|Year : 2015 | Volume
| Issue : 2 | Page : 65-74
Disorders of lung development
Department of TB and Chest Diseases, Guntur Medical College and Chest Physician, Government Fever Hospital, Guntur, Andhra Pradesh, India
|Date of Web Publication||12-Jun-2015|
Department of TB and Chest Diseases, Guntur Medical College and Chest Physician, Government Fever Hospital, Amaravathi Road, Guntur - 522 034, Andhra Pradesh
Source of Support: None, Conflict of Interest: None
Congenital lung malformations rarely occur but could represent an important cause of respiratory distress in the newborn. As the development of the respiratory tract occurs according to a strict timetable, abnormalities present at birth may be accurately dated to disturbances of intercellular relationships that have occurred at fixed points in intrauterine life. Major structural abnormalities of the respiratory tract are usually fatal or frequently diagnosed before birth or early in postnatal life. Other developmental anomalies may not manifest themselves until much later in life. Antenatal diagnosis by ultrasound scan permits early recognition and thus adequate management. After birth, thoracic-computed tomography is the most useful diagnostic tool. Management of the lesions is based on the characteristics of the lesion and the clinical status of the patient. The pathogenesis, clinical presentation, diagnostic tools, and management options of the important congenital lung malformations are briefly reviewed.
Keywords: Computed tomography, congenital lung malformations, intrauterine life, management
|How to cite this article:|
Raghu S. Disorders of lung development. J NTR Univ Health Sci 2015;4:65-74
| Introduction|| |
Human lung growth starts as a primitive lung bud in early embryonic life and undergoes several morphological stages which continue into postnatal life. Lung development requires the integration of multiple regulatory factors that mediate patterns of cell proliferation, differentiation, migration, and death. Understanding these processes could provide important insight into controlling cell differentiation and regeneration for therapeutic purposes.
Congenital lung malformations are a group of rare conditions that can be the source for important morbidity and mortality in infants and children. An understanding of their presentation and imaging features is important for the physician. Whenever we come across an abnormal chest X-ray (CXR) whose diagnosis is uncertain, one should rule out congenital lung anomalies.
| Stages of Lung Development and Associated Congenital Anomalies|| |
In the human embryo, development of lung starts as early as 3 weeks of embryonic life and continues into postnatal life up to 8 years.  The structural and vascular development of the lung is closely related and progresses simultaneously in the human fetus. The events of antenatal growth and development of human lung have been divided into five distinct but overlapping stages.  These stages and the associated lung anomalies have been described in [Table 1]. ,,,
For easy understanding, the developmental disorders can be divided into:
- Tracheobronchial anomalies.
- Anomalies of lung parenchyma.
- Anomalies of the pulmonary vasculature.
Some important disorders listed in [Table 2] have been discussed briefly in this article. Other disorders are not discussed because of their extreme rarity.
| Anomalies of Tracheobronchial Tree|| |
Developmental errors of the tracheobronchial tree cannot arise after the 16 th week of intrauterine life since by this time its formation is complete. 
Tracheal agenesis or aplasia refers to the absence of growth in the trachea or part of it. It has a male to female preponderance of 2:1 and is often incompatible with life. 
Three main anatomical patterns. Type II, in which the main bronchi join in the midline and communicate with the esophagus by a single fistula, is the most common variety accounting for 60% of cases [Figure 1]. Reconstructive surgery is needed. 
It is of five types [Figure 2]. Atresia of proximal esophagus and fistula between distal esophagus and trachea is most common variety.
|Figure 2: Figure showing the various types of tracheoesophageal fistulae|
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The H-type tracheoesophageal fistula, in which both the trachea and esophagus remain patent, may be sufficiently small to go undetected until adult life, despite the presence of recurrent symptoms from infancy.  May cause aspiration pneumonias, abdominal distension.  These can be corrected by early surgical intervention.
Three types are there:
- Diffuse stenosis (30%).
- Segmental stenosis (50%) (common).
