Discussion: Introduction
Pulmonary sequestration is defined as a segment of non-functioning lung parenchyma that is not in normal continuity with the tracheobranchial tree and almost always does not receive its blood supply from the pulmonary vasculature but rather from one or more anomolous systemic arterial vessels. The arterial supply is typically via the thoracic or abdominal aorta, but may arise from abdominal visceral or coronary vessels. The venous drainage depends on the type of sequestration.
There are two types of sequestration, extralobar (ELS) and intralobar (ILS). The distinction is made based on whether or not the sequestered segment lies outside the normal lung's visceral pleura, as in ELS, or within the normal visceral pleura as in ILS. Sequestration accounts for as many as 6.4% of all congenital pulmonary malformations. ILS accounts for about 75 to 80% of sequestrations and ELS the remaining 20 to 25%.
Presentation
ELS typically presents within the first year of life with some cases diagnosed in utero. It occurs more commonly in males. ELS is a distinct entity, and represents approximately 20 to 25 % of all pulmonary sequestrations. The malformation is typically found during the patients’ first days to weeks of life and, less frequently, in late infancy or early childhood. Sometimes theses are found coincidentally on imaging studies taken for respiratory tract infections to rule out pneumonia, preoperatively for surgery or for associated congenital anomalies. An extralobar sequestration consists of a discrete, accessory lobe of nonaerated lung tissue that is invested in its own pleural envelope. Although typically found in the posterior costodiaphragmatic sulcus between the lower lobe and hemidiaphragm (63%-77% of cases), the anomalous tissue may occur in the mediastinum, in the pericardium, and within or below the hemidiaphragm. Extralobar sequestration histologically resembles normal lung parenchyma, although all structures are dilated. A cut specimen of the lesion is typically firm, homogeneous, and may contain cystic areas. If the anomalous tissue contains air, one can infer the persistence of a patent connection to the gastrointestinal tract. This unusual feature allows the sequestration to be classified as a “bronchopulmonary foregut malformation”. Extralobar sequestration is typically (80% of cases) supplied by an anomalous artery that arises directly from the thoracic or abdominal aorta. The venous drainage of extralobar sequestration is usually systemic (80% of cases) through the azygos and hemiazygos system or superior vena cava into the right atrium
Diagnosis can be established in utero with ultrasound. However in previously undiagnosed cases, signs and symptoms such as maternal polyhydramnios, fetal ascites, and hydrothorax may suggest the presence of ELS. Immediately postpartum, patients may present with dyspnea, and cyanosis. Feeding difficulties arise secondary to connection to GI tract. In addition, patients may present with recurrent or chronic respiratory infections, or when anomolous blood supply arises from the coronary circulation, myocardial ischemia has been reported. ELS most often occurs in the left thoracic cavity. Less commonly some right sided cases and abdominally located segments are seen, usually found in the retroperitoneum where it may mimic an adrenal mass.
ILS typically usually presents later in life than ELS with as many as 50% being diagnosed after adolescence. Diagnosis after the age of 50 is very rare. ILS shows no difference in incidence between sexes. ILS typically presents with signs of infection with pulmonary involvement such as fever, cough and sputum production. ILS is almost always located in the lower lobes, and is more commonly found on the left side. The great majority of intralobar sequestrations are drained by normal pulmonary veins into the left atrium. There is significant evidence to support an acquired origin for most cases of intralobar sequestration. A likely explanation is that intralobar sequestration may simply represent a portion of lung that has acquired a systemic blood supply subsequent to chronic postobstructive pneumonia A`lower lobe bronchus becomes obstructed and a postobstructive pneumonia results This phase is followed by partial or complete interruption of the normal pulmonary arterial supply to the infected lung tissue and the acquisition of a systemic blood supply usually through parasitization of the neighboring pulmonary ligament arteries. These arteries supply the pleural granulation tissue and hypertrophy as they become incorporated into the underlying lung parenchyma. The phrenic vessels that originate from the celiac axis and supply the diaphragmatic surface of the pleura may be similarly recruited to supply the sequestration. Multiple episodes of pneumonia may need to occur before a substantial systemic arterial supply is established. However, a number of these lesions are clearly congenital in origin. There are rare reported cases of bilateral intralobar sequestration, coexistent intralobar and extralobar forms, neonatal congestive heart failure as a manifestation of intralobar sequestration, the (rare) occurrence of an associated bronchopulmonary malformation, and coincident intralobar sequestration and bronchogenic cyst. In these cases, the systemic arterial supply clearly represents an anomaly of embryonic development. The lesion is typically surrounded by nonsequestered,otherwise normal lung tissue. The incomplete, partially fibrous boundary between sequestered and normal lung tissue has long been considered to be the route of collateral air drift into the lesion. Other explanations for air within an intralobar sequestration represents either incomplete bronchial obstruction or partial recanalization of previously obstructed bronchi in the setting of chronic infection. At gross inspection, the visceral pleura overlying the sequestered segment appears thickened by fibrosis and has multiple irregular, cordlike adhesions to adjacent structures, including the mediastinum, diaphragm, and parietal pleura. On cut specimens, a segment of dense fibrotic and consolidated parenchyma that often contains multiple cysts filled with fluid or a thick gelatinous material is seen, which is sometimes purulent.
Both ILS and ELS may also present with hydrothorax which may occur due to torsion of the sequestered segement and subsequent obstruction of the pulmonary venous return. The left to left shunt which is formed can also lead to congestive heart failure. Both types of pulmonary sequestration have been associated with a variety of congenital malformations.
Diagnostic Evaluation
Diagnosis can be establiahed with a variety of imaging techniques. Prenatal diagnosis using ultrasonography, is suggested in patients with a lung mass with polyhydramnios. Post delivery chest radiographs will usually show a lower lobar homogeneous opacification which typically remains unchanged on serial studies.
CT can be effective in showing both the abnormal lung parenchyma as well as the anomalous blood supply. MR is a very effective study for diagnosing sequestrations as it can show both the lung abnormalities as well as the vascular supply with reasonable accuracy. Ultrasound can show the mass and sometimes delineate the vascular supply and drainage. Angiography is very rarely utilized for diagnosis.
Differential Diagnosis
In symptomatic patients, pneumonia, brochiectasis, TB, and diaphragmatic hernia. If asymptomatic, consider infected bronchogenic cysts, congenital lobar emphysema, cystic adenomatoid malformation, and lung malignancy.
Treatment
Definitive treatment for both ILS and ELS is surgical resection. A variety of thoracoscopic and laparoscopic approaches have been successfully employed. Surgery remains the only option which allows for definitive tissue diagnosis. However, transarterial embolization has been used both preoperatively to assist in control of blood loss during surgery as well as being used as definitive treatment in the minority of patients who present with hemoptysis and pulmonary hemorrhage.
