EMBRYOLOGY OF LUNG
Stages of Lung Development
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Embryonic period (0 to 7th weeks): lung bud branches off from the embryo’s primitive foregut.
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Pseudoglandular period (8th to 16th weeks): conducting airways form with differentiation including blood supply and mucus glands.
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Cannalicular period (17th to 24th weeks): gas exchange units form including terminal bronchioles, alveolar ducts, and alveoli.
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Terminal sac period (25th weeks to term): gas exchange units increase in number along with alveolar capillaries and differentiate into type I and II alveoli.
PULMONARY AGENESIS
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Definition: Pulmonary agenesis or aplasia is complete absence of one or both bronchi in which the lung tissue also fails to develop.
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Bilateral disease is a rare condition with only few cases described in literature.
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Unilateral disease is reported with higher incidence.
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Commonly associated with multiple anomalies, frequently cardiac malformation.
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Chest usually appears symmetric, although the trachea and mediastinum are deviated to the affected side.
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Lung function is impaired by agenesis.
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There can be severe respiratory distress if the normal lung is further impaired by infection, atelectasis, or immaturity.
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Radiographic features:
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Plain radiography: defective side is opacified; remaining lung extends across the midline to the contralateral chest.
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Computed tomography (CT) scan, echocardiography: only one bronchus and one pulmonary artery.
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PULMONARY HYPOPLASIA
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Definition: Pulmonary hypoplasia is low-volume lung development resulting in small lung size and low ratio of lung-to-body weight. There is also decreased number of bronchial generations and alveoli number.
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The hypoplastic lung is seen in association with other congenital anomalies that take up space in the chest, such as congenital diaphragmatic hernia or eventration and lung malformations.
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Pulmonary hypertension and persistent fetal circulation may be presenting clinical conditions that require intensive respiratory support.
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Radiographic features: similar to those of pulmonary aplasia.
PULMONARY ARTERIOVENOUS MALFORMATIONS
Definition and Epidemiology
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An arteriovenous malformation (AVM) is an anomalous direct communication between the pulmonary artery and vein.
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Connection vessels are thin walled and resemble aneurysmal sacs that can rupture.
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The male-to-female ratio is 1:2; it is a rare condition (about 4 cases/year at a large academic center).
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Peak age of presentation: 4th to 6th decades of life (90% identified in adults).
Pathobiology
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Most cases are congenital in nature. Approximately 70% to 90% are associated with hereditary hemorrhagic telangiectasia (HHT; also called Osler-Weber-Rendu disease). HHT is an autosomal dominant condition associated with gene mutations for ALK-1, a member of the tumor growth factor (TGF) β-1 superfamily of receptors, and endogolin. However , the precise pathobiology remains unknown. Note: In patients with HHT, pulmonary AVMs are found in 15% to 30%.
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A third of patients have multiple lesions, with bilateral lesions in a quarter. AVMs are located in the lower lobes in more than 50% and vary in size from 1 cm to giant 10-cm lesions. Diffuse microvascular lesions are rare.
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Classified morphologically as simple (single sac with single feeding vessel; 70%), complex (>1 feeding vessel), or multiple-type lesion (multichannel plexiform mass). Occasionally, a saccular tortuous connection. A rare presentation is a profusion of pulmonary AVMs throughout one or both lungs. This is termed diffuse or telangiectatic variety.
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Histology reveals a single layer of lining endothelial cells.
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Natural history is to slowly enlarge over time.
Clinical Features
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Classic triad: dyspnea on exertion, cyanosis, and clubbing of the fingers. This triad is no longer a common feature on more recent series.
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Epistaxis, related to associated HHT, is a common feature. This becomes manifest many years before clinical presentation of pulmonary AVMs per se.
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Exertional dyspnea is common (>50% of symptomatic cases). Mechanisms include : hypoxemia with right-to-left shunting, high-output cardiac decompensation and, occasionally, the development of pulmonary arterial hypertension.
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Platypnea (increase in dyspnea when upright) and orthodeoxia (increase in desaturation in the upright posture) has been described, owing to increased blood flow and shunting when upright.
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Bruits, which are loudest on inspiration and over dependent regions, can occasionally be heard arising from underlying AVMs.
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Hemoptysis complicates about 12% of cases and can be massive.
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Hemothorax described due to rupture of pleural AVMs.
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Mucocutaneous telangiectasia noted in patients with HHT
Diagnosis
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Imaging:
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Plain chest radiograph : well-defined round or oval, sometimes lobulated, lesion. Feeding vessel shadow or shadows (linear, curvilinear) and draining vein may be seen.
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CT scan : ultrafast contrast CT and noncontrast helical CT with three-dimensional reconstruction are sensitive tests used to detect feeding vessels and AVM morphology, and are the procedures of choice ( Fig. 14-1 ).
Figure 14-1
A series of figures related to diagnosis and treatment of a solitary pulmonary AVM. A, Original pulmonary angiogram showing large atrioventricuar malformation (AVM) (*) in the lingula with feeding vessel ( white arrows ) and draining vein ( black arrows ) well depicted. B, Follow-up angiogram following successful deployment of several occluding coils. C, Computed tomography (CT) scan depicting same lesion (*) before intervention. D, Follow-up CT scan 11 years later confirming long-lasting efficacy of initial coiling intervention.
(From Remy-Jardin M, Dumont P, Pierre-Yves B, Dupuis P, Duhamel A, Remy J. Pulmonary arteriovenous malformations treated with embolotherapy: helical CT evaluation of long-standing effectiveness after 2-21 year follow-up. Radiology 2006;239:576-585.)
