Pulmonary sequestration is a congenital malformation in which a mass of pulmonary tissue has no connection either to the parent tracheobronchial tree or the pulmonary vascular tree and receives its blood supply from a systemic artery. The systemic artery usually arises from the aorta either above or below the diaphragm; occasionally from an intercostal artery; or, rarely, from the brachiocephalic (innominate) artery. The sequestered tissu presents itself in two forms: intralobar and extralobar.
This type comprises a nonfunctioning portion of lung within the visceral pleura of a pulmonary lobe. In the majority of cases, it derives its systemic arterial supply from the descending thoracic aorta or the abdominal aorta. The venous drainage is invariably by way of the pulmonary veins, producing an arterio-arterial communication. Embryologically, it appears to be a failure of the normal pulmonary artery to supply a peripheral portion of the lung; hence, the arterial supply is derived from a persistent ventral branch of the primitive dorsal aorta.
This malformation may represent a secondary and more caudal development from the primitive foregut that is then sealed off and migrates caudally as the lung grows. Venous drainage is to the systemic circulation, usually the azygos, hemiazygos, or caval veins. Anatomically, it is related to the left hemidiaphragm in more than 90% of cases. It may be situated between the diaphragmatic surface of the lower lobe and the diaphragm or within the substance of the diaphragm.
On pathologic examination, the affected mass is cystic, and the spaces are ﬁlled either with mucus or, if infected, with purulent material. The cystic spaces are lined by either columnar or ﬂat cuboidal epithelium.
Only 20% of intralobar sequestrations present in the neonatal period; occasionally, there may be heart failure caused by massive arteriovenous shunting. Extralobar sequestration rarely presents in the neonatal period but may be found incidentally at operation to repair a congenital diaphragmatic hernia. Later presentations include secondary infection, pneumonia, pleural effusion, and empyema. When the sequestered lung becomes infected, it often appears to be a chronic pulmonary abscess accompanied by episodes of fever, chest pain, cough, and bloody mucopurulent sputum.
On antenatal ultrasonography, the abnormal lung can be seen as an echogenic intrathoracic or intra-abdominal mass. In 50% of cases, there is a pleural effusion, and polyhydramnios is a frequent complication. Postnatally, on radiographic examination the diagnosis should be suspected if there is a dense lesion on the posteromedial part of the left zone of the chest radiograph. Extralobar sequestration is usually seen as a dense triangular lesion close to the diaphragm.
Treatment for either type consists of surgical resection. Because of the threat of secondary infection and hemorrhage, surgery should be recommended even though the patient is asymptomatic at the time. When infection occurs, complete removal is mandatory.