- Funnel like stenosis (20%).
The condition usually presents in infancy with stridor and respiratory insufficiency. Surgical treatment with various plastic tracheal reconstruction procedures may be necessary. 
Indicate excessive weakness and collapsibility of the tracheal walls as a result of abnormally soft or pliable cartilages. 
Acquired forms of tracheomalacia occur as a result of prolonged endotracheal intubation and relapsing polychondritis.
Presents in childhood with expiratory airflow obstruction and apneic episodes. In severe cases, surgical intervention with aortopexy and various tracheal splinting procedures may be necessary. 
Characterized by unusual width of the trachea and main bronchi. Because of the ineffectiveness of cough, often complicated by lower respiratory infections and bronchiectasis.  Presents in young adults. Diagnosis with tracheo-bronchogram and on computed tomography (CT) or magnetic resonance imaging. 
| Abnormal Patterns of Bronchial Branching|| |
The abnormalities are:
- Additive (often).
The most common major anomaly is a supernumerary right upper lobe bronchus, which may arise anywhere from the trachea or right main bronchus.  Absence of an upper lobe bronchus is the most common of the major subtractive abnormalities (0.3%). 
The most common segmental anomaly is a double-stem apical lower lobe segmental bronchus (7%), which common on the right side. 
Normal pattern of bronchial branching in either the left or the right lung is mirrored in the contralateral lung, resulting in a so-called bilateral left or right lung. 
Result of a developmental interruption of normal bronchial continuity in which a length of bronchus, usually the apicoposterior segmental bronchus of the left upper lobe becomes sealed off from the larger proximal airways to which it may remain connected by thin, uncanalized, fibrous, vestigial strands. 
This sealed-off or atretic bronchus becomes distended by bronchial secretions, resulting in the formation of a cystic space or mucocele.
Asymptomatic and incidental finding in young adult. Diagnosis is by CT scan [Figure 3]. 
|Figure 3: Computed tomography scan chest showing right sided bronchial atresia|
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Surgical excision of the atretic segment by lobectomy or segmentectomy. 
| Bronchogenic Cysts (syn. Bronchial Cysts)|| |
Result of abnormal budding of the tracheobronchial tree during the course of its development between the 26 th day and 16 th week of intrauterine life, in which a tracheal or bronchial bud becomes detached from its parent, thereafter developing separately to produce a cystic structure.  Common in males and Yemenite Jews.
- Central (syn. mediastinal) - Arise earlier in the developmental. 
- Peripheral (syn. intraparenchymal, intralobar) - Arises at a later stage. 
Bronchogenic cysts vary in size and may reach 10 cm in diameter.  They are usually thin-walled, lined by ciliated respiratory epithelium and contain smooth muscle, elastic tissue, cartilaginous trabeculations, and mucous glands. The presence of cartilage in their walls serves to distinguish them from cysts of different origin. A fistulous communication with an adjacent airway with a ball-valve mechanism may cause it to enlarge and present in infancy or exceedingly rarely in young adult life by causing displacement of normal lung.  Intact and uninfected cysts contain clear fluid.
Frequently asymptomatic and may be an incidental finding. Symptoms occur from either a mass effect on adjacent structures or more frequently infection. Rhabdomyosarcoma may occur as a complication. 
Chest X-ray and CT scan: A well-circumscribed, rounded, homogeneous opacity situated either in the mediastinum close to a major airway or in the lung periphery [Figure 4].
|Figure 4: Computed tomography scan chest showing right-sided bronchogenic cyst|
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Elective surgical resection of the lesion is generally advised. Small asymptomatic cysts or in whom surgery is contra-indicated, cysts can be left in situ or needle aspirated. 
Congenital adenomatoid malformation of the lung
Congenital cystic adenomatoid malformation, congenital bronchiectasis or pulmonary cystic disease, accessory lung.
