Imaging in bronchopulmonary sequestration

Authors


  • P Abbey MD, DNB; CJ Das MD, DNB; G Pangtey MD, A Seith MD; R Dutta MS; A Kumar MS.

  • Conflicts of interest: None.

Dr Chandan J Das, Department of Radiodiagnosis, All India Institute of Medical Sciences, New Delhi-29, India.
Email: dascj@yahoo.com

Summary

Bronchopulmonary sequestration is an uncommon pulmonary disorder characterized by an area of non-functioning abnormal lung tissue, which receives its blood supply from a systemic artery and characteristically has no connection with the tracheobronchial tree. The abnormal lung tissue is located within the visceral pleura of a pulmonary lobe in the intralobar variety, whereas the extralobar form has its own visceral pleura. The venous drainage of the extralobar type is usually into the systemic veins, whereas the intralobar type drains into the pulmonary veins. Radiological imaging plays a vital role in establishing the diagnosis, and even more importantly, in providing to the clinician a vascular roadmap essential for surgical planning. We present here a review of bronchopulmonary sequestration and also discuss the role of various imaging methods in the early diagnosis and management of these cases.

Introduction

Bronchopulmonary sequestrations constitute approximately 0.15–6.4% of all congenital pulmonary malformations.1 A sequestration is generally defined as non-functioning lung tissue that is not in normal continuity with the tracheobronchial tree and derives its blood supply from systemic vessels. Sequestrations may be either intralobar or extralobar. An intralobar sequestration shares the visceral pleural covering of an otherwise normal pulmonary lobe, whereas an extralobar sequestration is separated from normal lung by its own pleural covering.

The term ‘sequestration’ was initially described by Pryce in 1946.2 Pryce described three types of intralobar abnormalities based on the distribution of the aberrant systemic artery. In type 1, the anomalous artery supplies functional lung tissue, which communicates with the tracheobronchial tree. In type 2, the artery supplies both normal lung tissue, as well as non-functional, non-communicating tissue. In type 3, the anomalous artery supplies the lung, which is isolated from the tracheobronchial tree.

Since then, many variants of sequestration have been described. In many cases, the distinction between intralobar and extralobar varieties is unclear, with presence of a mixed type of venous drainage to both pulmonary and systemic circulations. The ‘sequestration spectrum’ includes all anomalies in which there is an abnormal connection between one or more of the four important components of lung tissue, namely tracheobronchial airway, lung parenchyma, arterial supply, and/or venous drainage.3 Other pulmonary anomalies that may show some overlapping characteristics of bronchopulmonary sequestration include scimitar syndrome, cystic adenomatoid malformation, pulmonary arteriovenous fistula and systemic arterial supply to normal lung.

Intralobar sequestrations account for approximately 75% of cases and almost always within the lower lobes (98% cases), more commonly on the left and typically involving the posterior basal segment.1 Most lesions drain into the left atrium through the pulmonary veins.4

Extralobar sequestration usually manifests in early infancy and commonly occurs in association with other malformations. The sequestered lung is invested in its own pleural envelope.4 It classically occurs in the posterior costodiaphragmatic sulcus between the lower lobe and left hemidiaphragm (63–77% cases). It may uncommonly occur in the mediastinum and within or below the hemidiaphragm. Extralobar sequestration is typically supplied by a systemic artery arising directly from the aorta.5 Its venous drainage is usually systemic (80% cases) through the azygos–hemiazygos system or the superior vena cava.4

Pathogenesis

Many theories have been suggested to describe the aetiopathogenesis of bronchopulmonary sequestration. Historically, intralobar and extralobar sequestrations have been considered to be closely related developmental anomalies. Although extralobar sequestration is clearly a congenital anomaly, the pathogenesis of intralobar sequestration is controversial, with evidence to support an acquired origin for many of these cases.6

