Radiographic appearance of developmental anomalies of the lung

INTRODUCTION — Developmental anomalies of the lung can be categorized as bronchopulmonary anomalies, vascular anomalies, or combined anomalies (ie, hybrid pulmonary malformations) [1-3]. They are primarily due to aberrant intrauterine development; abnormalities during postnatal lung development are uncommon. Recognition of developmental anomalies is important because they are frequently mimicked by more ominous acquired abnormalities and they can cause complications during infancy, early childhood, or adulthood.

The radiographic appearances of the major developmental anomalies of the lung are discussed in this topic review. Other aspects of the developmental anomalies (eg, clinical presentation, diagnosis, treatment) are presented separately. (See "Congenital anomalies of the intrathoracic airways and tracheoesophageal fistula" and "Congenital lobar emphysema" and "Congenital pulmonary airway malformation" and "Clinical manifestations and diagnosis of hereditary hemorrhagic telangiectasia (Osler-Weber-Rendu syndrome)" and "Pulmonary arteriovenous malformations: Clinical features and diagnostic evaluation in adults" and "Pulmonary arteriovenous malformations: Epidemiology, etiology, and pathology in adults".)

BRONCHOPULMONARY ANOMALIES — Bronchopulmonary anomalies are caused by insults to the lung bud, the embryonic structure from which airways and parenchyma develop. Examples of bronchopulmonary anomalies include pulmonary agenesis, pulmonary hypoplasia, congenital bronchial atresia, congenital lobar emphysema, congenital cystic adenomatoid malformation (CCAM, now renamed as congenital pulmonary airway malformation [CPAM]), bronchogenic cyst, tracheal bronchus, tracheal diverticulum, and accessory cardiac bronchus.

Agenesis — Agenesis is explained by the failure of lung or lobe, bronchi, and blood vessels to develop. Radiographically, agenesis of a lung appears nearly identical to a pneumonectomy and agenesis of a lobe mimics lobar collapse or a lobectomy and leads to lung hypoplasia (image 1 and image 2) [1]. Associated anomalies of ribs or the spine are common.

Congenital bronchial atresia — Congenital bronchial atresia occurs with absence of a proximal bronchus, while the distal bronchial tree is preserved [4]. The apicoposterior segment of the left upper lobe (image 3), bronchus is involved most frequently, followed by segmental bronchi of the right upper lobe, right middle lobe, and right lower lobe [5]. Its radiographic appearance is due to mucoid impaction distal to the occluded bronchus and hyperexpansion of the affected pulmonary segment. Specifically, a perihilar mass is surrounded by a halo of lung that is hyperlucent due to collateral ventilation and gas trapping (image 4A-B and image 5 and image 6A-C). Congenital bronchial atresia is discussed in detail separately. (See "Congenital anomalies of the intrathoracic airways and tracheoesophageal fistula", section on 'Bronchial atresia'.)

Congenital lobar emphysema — Congenital lobar emphysema is more appropriately referred to as congenital lobar hyperexpansion. It is characterized by a hyperexpanded and hyperlucent lobe, adjacent atelectasis, and contralateral mediastinal shift (image 7 and image 8 and image 9) [6]. Congenital lobar emphysema is most frequently the result of a check-valve obstruction with gas trapping, not true emphysema. It can also, on rare occasions, be the result of a hyperalveolar lobe with an excessive number of alveoli of normal size. Occasionally, the lobe is opacified in newborns due to retained fetal lung liquid. Congenital lobar emphysema is discussed in detail separately. (See "Congenital lobar emphysema".)

Congenital pulmonary airway malformation (CPAM) — Congenital pulmonary airway malformation (CPAM; previously known as congenital cystic adenomatoid malformation [CCAM]) is characterized by adenomatoid proliferation of bronchioles resulting in cysts rather than alveoli. Radiographically, a large gas-filled cyst, a conglomerate of gas-filled cysts, or a solid mass (ie, liquid-filled cyst) can be seen (image 10A-B and image 11 and image 12 and image 13A-B) [7]. These lesions can cause mass effect with contralateral mediastinal shift. Pleuropulmonary blastoma can have a similar radiologic appearance. CPAM is discussed in detail separately. (See "Congenital pulmonary airway malformation".)

