Fetal Bronchopulmonary Sequestration

Course

"Sequestration" is from Latin, meaning to separate. Bronchopulmonary sequestration is divided into intralobar and extralobar. Both intralobar (ILS) and extralobar (ELS) sequestrations are separated from the normally developed bronchopulmonary tree. Embryologically, sequestrations arise from a supernumerary lung bud. If the lung bud develops before the pleura, it becomes intralobar in origin. If the lung bud develops after the pleura, it becomes extralobar in origin (Fig. 1)¹. Hence, extralobar sequestrations have their own visceral pleura.

Figure 1. Bronchopulmonary sequestration.

Some cases of ILS are acquired due to bronchial obstruction, distal infection, and recruitment of a systemic arterial blood supply. Small systemic arteries arising from the thoracic aorta traverse the pulmonary ligament to supply the lower lobes of the lung. These vessels are distinct from the pulmonary circulation. If the normal pulmonary arterial supply is compromised, it has been proposed that these smaller vessels are parasitized to supply the infected portion of a lower lobe. This proposed mechanism explains some of the distinct characteristics of intralobar, in contrast to extralobar, sequestrations; i.e., an absence of congenital anomalies; and the venous drainage of ILS into the pulmonary veins².

Rare cases of bilateral intralobar or coexistent intralobar and extralobar sequestrations represent an anomaly of embryonic development rather than an acquired lesion³.

The original embryologic connection between a pulmonary sequestration and the foregut involutes. Occasionally, it persists, allowing communication with the gastrointestinal tract in isolated cases of sequestration^4,5.

Bronchopulmonary sequestrations represent up to 23% of prenatally detected lung lesions⁶.

ELS have an aberrant systemic pulmonary drainage. 75% of venous drainage is through the vena cava or azygous system and 2% drain via the pulmonary veins.

In 80% of extralobar pulmonary sequestrations the arterial blood supply is a single vessel, usually derived from the thoracic or abdominal aorta (Fig. 2)⁷. Other atypical sources of blood supply include the celiac trunk, left gastric, subclavian, and intercostals¹

Figure 2. Left lower lobe pulmonary sequestration (markers) with an aberrant feeding vessel (arrow).

The feeding vessel to an ELS may come off the aorta at any angle. As a result, the vascular supply is not always identified prenatally⁸. Three-dimensional power Doppler has been utilized to improve the detection of an aberrant feeding vessel to a bronchopulmonary sequestration⁹. The diameter of the arterial feeding vessel is between 1 and 12 mm¹⁰. Aneurysms of the aberrant vessel with rupture and subsequent hemothorax have been described¹¹.

Since the blood supply to ILS and ELS overlap, they cannot generally be distinguished antenatally.

ELS occur more commonly in males (4:1) and 80-90% occur in the base of the left lung (Fig. 3)⁷. ILS are 75% of all sequestrations, do not have a gender preference, and 60% are left posterior basal in location¹².

Figure 3. Left bronchopulmonary sequestration (markers).

There are only a few cases of prenatally diagnosed intralobar bronchopulmonary sequestions².

Differential Diagnosis

Congenital Pulmonary Airway Malformation

Congenital pulmonary airway malformation (CPAM) is associated with ELS in 15% - 25% of cases^7,13. Microcystic congenital pulmonary airway malformation is difficult to distinguish from bronchopulmonary sequestration (Fig. 4). Pulmonary sequestrations account for up to 25% of chest masses that were thought prenatally to represent CPAMs².

Figure 4. Congenital pulmonary airway malformation (arrow) with secondary hydrops resulting in ascites (curved arrow).

A hybrid combination of a bronchopulmonary sequestration and CPAM are relatively common¹³. The presence of a large cyst with an echogenic left lung mass suggests a hybrid lesion. However, cysts of various sizes may be present in ILS¹⁰. Hybrid lesions have connections with both the systemic arterial and pulmonary supply. The detection of a feeding vessel to a lung lesion would, therefore, not exclude a combined lesion.

Lobar Emphysema

Congenital lobar emphysema (Fig. 5) is a second echogenic lung mass that is difficult to distinguish from bronchopulmonary sequestration. Postnatally, a diagnosis of congenital lumbar emphysema can be confirmed by computed tomography that shows emphysematous alveoli, in contrast to the distorted lung parenchyma of cystic pulmonary airway malformation¹⁴.

Figure 5. Congenital lobar emphysema (arrow).

Neuroblastoma

A subdiaphragmatic fetal mass is frequently presumed to be a neuroblastoma. However, a subdiaphragmatic ELS occurs once for every 2.5 neuroblastomas¹⁵. A left-sided subdiaphragmatic mass that is detected in the 2nd or early 3rd trimester is most likely an ELS. A right-sided subdiaphragmatic mass detected at, or after, 30 weeks' gestation should presumptively be considered a neuroblastoma. A bronchopulmonary sequestration characteristically decreases in size over time, while the size of a neuroblastoma increases¹⁵

Adrenal Hemorrhage

An adrenal hemorrhage (Fig. 6) is the most common etiology for a suprarenal mass. It can be readily distinguished from extrapulmonary sequestration by its changing echo pattern over serial examinations¹⁶. Calcifications may eventually develop after a long-standing hemorrhage.

Figure 6. Adrenal hemorrhage (markers).