Extralobar sequestration is thought to develop from an anomalous or supernumary lung bud that derives its blood supply from primitive splanchnic vessels that surround the foregut.4 These vessels give rise to the anomalous systemic arterial supply to the sequestered lung. The original connection with the foregut involutes in most cases, but it may sometimes persist, giving rise to a communication with the gastrointestinal tract. There are many factors that support the congenital nature of extralobar sequestration. Most of the cases present during infancy – approximately 60% are diagnosed before the age of 6 months.7 Diagnosis may sometimes be established in utero; an extralobar sequestration may be associated with polyhydramnios or fetal hydrops.4 More than half (approximately 50–65%) of patients with extralobar sequestration have associated congenital anomalies, the most common of these being congenital diaphragmatic hernia – seen in 20–30%.7 Diaphragmatic eventration or paralysis may also be commonly associated. Various other anomalies have been reported in association including bronchogenic cyst, foregut duplication, ectopic pancreas, vertebral anomalies, and various cardiovascular, genito-urinary and gastrointestinal anomalies.4

Intralobar sequestrations do not usually present in infancy; neither do they commonly occur in association with other congenital malformations. Patients usually present in late childhood or as young adults. Virtually all intralobar sequestrations are characterized histologically by chronic inflammation and fibrosis.7 These factors point towards an acquired origin of many of these cases. Stocker and Malczak suggested a relationship between the occurrence of normal, small, systemic pulmonary ligament arteries and the development of intralobar sequestration in a segment of lung affected by chronic infection.8 They have described the normal occurrence of small systemic arteries, which arise from the aorta and traverse the pulmonary ligament to ramify in the visceral pleura of the lower lobes of the lungs – they found these arteries in 90% of randomly selected pediatric autopsy specimens. They have suggested that in cases where a lower lobe bronchus gets obstructed and a portion of the lung is affected by chronic post-obstructive pneumonia, there may be a partial or complete interruption of the normal pulmonary arterial supply to the infected lung tissue. Subsequently, sometimes after many episodes of infection, there may be parasitization of the neighbouring pulmonary ligament arteries and establishment of an anomalous systemic arterial supply – thus creating an intralobar sequestration. There are reports in the published literature that describe the detection of an aspirated foreign body within an obstructed bronchus proximal to an intralobar sequestration.8

However, a few cases of intralobar sequestration show evidence supporting a congenital origin – the simultaneous occurrence of both intralobar and extralobar forms of sequestration has been described, as also the uncommon association with other congenital malformations – like vertebral and rib anomalies, oesophagobronchial fistulas and diverticulae.9

The development of systemic arterial supply to lung with normal bronchial connections and with coexistent infection has also been referred to as pseudosequestration. It is suggested that chronically inflamed lung activates neovascularization of the systemic circulation.6 These aberrant vessels may disappear following antibiotic therapy.

Clinical presentation

Patients with intralobar sequestration usually present in late adolescence or early adulthood with recurrent episodes of productive cough and fever due to infection. Half of all patients are diagnosed after the age of 20 years.9 The male and female sexes are equally affected.1 Minor haemoptysis is a common presenting complaint, usually in association with infection. Some cases of severe, even life-threatening haemoptysis have been reported.10 A small percentage of patients are asymptomatic; the lesion may be an incidental detection on a chest radiograph obtained for an unrelated indication. The findings on physical examination are usually non-specific, with signs of chest infection.

Presentation in infancy or early childhood is unusual. Congestive cardiac failure is an uncommon presentation of intralobar sequestration, which has been described in the neonatal period. It may occur due to the ‘left to left’ shunting of blood, which occurs through the systemic artery supplying the sequestration, from the aorta into the left atrium. Infrequently, other malformations like bronchopulmonary foregut malformations and skeletal anomalies (like scoliosis and rib anomalies) may be associated.

Extralobar sequestration occurs more frequently in the male sex with a male : female ratio of 4:1. Patients typically present during the first few months of life, often with respiratory distress and feeding difficulties.4 Secondary infection usually does not occur (unlike intralobar sequestration). Associated congenital anomalies are very common, most common being congenital diaphragmatic hernia.7

Pathological features

Intralobar sequestration

As a result of multiple episodes of infection, there is fibrosis and thickening of the visceral pleura overlying the sequestered lung, with formation of adhesions. On cut sections, the sequestration comprises fibrotic and consolidated lung parenchyma that frequently contains multiple cystic areas.7 The cysts may contain fluid, infected purulent material or may be predominantly air filled – in which case, the cysts have at least a partial communication with the tracheobronchial tree. The sequestration is surrounded by normal lung parenchyma, which may show emphysematous changes.9 The boundary between the sequestered segment and normal lung tissue is often ill defined, incomplete and partially fibrous and is considered to be the route of collateral air drift into the lesion.