Bronchogenic cyst — Bronchogenic cysts can have a mediastinal or intrapulmonary location. Mediastinal bronchogenic cysts appear as a subcarinal, paratracheal, para-aortic, or aortico-pulmonary window mass (image 14A-B and image 15A-C and image 16A-C and image 17A-B and image 18A-B and image 19). On computed tomography (CT), most are low attenuation (like water), but some display higher attenuation (like soft tissue) due to protein or calcium salts [1,2].

Intrapulmonary bronchogenic cysts appear as sharply outlined masses, which may contain gas due to previous or current infection (image 20 and image 21 and image 22), and sometimes contain mycetomas. A rudimentary bronchus may appear as a tail. On a CT scan, adjacent strands of atelectasis or scarring may be visible. Mosaic attenuation in the lung parenchyma surrounding a bronchogenic cyst has also been described. Bronchogenic cysts are discussed in detail separately. (See "Congenital anomalies of the intrathoracic airways and tracheoesophageal fistula", section on 'Bronchogenic cyst'.)

Tracheal bronchus — A tracheal bronchus (ie, "pig bronchus") is an abnormal, ectopic bronchus that originates from the trachea and usually supplies the right upper lobe or one of its segments (image 23 and image 24). Tracheal bronchi are discussed in detail separately. (See "Congenital anomalies of the intrathoracic airways and tracheoesophageal fistula", section on 'Tracheal bronchus'.)

Tracheal diverticulum — A tracheal diverticulum (ie, tracheocele) is a gas-containing outpouching of the proximal trachea (image 25). It is usually encountered on the posterolateral right side, at the level of the thoracic inlet, due to absence of cartilage in the posterior membranous wall and lack of support provided by the esophagus on the left posterolateral side.

Accessory cardiac bronchus — An accessory cardiac bronchus is an abnormal bronchus that arises from the inferomedial wall of the right main bronchus, bronchus intermedius, or right lower lobe bronchus (image 26) [8]. It either ends blindly or has a small extra segment of lung parenchyma attached to it.

VASCULAR ANOMALIES — Pulmonary vascular anomalies are caused by aberrant angiogenesis. Examples of pulmonary vascular anomalies include proximal interruption of a central pulmonary artery, an anomalous origin of the left pulmonary artery from the right pulmonary artery (pulmonary sling), partial or total anomalous pulmonary venous return, pulmonary arteriovenous malformation, bronchial artery aneurysms and malformations, azygos vein continuation of the interrupted inferior vena cava (image 27) and maldevelopment of lymphatic vessels, such as cystic hygroma.

Proximal interruption of a central pulmonary artery — Blood flow through the central pulmonary arteries (ie, the main pulmonary artery, right pulmonary artery, and left pulmonary artery) may be interrupted due to pulmonary artery agenesis, pulmonary atresia, or pulmonary artery stenosis. Radiographically, the ipsilateral hilum and lung are small and receive arterial blood from systemic collaterals (image 28A-C). Collateral vessels may include bronchial arteries, intercostal arteries, internal thoracic arteries, coronary arteries, and/or a patent ductus arteriosus. The pleura may thicken due to systemic collateral vessels and rib notching may exist. These intercostal collateral artery branches anastomose with the peripheral pulmonary artery branches via pleural adhesions. The right pulmonary artery is most frequently involved [9].

Anomalous origin of the left pulmonary artery — In rare instances, the left pulmonary artery can originate from the right pulmonary artery (ie, pulmonary artery sling); it reaches the left hilum by passing anterior to the right mainstem bronchus and then between the trachea and esophagus. On a chest radiograph, the aberrant left pulmonary artery can appear as a right paratracheal or retrotracheal mass, while the left hilum appears smaller and more caudal in position than usual (image 29). A computed tomography (CT) scan or magnetic resonance angiogram (MRA) can directly image this anomaly (image 30). Anomalous origin of the left pulmonary artery can be associated with complete cartilaginous tracheal rings (the ring-sling syndrome) or with other congenital cardiac anomalies [10].