Mesoblastic Nephroma

Mesoblastic nephroma (Fig. 7) is the most common renal tumor in the neonatal period. It occurs once in every 12,500 – 27,500 live births and accounts for 50% of neonatal tumors^17,18. Mesoblastic nephromas are homogeneously solid and should not, therefore, be confused with an extralobar pulmonary sequestration. They are characteristically associated with polyhydramnios – an uncommon sonographic finding with extrapulmonary sequestrations¹⁹.

Figure 7. Mesoblastic nephroma (arrow).

Nephroblastoma (Wilm's Tumor)

A nephroblastoma is a renal carcinoma that typically occurs in children. It is rarely detected in-utero and always in the 3rd trimester²⁰.

Sonographic Findings

ELS is classically a diffusely echogenic mass in the base of the left lung. The shape of the mass either conforms to the lower lobe or is triangular (Fig. 8). While the majority of extralobar pulmonary sequestrations are small to medium in size, larger lesions will occasionally occur (Fig. 9). A bronchopulmonary sequestration is usually unilateral; bilateral cases have been reported²¹.

Figure 8. Right bronchopulmonary sequestration (markers)

Figure 9. Right bronchopulmonary sequestration (arrow)

As with CPAM, ELS has not been diagnosed until after 16 weeks' gestation^22,23.

Over 85% of ELS have dilated suprapleural lymphatic vessels that are responsible for the ipsilateral pleural effusion noted in some of these cases.

The identification of a feeding vessel to the thoracic mass from the thoracic or abdominal aorta help to differentiate ELS from CPAM, or other echogenic lesions (Fig. 10).

Figure 10. Bronchopulmonary sequestration with an aberrant blood supply (green arrow)

Associated anomalies occur in over 50% of extrapulmonary sequestrations (Table I)^10,24. As previously stated, ILS is not associated with an increased risk of additional anomalies¹⁵. Extrapulmonary sequestrations are most frequently associated with congenital diaphragmatic hernia. Hence, a left-sided abdominal mass in a fetus with a diaphragmatic hernia should be considered an extrapulmonary sequestration until proven otherwise²⁵.

Table I. Associated congenital anomalies with extralobar sequestrations.

• Congenital heart disease
• Diaphragmatic hernia
• Eventration of the diaphragm
• Accessory spleen
• Gastric duplication

A correct prenatal diagnosis of pulmonary sequestration is made approximately 29% of the time²⁶.

Intra-thoracic pulmonary sequestrations are much more likely to have associated sonographic findings, i.e. polyhydramnios, pleural effusions, mediastinal shift, and hydrops. None of these sonographic findings are generally found with intra-abdominal pulmonary sequestrations²⁶.

Complete resolution or regression of a fetal pulmonary sequestration may occur in up to 70% of cases. Postnatal CT has confirmed the resolution of antenatally diagnosed masses^5,27,28. However, some presumed bronchopulmonary sequestrations that appeared to resolve in-utero are detected on postnatal computed tomography. The reduction in size or disappearance of an in-utero bronchopulmonary sequestration is thought to be due to torsion and/or clotting of the vascular pedicle, or decompression into the normally expanding lung^15,29.

The rate of hydrops with bronchopulmonary sequestration ranges from 7%²⁴ to 35%²⁶. The etiology of hydrops in bronchopulmonary sequestrations may be either from: 1) a mass effect; 2) a tension pneumothorax due to fluid secretion by the mass, resulting in cardiac compression and/or vena caval obstruction; or 3) large volume shunting through the systemic arterial feeder vessel to the mass.

Fetal Management

Once the diagnosis of bronchopulmonary sequestration has been made, the presence or absence of hydrops becomes the most important prognostic feature³⁰. While there are isolated case reports of survival after the sonographic detection of hydrops, this combination of findings is generally fatal³¹. Survival rates are significantly higher among hydropic fetuses with bronchopulmonary sequestration who have in-utero intervention³².

The 6-10% of bronchopulmonary sequestrations with an isolated pleural effusion^5,28 can be managed with serial thoracentesis or a thoraco-amniotic shunt. Survival is improved after shunting^10,12. Complications associated with a shunt procedure include chest wall trauma, fetal hemorrhage, displacement or plugging of the catheter, placental abruption, preterm premature rupture of the membranes, preterm labor, and intrauterine demise^33,34.

Since the development of a pleural effusion cannot be predicted, patients with a diagnosis of bronchopulmonary sequestration should be followed with serial ultrasound examinations until delivery.

One approach to the treatment of a pulmonary sequestration with symptomatic pleural effusion is YAG laser coagulation of the feeding vessel²³.

For fetuses > 32 weeks' gestation, delivery with immediate postnatal surgical resection should be considered in the presence of a presumptive bronchopulmonary sequestration and hydrops²⁸.

In a recent series of 39 antenatally diagnosed bronchopulmonary sequestrations, regression was documented in 72% of cases; there was one prenatal mortality⁵. This success may not be reproduced in additional series with a lower prevalence of lesions that regress.

Neonatal Management

Large pleural effusions should be drained immediately. However, the timing of surgical resection is highly variable and dependent upon neonatal status. The resection of an extralobar lesion does not affect the amount of functioning lung tissue. Pre-operative evaluation of the arterial and venous supply is mandatory to avoid ligation of anomalous veins that may constitute the only drainage for the ipsilateral lung³⁵.

If a bronchopulmonary sequestration has regressed in utero, a postnatal CT should be performed to confirm resolution. If a mass is still detected, management varies from prophylactic removal due to the theoretical risk of infection or hemorrhage, to observation with serial imaging³⁶. Surgical removal of an extrapulmonary sequestration may be by either open laparotomy or laparoscopy³⁷.

References

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