Histologically, intralobar sequestration is characterized by changes of chronic inflammation and fibrosis (Fig. 1) with cystic changes.7 Superimposed changes of acute infection may be present. The alveoli that border the sequestered lung may be emphysematous. Changes of vascular sclerosis may be seen. Atherosclerotic changes are commonly observed within the systemic arterial branches supplying the sequestered lung, even in children.1

Figure 1.

Postoperative histopathology of a patient with intralobar sequestration (haematoxylin–eosin stain, 40× magnification). The alveolar spaces are filled with haemorrhage, oedema and chronic inflammatory infiltrate. Few thick-walled blood vessels and areas of bronchial dilatation are also seen.

Extralobar sequestration

An extralobar sequestration is typically a single, well-circumscribed lesion covered by a mesothelial layer (pleural envelope). Its surface may show a reticular pattern due to presence of dilated subpleural lymphatics.4 On cut section the lesion is firm and homogenous. Cystic areas may be present rarely. The sequestered segment does not contain air – unless it has a patent communication with the gastrointestinal tract.4

Microscopically, extralobar sequestration resembles normal lung tissue, except for dilatation of bronchioles and alveolar ducts and alveoli within the lesion. A well-formed bronchus is identifiable in approximately half the cases, typically located at one edge of the lesion.4,7 The anomalous supplying systemic artery often shows evidence of hypertensive changes.

Imaging findings

The first and foremost objective of imaging in every case of suspected bronchopulmonary sequestration should be to show the aberrant vascular anatomy. The presence of an abnormal systemic arterial supply to the sequestered lung not only clinches the diagnosis, but also provides a preoperative vascular roadmap to the surgeon, thus minimizing chances of inadvertent vascular injury. Other features – like presence or absence of a separate pleural envelope or any associated congenital malformation – are not of primary importance for treatment planning.6

The aberrant artery supplying the sequestered lung most commonly arises from the descending thoracic aorta in approximately 73% cases, followed by the upper abdominal aorta, coeliac or splenic arteries in 21% cases.11 Less commonly, it may arise from intercostals, subclavian, internal thoracic or coronary arteries. Multiple supplying vessels are identified in 16% cases. The feeding vessels that originate from the descending thoracic aorta are usually located within the leaves of the inferior pulmonary ligament, and vessels that originate from the abdominal aorta pierce the hemidiaphragm or traverse the aortic or oesophageal hiatus to reach the sequestered segment.1,12 In 95% cases, venous drainage occurs through the pulmonary vein into left atrium, thus creating a unique ‘left-to-left’ shunt. Venous drainage into systemic circulation may rarely occur through intercostal veins, azygous–hemiazygous system, superior vena cava or inferior vena cava. Mixed patterns of venous drainage may be present.

Plain radiographs

The chest radiograph is usually the first investigation carried out in cases of pulmonary sequestration and the diagnosis can often be suspected on the basis of the radiographic findings.

The classical abnormality found in cases of intralobar sequestration is an intrapulmonary lesion involving the posterobasal segment of left lower lobe. The right lower lobe may be affected in approximately one-third of cases. A range of radiographic appearances may be seen. Uncomplicated intralobar pulmonary sequestration may be seen on a chest radiograph as a homogenous opacity or a patch of consolidation (Fig. 2a,b). Areas of cavitation may develop, which do not always signify active infection.9 Cystic spaces may develop as a result of chronic inflammation (Fig. 7a) – the radiograph may show single or multiple cysts that may vary in size and may show air fluid levels or be completely air filled – in which case, a communication with the tracheobronchial tree must have developed. A prominent vascular shadow may sometimes be the only finding, seen as a tubular or branching opacity (Fig. 3a,b). Localized hyperinflation of the lung may be seen surrounding the sequestered segment. After administration of antibiotics, there may be partial resolution of the lesion; however, some residual abnormality usually remains.

Figure 2.

