gestation or ex-utero-intrapartum treatment (EXIT) to neonatal resection if hydrops developed after 32 weeks.  This paradigm has changed after the serendipitous discovery that short courses of maternal steroids seem to be much more effective at reversing hydrops than surgery. [7, 17, 18]
Several investigators have demonstrated a survival rate of approximately 50% for hydropic fetuses with microcystic CPAM after surgery.  Hydropic fetuses treated with steroids, however, have survival rates near 85%.  The superiority of steroid therapy combined with its complete abrogation of serious maternal risk has made it the standard of care for hydrops caused by microcystic CPAM before 32 weeks’ gestation. If hydrops persists or emerges past 32 weeks, EXIT and neonatal resection remain options. [11, 19]
Macrocystic lesions that cause hydrops can be treated with catheter-based drainage techniques of the dominant cyst. Simple aspiration of the cyst is usually a temporizing measure but can slow down disease progression and help determine if a thoracoamniotic shunting will be effective.  In lesions without a significant solid component, placement of a thoracoamniotic shunt can effectively decrease the CVR and reverse hydrops (see image below). 
The cause of CPAM is unknown, but several hypotheses have been made. One prevailing theory is that CPAM is caused by airway obstruction. The different presentation and types of lesions are accounted for by the timing and location of obstruction. [5, 20] It has also been proposed that an imbalance between cell proliferation and apoptosis during airway branching morphogenesis may lead to CPAM. [21, 22] Dysregulation the genes HOXB5  and glial cell-derived neurotrophic factor  have also been implicated in CPAM pathogenesis.
The use of steroid therapy is clearly indicated for microcystic CPAMs that have resulted in hydrops.  There is minimal maternal morbidity associated with this intervention, and efficacy is superior to fetal resection. Uncertainty remains about nonhydropic fetuses with large microcystic CPAMs. A UCSF-led randomized trial attempted to address the use of steroids in this context, but recruitment was hampered because of the increasing use of maternal steroids for nonhydropic microcystic CPAMs.
Drainage of macrocystic CPAMs decreases the size and compressive effects of CPAMs; it is indicated in lesions that cause hydrops. 
Simple cyst aspiration usually precedes thoracoamniotic shunt placement to ensure the efficacy of CPAM fluid removal.  Gestational age should be greater than 20 weeks and less than 32 weeks.
The risk of chest wall deformity is extremely high if catheter-based interventions are pursued before 20 weeks. After 32 weeks, delivery and neonatal resection are indicated rather than prenatal therapy.
Fetal resection of CPAMs are much less common than steroid therapy owing to the efficacy of maternal steroid administrations, and the general consensus is that large microcystic lesions that threaten fetal well-being are best treated with steroid therapy.  Macrocystic lesions are best treated with catheter-based therapies.  If steroids are ineffective, open fetal resection can be used as salvage therapy.
Ex-utero intrapartum therapy (the EXIT procedure)
Although rare, hydrops and fetal compromise can occur in or persist into the third trimester. In these cases, the CVR is typically greater than 1.6-2.0, and significant respiratory distress is anticipated at birth. The EXIT procedure utilizes the placenta for gas exchange so that the fetal lungs can be bypassed while airway access is gained and lung lesions can be resected.
Contraindications to macrocystic drainage include a predominately solid CPAM, abnormal fetal karyotype, and fetal cardiac abnormalities. Other associated congenital abnormalities may also represent contraindications but need to be considered on a case-by-case basis.
Contraindications to fetal resection include significant maternal operative risk, abnormal fetal karyotype, and fetal cardiac abnormalities. Other associated congenital abnormalities may also represent contraindications but need to be considered on a case-by-case basis.
Ex-utero intrapartum therapy (the EXIT procedure)
The EXIT procedure requires maternal laparotomy and hysterotomy. If maternal health is significantly endangered by these interventions, EXIT should not be performed.
The assembly of a multidisciplinary team including an expert sonologist, anesthesiologist, and fetal surgeon is critical for intervention success in catheter-based therapies, fetal resections, and EXIT procedures.
Overall survival data for thoracoamniotic shunts for macrocystic CPAM can be derived from the aggregation of several small reports of its use. Of the 41 patients treated, 26 survived (63%). 
Maternal and fetal trauma due to trocar and shunt placement are rare but have been reported.  These include both vascular and structural injuries. Catheter insertion can also cause premature rupture of membranes, preterm labor, and chorioamnionitis.
Possible complications during and after fetal CPAM resection are significant for both the mother and fetus. Maternal complications include bleeding, infection and wound issues. Uterine rupture is a risk after the hysterotomy and necessitates Cesarean delivery for the fetus with CPAM and for any subsequent pregnancies. For the fetus, the physiologic stress of the operation is significant, and perioperative fetal demise is not infrequent. Subsequent chest wall deformity is also possible.
Ex utero intrapartum therapy (the EXIT procedure)
Blood loss and wound infections are the most common maternal complications with the EXIT procedure.  Long-term maternal fertility does not appear to be affected.
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