It may involve either a part or the whole of a lobe or rarely an entire lung. The affected lobe or lung derives its blood supply from the pulmonary circulation and comprises a firm and airless mass of disorganized pulmonary tissue that lacks a properly defined bronchial system but contains an excess of air passages resembling terminal bronchioles and often includes mucous cysts of varying sizes. 
Stocker et al. classification: 
Type I characterized by single or multiple large cysts, was the most common form, accounting for 50% and carrying a good prognosis [Figure 5].
|Figure 5: Computed tomography chest showing cystic adenomatoid malformation|
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Type II consisting multiple small cysts of <1 cm in diameter, accounted for 40% of cases and carried a poor prognosis.
Type III carried a very poor prognosis and consisted of a solid airless mass of tissue.
Results from a developmental failure of the proximal bronchial system, arising from the epithelial laryngotracheal bud, to unite in a normal manner with mesodermally derived distal alveolated tissue.  About 50% of cases are born prematurely, and 25% are stillborn.  Many of those born alive die within a few hours, often with anasarca caused by vena caval compression due to the space-occupying effect of the abnormal tissue mass. Infection of the cysts commonly occurs if the infant survives beyond the neonatal period, although the onset of symptoms may be delayed for 2 or 3 years.  The condition may be complicated by pneumothorax. Can be diagnosed antenatally by ultrasound examination. Treatment by surgical resection of the affected parts of the lung. 
| Anomalies of The Lung Parenchyma|| |
Agenesis and hypoplasia of the lung
Complete bilateral agenesis of the lungs is extremely rare.  Unilateral agenesis of the lung is much less rare and may be present to varying degrees of severity.  The left lung is affected more frequently and exhibit other congenital abnormalities, one of the most common being patent ductus arteriosus.  Occur in three forms. In the most common form of unilateral agenesis, there is rudimentary bronchus but a complete absence of lung tissue.
Mostly etiology is unknown.
- Unilateral hypoplasia
- Conditions that decrease intrathoracic space, 
- Congenital diaphragmatic hernia,
- Ipsilateral congenital vascular anomalies.
- Bilateral hypoplasia
- Oligohydramnios (renal tract disorders or by amniotic fluid leak), 
- Premature rupture of the membranes,
- Maternal treatment with ACE inhibitors. 
Recurrent episodes of lower respiratory tract infection resulting from either imperfect drainage of lung secretions or the spillover of pooled secretions from a blind bronchial stump into initially normal lung tissue. 
A hemithorax of diminished volume with crowded ribs, elevated hemidiaphragm, and mediastinal displacement to the affected side. The contralateral lung tends to overinflate and may expand across the midline [Figure 6].
|Figure 6: Chest X-ray PA view showing agenesis of the right lung with compensatory hyper-inflation of left lung|
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Bronchoscopy, CT and occasionally pulmonary angiography. 
| Congenital Abnormalities of Lobulation|| |
Most common is the azygos lobe.  Present in about 0.25% of chest radiographs An azygos lobe is really the medial portion of a bifurcated right upper lobe regarded as an anatomical variation. Normally, the entire right upper lobe lies lateral to the azygos vein. But in the case of this anomaly the right upper lobe assumes a bifid configuration so that a portion of the lung lies on either side of the venous arch formed by the azygos vein.  The azygos fissure is a supernumerary fissure and is bounded on each side by a layer of visceral and parietal pleura. CXR: Thin outwardly convex line ending in a small triangular shadow at its lower end [Figure 7].
| Lung Sequestration|| |
Characterized by the formation of an island of abnormal unventilated lung tissue that has no normal communication with the bronchial system and derives its arterial supply from the systemic rather than the pulmonary circulation.
Develops as a result of the formation of an accessory lung bud distal to the normal laryngotracheal bud on the ventral aspect of the primitive foregut. As this anomalous bronchial tissue grows, it becomes invested by primitive respiratory mesenchyme. The amount of mesenchyme determines the size of the sequestration and results in the development of its respiratory bronchioles and alveolar tissue. 