A 11-year-old boy presented with recurrent chest infection. Initial chest radiograph (a) showed an ill-defined opacity in the left parahilar region (arrow). Repeat (digital) radiograph after 5 months shows persistence of the left parahilar opacity (b) extending into the left retrocardiac region (long arrow). Contrast-enhanced CT (mediastinal window) shows a heterogeneous opacity in the left lower lobe, extending into the parahilar location (c) with cyst formation (d), presence of air specks (c) as well as a tiny speck of calcification (arrow in e). DSA of the patient shows blush within the sequestered lung segment on the descending thoracic aorta run (arrow in f). On selective cannulation of the supplying artery arising from the thoracic aorta (g), the aberrant vessel is better seen.

Figure 7.

Cystic intralobar sequestration. A 21-year-old man presented with recurrent hemoptysis. Chest radiograph (a) shows a cavitary lesion in right lower zone, in the retrocardiac location (arrows). The CT axial image in lung window (b) shows a large thin-walled cavity with multiple septations and an air-fluid level in the medical basal segment of right lower lobe (arrow). Digital subtraction angiography images show a large aberrant vessel arising from the aorta and supplying the cystic lesion (c,d).

Figure 3.

This 25-year-old woman presented with episodes of streaky hemoptysis and cough. Chest radiographs (a, b) show a tubular opacity in the left lower lobe (arrows), suspicious for a vascular lesion. The CT shows presence of a large abnormal artery arising from the thoracic descending aorta (c) and supplying a portion of the left lower lobe including the posterior basal segment. Lung window (d) shows an area of emphysema in the left lower lobe, around the abnormal vessel.

Extralobar sequestration usually manifests as a well-defined, homogenous opacity seen in close relation to the posterior medial hemidiaphragm on the left side (its typical location is in the pleural space between the lower lobe and the diaphragm, although it may uncommonly occur within the substance of the diaphragm, in a subdiaphragmatic location, within the mediastinum or pericardium). As it lacks a communication with the normal tracheobronchial tree, the development of cystic areas within the lesion is rare. Presence of air within the lesion suggests a patent communication with the gastrointestinal tract. Associated abnormalities – like an associated diaphragmatic hernia may also be detected on the chest radiograph.

Pulmonary sequestration should always be considered in the differential diagnosis of any persistent lung opacity involving the lower lobe of a child or a young adult. However, in some cases, the findings may be very subtle (Fig. 4a) and the radiograph may appear normal to the inexperienced viewer.

Figure 4.

A 31-year-old man presented with recurrent chest infection. Chest radiograph shows a subtle nodular opacity in the right lower zone (arrow in a). Axial CT sections (b,c) show an aberrant systemic artery arising from the descending thoracic aorta supplying a sequestered segment in right lower lobe.

Angiography

Traditionally, angiography has been considered as the gold standard for diagnosis of pulmonary sequestration, by virtue of its excellent depiction of the arterial and venous anatomy. However, angiography is an invasive procedure, involves considerable radiation exposure and requires general anaesthesia when carried out in children. With the advent of non-invasive angiographic techniques using multidetector CT and MR imaging, the imaging technique of choice has evolved from invasive conventional or digital subtraction angiography (DSA) to non-invasive multidetector CT or MR angiography for the observation of the anomalous vascular supply. A diagnostic DSA can be useful in the occasional problematic cases (Fig. 2f,g) where CT or MR angiography could not provide a definitive diagnosis. DSA is also carried out when a preoperative or therapeutic embolization of the supplying artery is considered as a treatment option.13,14

Computed tomography

In the present day scenario, multidetector CT angiography has emerged as the imaging technique of choice for preoperative evaluation of pulmonary sequestration, both in the paediatric and in the adult population.15–17 Computed tomography has the advantage of being able to show the pulmonary parenchymal abnormality (Figs 2c–e,3d) as well as the arterial and venous anatomy (Figs 3c,4b,c), all in a single examination. Computed tomography has an advantage over MR angiography, as CT scan times are significantly shorter, which makes it especially easier to evaluate children. CT also has a better spatial resolution for evaluation of small vessels and lung parenchyma. The main disadvantage of CT remains the radiation dose involved. While evaluating paediatric patients, care should be taken to scan at low exposure settings,16 approximately 25–35 mAs and 80–100 kV, depending on the age and the weight of the child. Computed tomography scanning in a single phase of contrast injection is adequate for evaluation of both the arterial and the venous anatomy; this also helps to minimize the radiation exposure.16

