Sequestrations of the lung.

Sequestration of the lung is a well-recognized entity usually subclassified into one of two groups based on its location: intralobar sequestration (ILS), within the normal pleural investment, and extralobar sequestration (ELS), outside the normal pleural investment. ELS is clearly congenital in origin; it is seen most frequently in the neonatal period, is associated with other anomalies, and is supplied by systemic and/or pulmonary arteries. ILS, on the other hand, in the majority of cases, probably results from repeated episodes of chronic pneumonia producing, through the process of granulation tissue formation, parasitization of normally occurring pulmonary ligament arteries resulting in a systemic artery supply to the infected area of lung. Its location (almost exclusively in the lower lobes) is dependent on the availability of systemic arteries situated only in pulmonary ligaments between the lower lobes and the mediastinum. Further support for the acquired nature of this lesion is its almost complete absence in neonates and infants, the infrequency of associated anomalies, and the consistent features of chronic inflammation and fibrosis within resected specimens.