Two types [Table 3]:
Extralobar sequestration. 
Intralobar sequestration: More common. The abnormal lung tissue is contained within the substance of normal lung tissue that completely surrounds it. 
Extralobar sequestration (accessory lung without a bronchial connection'). 
Abnormal lung tissue, although lying close to normal lung, is situated outside the pleural coverings of the lung and is indeed invested by its own separate pleural membranes.
Both forms occur more commonly in the left hemithorax.  Probably due to delayed closure of the primitive pleuroperitoneal canal on the left side and also the absence of hepatic tissue on the left provides additional space in which abnormal tissue may develop.
Extralobar sequestrations are frequently diagnosed in the first year of life because of associated multiple congenital abnormalities.
Manifest as nonspecific symptoms of lower respiratory tract infection or with recurrent pneumonic episodes, occasionally hemoptysis, which may be massive, mucocele, and fistula formation.
Persistent homogenous opacity situated in either of the posterior basal segments. Air-fluid level appears if the cysts get infected.
Small homogenous opacity contiguous with the left hemidiaphragmatic shadow.
Computed tomography scan confirms the diagnosis [Figure 8]. Retrograde thoracic aortography to demonstrate feeding vessel.
|Figure 8: Contrast computed tomography chest showing pulmonary sequestration|
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Resection of a symptomatic pulmonary sequestration is indicated. Any infection that might be present is first treated with appropriate antibiotics. 
Intralobar sequestrations: Segmental resection or lobectomy.
Extralobar sequestration: Resected without disturbing normal lung.
| Anomalies of The Pulmonary Vasculature|| |
Persistent pulmonary hypertension of new-born
Due to disturbances of remodeling that result in the excessive muscularization of intra-acinar arteries. Disturbances are due to maternal ingestion of aspirin which leads to the premature closure of ductus arteriosus. 
Absent pulmonary artery trunk
The pulmonary artery trunk may be completely absent as a result of agenesis of the primitive sixth branchial arch or of developmental failure of the septum that normally divides the truncus arteriosus into the aorta and pulmonary artery trunk. 
Absent unilateral pulmonary artery
Consequence of the failure of either the left or right side of the primitive sixth branchial arch to develop in the embryo. The lung parenchyma on the affected side receives its blood supply systemically, most frequently from enlarged bronchial vessels. The left and right pulmonary arteries are affected with equal frequency but the right-sided lesion is more common in adult life because absence of the left pulmonary artery is strongly associated with congenital heart disease and therefore with increased early mortality. ,
Pulmonary artery stenosis 
Whereas patients who present in infancy usually due to pulmonary hypertension as a result of associated cardiac disease. Adults may be asymptomatic, the diagnosis being made following a chest radiograph requested for some other reason. The chest radiograph shows a small hilar shadow and a small hyperlucent lung on the involved side. The diagnosis may be confirmed and differentiated from Macleod's syndrome by a perfusion lung scan or pulmonary angiogram, both of which show a complete lack of perfusion on the affected side.
| Anomalous Origin of The Left Pulmonary Artery|| |
The failure of the pulmonary arterial plexus of the left lung and the left part of the sixth primitive branchial arch to connect in the normal way may result in the left pulmonary artery arising in anomalous fashion from the right pulmonary artery.
This aberrant left pulmonary artery loops around the right main bronchus like a sling and runs a peculiar course between the lower end of the trachea anteriorly and the esophagus posteriorly, thereby crossing the midline to enter the left hilum. A condition known as "sling" pulmonary artery syndrome. 
Symptoms are due to the pressure on the left main bronchus, trachea, and esophagus.
On the lateral view appears as a rounded opacity between the air-filled trachea and esophagus at the level of the carina. A barium swallow may provide supportive evidence.
The diagnosis is confirmed by pulmonary angiography.
| Anomalous Systemic Pulmonary Perfusion|| |
Anomalous venous drainage of a lung arises as a result of persistent communications between the embryonic system of cardinal veins and the pulmonary venous plexus. These communications develop into aberrant pulmonary veins that drain systemically, usually into the inferior vena cava that creates a left-to-right shunt. 