Similar to the varied radiographic appearances in cases of intralobar sequestration, CT may show the sequestered lung segment as a homogenous or heterogeneous soft tissue opacity (Fig. 2c–e). Areas of cavitation may be present. Some lesions show a predominantly cystic appearance; few or multiple cysts filled with fluid or showing air fluid levels may be present (Figs 2c–e,7b). Emphysematous lung changes are often observed around the lesion (Fig. 3d) – possibly due to air trapping within the ‘transition zone’ between the sequestered segment and the normal lung. Areas of calcification may rarely be seen within the sequestered segment (Fig. 2e)18 or in the anomalous systemic artery. Some lesions show partial, heterogeneous enhancement post-contrast administration.

Computed tomography chest in cases of extralobar sequestration shows the lesion as a homogenous, well-circumscribed soft tissue density mass. Emphysematous changes within the adjacent lung have been described.18

The aberrant artery can often be identified on the axial CT images (Figs 3c,4b,c). The exact origin and course of the vessel can be better depicted using multiplanar reconstruction techniques, which can very well depict the arterial as well as venous anatomy in any desired plane (Fig. 5). The sequestered segment and the aberrant vessels can be depicted using 3-D maximum-intensity projection, volume rendered or surface-shaded display reconstructions (Fig. 6).

Figure 5.

Computed tomographic angiography images of a 7-year-old male show the origin of an aberrant systemic artery from the aorta (single arrow in a) supplying a sequestered segment in the right lower lobe and venous drainage of the lesion through the pulmonary vein into left atrium (double arrows in b) –classical features of intralobar sequestration.

Figure 6.

Maximum intensity projection (a) and surface-shaded display (b) images of the patient shown in Figure 4; the aberrant vessel (arrow) is seen arising from the descending thoracic aorta.

Magnetic resonance imaging

This is a safe and non-invasive alternative imaging method, which may be useful.19–21 With its multiplanar capability, MRI provides an excellent observation of the supplying systemic arterial anatomy6 (Figs 8,9). MR imaging can also depict the pulmonary venous return of the lesion and the relationship of the draining vein to the cardiac chambers.21 Breath-hold contrast-enhanced MR angiography can offer excellent delineation of the aberrant vessels without flow or respiratory artefacts.22 The advantage of MRI over CT is in the absence of radiation risks. However, long scan times combined with need for prolonged sedation time in children, as well as suboptimal evaluation of lung parenchyma are the disadvantages of this method.

Figure 8.

An MRI of the patient shown in Figure 3. Axial MR images show the vessel as bright on TruFISP (a) and as a flow void on T2 turbo spin-echo images (b). Contrast-enhanced MR angiography maximum intensity projection image (c) also shows the abnormal systemic artery supplying the left lower lobe (arrow) with venous drainage into the left hemiazygos system (arrow in d).

Figure 9.

A 20-year-old man presenting with recurrent chest infection. The MRI coronal TruFISP image shows the sequestered segment (a). Contrast-enhanced MR angiographic maximum intensity projection image (b) also shows the supplying artery.

Magnetic resonance imaging can show the cystic nature of many intralobar sequestrations and can depict the hemorrhagic as well as solid components that may be present.

Nuclear scan

Radionuclide angiography is a non-invasive technique that has been used to show the systemic arterial supply to a pulmonary sequestration.23 The sequestration may be seen as an area of lung having a relative lack of perfusion (Fig. 10) during the pulmonary arterial phase followed by perfusion during the systemic phase. The addition of Fourier phase analysis of the flow study data greatly enhances the sensitivity for detection, especially for smaller lesions.24

Figure 10.

99Tc pertechnetate scan of the patient shown in Figure 9. An aberrant vessel is seen running parallel to the descending aorta in the blood pool phase of 99Tc pertechnetate scan (arrow in a). Perfusion scan shows a defect in the posteromedial basal segment of right lower lobe (b).

A ventilation scan is useful to show collateral ventilation to a sequestration. The sequestered lung segment has no direct communication with the tracheobronchial tree and therefore shows absent uptake in the inhalational phase. Because of the presence of collateral channels, it shows delayed entry and subsequent trapping of radiogas.25

However, the lack of anatomical detail provided by radionuclide scans makes it unsuitable for adequate preoperative assessment and surgical planning. Therefore, scintigraphy is not routinely recommended for imaging suspected cases of lung sequestration.