Anomalous pulmonary venous drainage — Anomalous pulmonary venous drainage exists when oxygenated pulmonary venous blood drains into a vertical vein (that flows into the left brachiocephalic vein), the superior vena cava, the right atrium, the inferior vena cava, or the hepatic veins, instead of the left atrium [11]. The aberrant venous return may involve all four pulmonary veins (ie, total return) or only selected pulmonary veins (ie, partial return).

Radiographically, the cardiac silhouette is enlarged (due to right atrial and right ventricular enlargement resulting from the left-to-right shunt), and the central and peripheral pulmonary vessels are dilated due to shunt vascularity. If the pulmonary venous return is obstructed, the cardiac silhouette is normal with signs of interstitial pulmonary edema (thickening of interlobular septa, peribronchial cuffing, and thickening of interlobar fissures).

If the pulmonary venous blood drains into a vertical vein, the anomalous vertical vein can be mistaken for a left superior vena cava (image 31). In case the anomalous pulmonary vein drains into the inferior vena cava, the anomalous vascular structure can look like a scimitar along the right heart border in the right lower lung region. In cases of partial anomalous pulmonary venous return from the right upper lobe to the superior vena cava, the anomalous veins produce a configuration that is described as the "pineapple sign" on CT. In cases of total anomalous pulmonary venous return, the chest radiograph can display a "snowman" configuration.

Pulmonary arteriovenous malformations — Pulmonary arteriovenous malformations (AVMs) are abnormal connections between pulmonary arteries and pulmonary veins. They allow blood to bypass the pulmonary capillaries, thus creating an extracardiac right-to-left shunt. Radiographically, pulmonary AVMs appear as round, oval, or scalloped homogeneous nodules or masses with feeding arteries and draining veins. They are most common in the lower lobes and in the periphery of the lung (image 32A-D and image 33A-B and image 34A-D and image 35 and image 36 and image 37). It is important to measure the diameter of the feeding artery, since embolization of the arteriovenous malformation is possible only if the diameter exceeds 3 mm [12].

Pulmonary AVMs are discussed in detail separately. (See "Clinical manifestations and diagnosis of hereditary hemorrhagic telangiectasia (Osler-Weber-Rendu syndrome)" and "Pulmonary arteriovenous malformations: Clinical features and diagnostic evaluation in adults" and "Pulmonary arteriovenous malformations: Epidemiology, etiology, and pathology in adults".)

Congenital bronchial artery malformations are unusual and can manifest as aneurysms or malformations with connections to the pulmonary arteries, coronary arteries or cardiac chambers (image 38).

Cystic hygroma — Cystic hygromas (ie, lymphangiomas) are an abnormal collection of lymphatic vessels. They frequently dilate, collect lymph, and resemble liquid-filled cysts. Most are located in the neck, but they can also exist in the axilla and mediastinum. Isolated mediastinal cystic hygromas are rare.

Magnetic resonance imaging (MRI) is the best way to evaluate a suspected cystic hygroma. Characteristics include signal hyperintensity on T2 images, only slightly increased signal intensity on T1 images, and an absence of enhancement after injection of gadolinium-based contrast material. CT scans demonstrate solitary or multiple liquid-filled cysts with rare interspersed calcifications and occasional enhancement of septa. Chest radiographs usually demonstrate a soft tissue mass (image 39 and image 40A-B and image 41A-B and image 42A-B and image 43 and image 44A-B and image 45). Occasionally ectasia of the superior vena cava can be identified in patients with mediastinal hygromas.

COMBINED ANOMALIES — Combined pulmonary and vascular malformations (ie, hybrid abnormalities) include hypogenetic lung syndrome and bronchopulmonary sequestrations.

Hypogenetic lung syndrome — Hypogenetic lung syndrome (ie, congenital venolobar syndrome, scimitar syndrome) is characterized by a small lung, ipsilateral hypoplastic pulmonary artery, and partial anomalous pulmonary venous return [13]. The aberrant pulmonary vein (ie, the scimitar vein) usually drains to the inferior vena cava, but can also connect to the hepatic veins, portal veins, coronary sinus, or right atrium [14,15]. The bronchial tree can be a mirror image of the left side (left or L-isomerism). Coexisting congenital cardiac anomalies are common, as well as scoliosis and hemivertebrae.