Scimitar syndrome: (Syn: Congenital pulmonary venolobar syndrome/vena cava bronchovascular syndrome).
The most consistent feature is the drainage of part of the right lung by an anomalous scimitar-shaped pulmonary vein that passes through the lung running parallel to, but separate from, the mediastinal structures before joining the inferior vena cava, usually below the diaphragm. 
- An anomalous systemic arterial supply to the territory drained by the scimitar vein
- Hypoplasia of the right lung 
- Cardiac dextroposition
- Diaphragmatic abnormalities.
The condition is most frequently diagnosed in adult life in asymptomatic subjects following a routine chest radiograph.
Shows a characteristic curvilinear vascular density in the right lower lung with features of ipsilateral loss of lung volume [Figure 9].
|Figure 9: Chest X-ray PA view showing curvilinear vascular density in right lower zone|
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Pulmonary arteriovenous malformations and telangiectasia
(Syn: Pulmonary arteriovenous fistula/pulmonary arteriovenous aneurysm/pulmonary angioma/pulmonary haemangioma/pulmonary cavernous angioma).
Probably originate during the development of the primitive pulmonary vascular plexus, when an anastomotic vessel arises which presents a significantly lower resistance to flow than that provided by the surrounding pulmonary vascular bed.  About 95% of arteriovenous malformations derive their blood supply from branches of the pulmonary artery and drain into the pulmonary venous circulation. 
Pulmonary telangiectasis results if there is a fistulous communication between a small peripheral arteriole and venule. If large numbers of these defects are present and if they are diffusely distributed, then the condition may be referred to as pulmonary telangiectasia. 
Arteriovenous malformations are slightly more common in women than men. 
Mean age at diagnosis was 40 years.  Occasionally, the condition is detected in infancy, especially in patients with hereditary telangiectasia. More than half of all patients have no symptoms. 
The respiratory symptom most commonly present at the time of diagnosis is dyspnea, haemoptysis being less frequent.  A bruit is audible over the lesion in more than half of all patients with arteriovenous malformations.  Cyanosis may be present as a result of right-to-left shunting and, when this is present, finger clubbing is a frequent accompaniment.
Symptoms are unlikely to occur in the case of single discrete lesions provided that the radiographic diameter of the malformation is <2 cm.
One or more rounded lobulated or tortuous opacities, frequently occupying the periphery of the lower lung fields [Figure 10].  Serial radiographs may show gradual enlargement of the malformations with the passage of time. The shadows are often well-defined, and the radiograph may be misinterpreted as indicating metastatic tumor. 
|Figure 10: Chest X-ray PA view of a patient with pulmonary A-V malformation showing the opacity at the periphery of the left lower zone|
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The radiographic size of the discrete lesion may be shown to vary according to intrathoracic pressure, increasing in size during a Muller maneuver and becoming smaller with a valsalva maneuver. 
Computed tomography chest
Useful method for identification and delineation of lesions and for identification of feeding or draining the vessel. 
Bilateral pulmonary angiography
Confirms the diagnosis in all but a few cases. This investigation, together with CT, should always be undertaken if surgical treatment is being considered. 
Active treatment is desirable for lesions with feeding vessels >3 mm in diameter. 
Discrete peripheral arteriovenous malformations may be removed by local resection with ligation of the afferent and efferent vessels. Deeper and more extensive lesions may be removed by wedge resection, segmental resection or lobectomy with the general aim of conserving as much lung tissue as possible. 
When the arteriovenous malformations are multiple or bilateral, selective treatment is indicated and embolization or occlusion techniques, using balloons or steel coils, have been used with success.  In some cases of multiple lesions and telangiectasia with cyanosis, lung transplantation can be tried. 
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]
[Table 1], [Table 2], [Table 3]