Barium upper gatsrointestinal study

In cases where a patent communication with the oesophagus or stomach is suspected, for example, in an extralobar sequestration containing foci of air, barium studies are useful to show the same.6 Otherwise, it has no significant role in the evaluation of bronchopulmonary sequestration.

Sonography

Ultrasound evaluation is ideally suited for evaluation in the antenatal period or in neonates.26,27 Sequestration should be considered in the differential diagnosis in any fetus with a lung mass. The lesion usually has a homogenously echogenic appearance on ultrasound, but may show cystic changes.6 Doppler ultrasound may be useful for evaluation of the supplying systemic artery.28 However, a small aberrant systemic artery may not be identified and the acoustic window may be impaired by surrounding air-filled lung or the bony thorax.

Imaging guidelines

A chest radiograph is very useful as a screening investigation in virtually all patients presenting with chest complaints. Most cases of pulmonary sequestration show findings appreciable on chest radiographs. The radiographic appearances vary widely; therefore, a high index of suspicion should be kept, especially in the clinical setting of recurrent chest infections.

After a plain radiograph, which shows findings suspicious for a pulmonary sequestration, a multidetector CT angiography is the current diagnostic imaging method of choice for optimum evaluation of the sequestered lung and its vascular supply. Computed tomography can also help in diagnosing other lung conditions that may mimic sequestration on a chest radiograph. Multiplanar CT reconstructions and 3-D reconstructions are useful in depicting the arterial and venous anatomies of a sequestered lung segment.

Magnetic resonance angiography may be useful in select cases where CT is undesirable or contraindicated – like in patients with hypersensitivity to iodinated contrast media, or when exposure to radiation is undesirable – for example, in pregnancy and adolescence (although the use of gadolinium-based MR contrast media is not recommended during pregnancy). Although MR angiography provides good delineation of the vascular anatomy, evaluation of lung parenchyma is suboptimal.

Invasive catheter angiography may be helpful as a diagnostic procedure in those select cases where the clinical and radiological suspicion for sequestration is high, but a supplying systemic vessel cannot be shown using CT or MR angiographic techniques. DSA is also required before therapeutic or preoperative embolization.

Treatment

Surgical resection remains the treatment of choice for pulmonary sequestration (Fig. 11). Surgery is usually necessary as infection almost universally occurs if the abnormal lung is not removed.6 Although sequesterectomy is possible for extralobar lesions, in cases of intralobar sequestration resection usually means a lower lobectomy. Thoracotomy is usually carried out, although thoracoscopic procedures have also been attempted. Lobectomy by video-assisted thoracoscopic surgery has been successfully used to treat intralobar sequestration.29 Embolization of the anomalous vessel has been suggested for treatment of extralobar pulmonary sequestration 13 and has also been used in Pryce’s type 1 cases. Preoperative elective coil embolization has been described before resection of intralobar pulmonary sequestration.14

Figure 11.

Peroperative photograph of the case shown in Figures 3 and 8. Large aberrant artery (thick arrow) is seen arising from the aorta (A), which is exposed after retracting the heart (H), and left hemidiaphragm (D). The draining vein (thin arrow) is seen medial to the abnormal segment of lung (L).

Conclusion

Pulmonary sequestration is a heterogeneous group of lesions with anomalous connections between the various components of the lung. The primary role of imaging in an individual patient is to depict the aberrant vascular anatomy for diagnosis and accurate surgical planning. A high index of suspicion is required, both by the clinician and the radiologist, in making a presumptive diagnosis. Any persistent intrapulmonary opacity in the lower lobes of a child or young adult, on a chest radiograph, should prompt further evaluation with cross-sectional imaging. Non-invasive CT and MR angiographic techniques are useful for depiction of vascular anatomy. Multidetector CT angiography, which allows simultaneous imaging of the aberrant vascular anatomy as well as the lung parenchyma, has now become the first-line examination in the preoperative assessment of pulmonary sequestration. Diagnostic catheter angiography is only required in select problematic cases.

Acknowledgements

The authors thank Dr Prasenjit Das (MD, DNB, Department of Pathology, All India Institute of Medical Sciences, New Delhi, India) for his valuable help with the pathological findings.

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