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2007 What is Pulmonary Sequestration?

Dr Lo Yi Tat,
Department of Medicine, Alice Ho Miu Ling Nethersole Hospital

Pulmonary sequestration (PS) was first used by Pryce1 in 1946, in which sequestare in Latin means “to separate”. It is defined as a non-functioning lung tissue that is not in normal continuity with the tracheobronchial tree and that derives its blood supply from systemic vessels2.
Pulmonary sequestration is divided into two types based on their pleural covering, extralobar and intralobar. In extralobar pulmonary sequestration, the bronchopulmonary tissue has a distinct pleura that maintains complete anatomical separation from adjacent normal lung tissue. In contrast, intrapulmonary sequestration is the lung tissue that contained within the normal visceral pleura. The aetiology of sequestration has been controversial and including five groups of proposals: vascular traction, vascular insufficiency, co-incidental occurrence, acquired pathology following infection, and common developmental theory3. It is a rare abnormality that accounts 0.15%-6.4% of all congenital pulmonary malformation4. Intralobar sequestration is more common (75% of cases) than extralobar type.

Clinical features
Extralobar type usually presented at infancy or diagnosed antenatally. There may be co-existing congenital anomalies such as diaphragmatic hernia, lung hypoplasia, congenital cystic adenomatoid malformation, bronchogenic cyst. On the contrary, intralobar sequestration typically presented in early adulthood with chronic cough, mucopurulent sputum production, recurrent pneumonia, or as incidental finding of lung mass on radiograph. Both types usually involve lower lobes with left side being more common in particular.

Radiologic appearance5, 6
Chest radiograph may find a homogeneous consolidation with irregular margins, uniformly dense mass with smooth or lobulated contours in the posterior basal portion of a lower lobe, or cavitatry and cystic lesions. Focal bronchiectasis, subsegmental atelectasis, reduced lung volume, mediastinal shift and prominent pulmonary hilum have been described. Whilst, pleural effusion is rare (less than 4 % of cases).

CT not only demonstrates lung parenchymal lesions but also can delineate the anomalous systemic arteries in about 80% of cases. Helical CT had been reported to facilitate the display of the aberrant artery relative to conventional CT by volume acquisition through slice reconstruction with narrow intervals and multiplanar and 3-dimensional reformatting.

MRI and MRA can evaluate the thoracic aorta and pulmonary vasculature of the sequestration. However, it may not be able to detect the small anomalous vessels.
Retrograde aortography has been the traditional approach in diagnosing pulmonary sequestration by defining an aberrant systemic artery. However, it remained invasive and is becoming less favour with the availability of other imaging modalities.
Sonography is suited for evaluating the prenatal and post-natal chest.

Surgical resection as either segmentectomy, or a full lobectomy is the main treatment mode. Radiological delineation of the systemic vascular supply and venous drainage of the sequestration would assist the surgeon in planning a successful ligation of vessels and avoiding inadvertent bleeding.

1. Pryce DM. Lower accessory pulmonary artery with intralobar sequestration of lung: a report of seven cases. J Pathol Bacteriol. 1946; 58:457-67.
2. Stocker JT. Sequestrations of the lung. Semin Diagn Pathol. 1986; 3:106-121.
3. Corbett HJ, Humphrey GME. Pulmonary sequestration. Paed Resp Rev. 2004; 5:59-68.
4. Savic B, Birtel FJ, Tholen W, et al. Lung sequestration: report of seven cases and review of 540 published cases. Thorax 1979; 34:96-101.
5. Frazier AA, Rosado de Christenson ML, Thomas Stocker J, Templeton PA. Intralobar sequestration: Radiologic-Pathologic Correlation. Radiographics. 1997; 17:725-745.
6. Peterson G, Martin U, Singhal A, Criner GJ. J Thorac Cardiovasc Surg. 2003; 126: 2086-90.