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Magnetic resonance imaging (MRI): a variety of MRI techniques are sensitive and specific for detecting AVMs and feeding vessels larger than 5 mm and 3 mm respectively.
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Lung perfusion scan: can show uptake systemically over the brain and kidney.
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Conventional pulmonary angiography: can delineate the AVM, together with a therapeutic intervention (see later) (see Fig. 14-1 )
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Echo : Contrast bubble study shows contrast in left atrium after three to eight cardiac cycles
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Shunt study: 100% oxygen study detects an elevated shunt fraction in the majority of patients (sensitivity 87.5%).
Treatment
Treatment should be considered in all symptomatic cases, as well as in asymptomatic patients with sizable AVMs (>20 mm) or large feeding vessels (>3 mm).
Percutaneous Embolization Approach
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Employs conventional selective angiography to visualize AVM and to deploy either coils, balloons, or polyvinyl alcohol wool coils to obstruct the feeding vessel or vessels and is the procedure of choice (see Fig. 14-1 ).
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More limited experience in children.
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Multiple AVMs can be embolized at a single session or in serial sessions.
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Immediate impact on gas exchange and symptoms can occur.
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Procedure of choice, particularly with multiple AVMs, AVMs in multiple lobes, and bilateral disease.
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Large and complex lesions may be technically challenging.
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Microvascular diffuse AVMs are not amenable.
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Air embolism, pulmonary infarction, and balloon migration can rarely complicate the procedure.
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Effects of embolotherapy can be long-lasting (see Fig. 14-1 )
Surgical Approach
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Rarely performed electively in adults
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May be required with failed attempts at embolization; with a single large, often complex, or plexiform tangle; or in emergent cases.
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A variety of ligation and resectional procedures may be required.
Complications
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Neurologic
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Occur in up to 40%.
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Stroke, transient ischemic attack, and brain abscess are likely secondary to paradoxical embolism.
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Seizure, migraine
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Cardiovascular:
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Pulmonary arterial hypertension (HHT).
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High-output cardiac failure
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Respiratory:
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Massive hemoptysis
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Massive hemothorax
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CONGENITAL CYSTIC ADENOMATOID MALFORMATION
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Definition: A CCAM is considered a hamartomatous lesion of the lung with excessive proliferation of the bronchial structures with alveoli. CCAMs are connected to the tracheobronchial tree because the conducting airways have already formed.
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As described by Stocker, lesions are classified based on the appearance of the cysts according to the size and shape.
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Type I (70%): single or multiple large cysts larger than 2 cm in diameter without adenomatoid tissue.
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Type II (20%): mixture of intermediate sized cysts between 0.5 cm and 2 cm in diameter with some adenomatoid tissue.
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Type III (10%): multiple small cysts less than 0.5 cm in diameter, almost solid like with predominately adenomatoid tissue.
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Most patients are diagnosed prenatally with antenatal care. Development of hydrops fetalis, which is essentially fetal congestive heart failure, is a poor prognostic sign.
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Newborns typically present with poor feeding, respiratory distress, or recurrent pneumonias.
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Pulmonary hypoplasia and mediastinal shift are frequently presented.
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Prognosis: related to the lesion’s size and the compression on the remaining lung.
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Type I CCAMs have good prognosis.
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Type II and III lesions have mortality rate greater than 50%.
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Radiographic features:
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Prenatal ultrasound (US): detects CCAM as early as the 12th to 14th week of intrauterine life; distinguishes type I lesion from other CCAMs.
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Plain radiography: single or multicystic structure.
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CT scan: highlights lung markings and septations within cystic lesion ( Fig. 14-2 ).
Figure 14-2
Chest computed tomography scan showing a congenital cystic adenomatoid malformation in the left side of the chest.
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PULMONARY SEQUESTRATION
Definitions
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Sequestration refers to nonfunctioning lung tissue that has no connection to the tracheobronchial tree and that receives its blood supply from a systemic artery.
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Intralobar sequestration (75%–90%):
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Abnormal tissue within the lung parenchyma with no separate visceral pleural lining.
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Arterial supply typically arises from the aorta; venous drainage is via the pulmonary vein or azygos vein (see below for more details).
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Extralobar sequestration (10%–25%):
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Abnormal tissue has its own separate visceral pleural lining and therefore is separate from the normal lung
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The arterial supply typically arises from the aorta; venous drainage is mainly into systemic veins.
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Epidemiology
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Low prevalence: 0.15% to 6.4% of congenital lung malfor-mations.
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No gender preference for intralobar; male dominance for extralobar.
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Intralobar: presents in patients older than age 20 in 50%, but can present in patients from infancy to 65 years.
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Extralobar usually presents in early childhood
Pathobiology
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Intralobar sequestration:
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May be congenital (a branch of the developing bronchial tree becomes cut off and retains its systemic blood supply) or acquired (bronchial obstruction with repeated infection and parasitization of hypertrophied systemic vessels to maintain viability of the segment).
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Histology reveals normal lung elements but with the added presence of inflammation, cysts, and fibrosis
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Blood supply details: lower thoracic aorta: 75%; abdominal aorta: 20%; intercostal artery: 5%. Note: multiple vessels in 16% to 32%. Venous drainage is into pulmonary veins. Rarely into systemic veins (e.g., azygos, inferior vena cava, superior vena cava, intercostals, and so on).
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Extralobar sequestration:
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Congenital in origin
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Associated abnormalities include diaphragm disorders (eventration, congenital hernia), congenital heart disease, congenital vertebral disorders, and so on.
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Although mainly in the thorax, it can be situated within or below the diaphragm. Extrapulmonary sequestrations in the retroperitoneum have also been reported.
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Note: Distinguishing feature: anomalous vascular supply ( Fig. 14-3 ).