Radiographic features include a small hyperlucent lung, an abnormal arrangement of pulmonary arteries, and an aberrant pulmonary vein coursing towards the inferior vena cava (image 46 and image 47A-B).

Bronchopulmonary sequestration — A bronchopulmonary sequestration represents a nonfunctioning mass of lung tissue that lacks normal communication with the tracheobronchial tree and receives its arterial blood supply from the systemic circulation (picture 1). The systemic arterial supply typically originates from the lower thoracic aorta, the upper abdominal aorta, or major aortic branches [16,17]. (See "Bronchopulmonary sequestration".)

Sequestrations are classified anatomically as follows:

●Intralobar sequestration (also known as intrapulmonary sequestration) – The lesion is located within a normal lobe, lacks its own visceral pleura, and lacks connection to the bronchial tree

●Extralobar sequestration (also known as extrapulmonary sequestration) – The mass is located outside the normal lung, has its own visceral pleura, and lacks connection to the bronchial tree

Both have a predilection for the lower lobes, especially the left lower lobe. Venous drainage from an intralobar sequestration is typically into the pulmonary veins and left atrium. In contradistinction, venous drainage from an extralobar sequestration leads typically to systemic veins like the azygos vein, the superior vena cava, the inferior vena cava, the right atrium, or, rarely, the portal vein.

The radiographic appearance of a sequestration is variable and relates to its stage when imaged. Sequestrations initially appear as solid masses since they do not communicate with the bronchial tree. Absent drainage of secretions and recurrent infection eventually lead to communication with the adjacent airways, particularly with intralobar, intrapulmonary sequestrations that are not protected by their own pleural cover. At this stage, the sequestration appears as a gas-containing multilocular cystic structure, with or without gas-liquid levels (image 48A-D and image 49A-B). Thus, the appearance of gas or gas-liquid levels in a sequestration suggests infection.

Conventional computed tomography (CT) does not consistently demonstrate the aberrant systemic arterial supply. Contrast-enhanced helical CT or multidetector row CT angiography have improved the likelihood of simultaneous visualization of the arterial supply, venous drainage, and lung parenchyma. Magnetic resonance imaging (MRI) can demonstrate the location of the lesion and define the aberrant arterial supply and venous drainage, especially if gadolinium enhanced three-dimensional magnetic resonance angiography (MRA) is used. An invasive catheter aortogram was previously the only method able to define the arterial blood supply (image 50).

Bronchopulmonary sequestrations are discussed in detail separately. (See "Bronchopulmonary sequestration".)

SUMMARY AND RECOMMENDATIONS

●Developmental anomalies of the lung can be categorized as bronchopulmonary anomalies, vascular anomalies, or combined (hybrid) anomalies. They are primarily due to aberrant intrauterine development; abnormalities during postnatal lung development are uncommon. (See 'Introduction' above.)

●Bronchopulmonary anomalies include pulmonary agenesis, hypoplasia, congenital bronchial atresia, congenital lobar emphysema (more appropriately called congenital lobar hyperexpansion), congenital pulmonary airway malformation (previously called congenital cystic adenomatoid malformation), a bronchogenic cyst, a tracheal bronchus (ie, "pig bronchus"), a tracheal diverticulum, and an accessory cardiac bronchus. (See 'Bronchopulmonary anomalies' above.)

●Pulmonary vascular anomalies include proximal interruption of a central pulmonary artery, an anomalous origin of the left pulmonary artery from the right pulmonary artery (pulmonary artery sling), partial or total anomalous pulmonary venous return, pulmonary arteriovenous malformation, bronchial artery anomalies and maldevelopment of lymphatic vessels, such as cystic hygroma. (See 'Vascular anomalies' above.)

●Combined (hybrid) pulmonary and vascular malformations include hypogenetic lung syndrome and bronchopulmonary sequestrations. (See 'Combined anomalies' above.)