Diseases of the Airways

Chapter 22

Diseases of the Airways

Dennis M. Marchiori

ATELECTASIS

BRONCHIAL ASTHMA

BRONCHIECTASIS

BRONCHOPULMONARY SEQUESTRATION

CONGENITAL BRONCHOGENIC CYSTS

EMPHYSEMA


Atelectasis

Background

General.

Atelectasis is defined as incomplete air filling and underexpansion of pulmonary tissue. It should be distinguished from consolidation, which also represents incomplete air filling of the lung. However, in consolidation, the missing air is replaced by blood, edema, pus, or other space-occupying substance, but missing air of atelectasis is not replaced, resulting in segmental collapse. Atelectasis may involve the entire lung or appear localized to a lobe, segment, or subsegment of the lung. It is not a disease itself but rather a radiographic sign suggesting the presence of another pathology. Based on the underlying mechanism, atelectasis is divided into five categories: obstructive, passive, compressive, adhesive, and cicatrization.

Obstructive Atelectasis.

The most common form of atelectasis is obstructive or resorptive atelectasis. It is caused by intrinsic or extrinsic obstruction of an airway by conditions as neoplasm, infection, foreign body, or heavy secretions.2,6 Over time, air distal to the airway obstruction is resorbed and not replaced, leading to an airless, radiolucent, collapsed portion of lung tissue (Figs. 22-1 to 22-5). Often, the lung distal to bronchial obstruction exhibits increased radiopacity correlating to replacement of the alveolar air with inflammatory transudate, exudate, or other substance. Less commonly, a check valve or obstruction develops, creating a hyperlucent, overinflated portion of lung tissue.

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FIG 22-1 Pulmonary tissue may collapse secondary to a bronchial obstruction (obstructive), expanding intrapleural lesion (passive), expanding intrapulmonary lesion (compressive), and scarring contracture of the lung tissue (cicatrization). A fifth type, adhesive, is less common and is seen in infants with hyaline membrane disease.
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FIG 22-2 Collapse of the patient’s left lung with proximal shift of the patient’s left hemidiaphragm, heart, and trachea. Courtesy John A.M. Taylor, Seneca Falls, NY.
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FIG 22-3 A 53-year-old male patient with a fracture of the left sixth rib (arrow). The elevated left hemidiaphragm suggests volume loss of the left lung. Also, blunting of the lateral costophrenic angle denotes pleural effusion (crossed arrow). Courtesy John A.M. Taylor, Seneca Falls, NY.
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FIG 22-4 A, Atelectasis of the left lower lobe presenting as a hazy radiodense shadow in the left lower margin of the lung field (arrow) and, B, posterior shift of the oblique fissure (arrows). Courtesy John A.M. Taylor, Seneca Falls, NY.
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FIG 22-5 Tracheal carcinoma with left hilar mass (white arrow) and partial obstructive atelectasis of the left lung and elevation of the left hemidiaphragm (black arrow). Courtesy Steven P. Brownstein, MD, Springfield, NJ.

A check-valve mechanism involves accumulation of air distal to an airway obstruction. Airways dilate during inhalation, permitting air to pass by a partial bronchial obstruction. Airways constrict tightly around an obstruction during exhalation, effectively limiting the expiration of air distal to the obstruction. With each breathing cycle, more air is “pumped” into the overinflated lung tissue distal to the obstruction. Check-valve obstructions in a small airway are of limited clinical significance; however, they may result in respiratory distress when occurring in a large airway.

Passive Atelectasis.

Passive (relaxation) atelectasis is the second form of atelectasis. It results from the presence of a space-occupying lesion external to the lung. Pleural fluid (blood, exudate, transudate, and chyle) or air may accumulate to such an extent that the adjacent lung is “pushed” aside, resulting in partial or complete pulmonary collapse. Diaphragmatic elevation or herniation of abdominal viscera also may compress the lung. The amount of collapse is proportional to the size of the space-taking lesion.

Compressive Atelectasis.

Compressive atelectasis is a form of passive atelectasis in which the space-occupying lesion is located within the involved lung. Large bullae, neoplasms, abscesses, or other large lesions may result in compressive atelectasis of adjacent lung tissue.

Adhesive Atelectasis.

Adhesive atelectasis is nonobstructive, noncompressive pulmonary collapse secondary to decreased surfactant production by the type II pneumonocytes.31 Pneumonocyte damage results from genetic defects, general anesthesia, ischemia, or radiation damage. Adhesive atelectasis is seen in neonates with hyaline membrane disease.

Cicatrization Atelectasis.

Finally, cicatrization atelectasis represents scarring and contracture of pulmonary tissue after infection, pneumoconioses, scleroderma, radiation, idiopathic pulmonary fibrosis, and so on. This may appear localized (tuberculosis in the lung apices) or generalized (interstitial pulmonary fibrosis).

Imaging Findings

The presence of atelectasis is suggested by direct and indirect radiographic signs (Box 22-1). Displacements of intralobar fissures are the most reliable findings. The radiographic appearance differs based on the degree of lung involvement, location, and type of atelectasis (Table 22-1). Combined lobar collapse may complicate the radiographic appearance.16 Rounded atelectasis is a form of peripheral pulmonary collapse that must be differentiated from neoplastic masses with similar radiographic appearance.20,32

Box 22-1   Direct and Indirect Signs of Lobar Atelectasis

Direct signs

Displacement of interlobular fissures

Increased radiopacity*

Indirect signs

Elevation of the diaphragm

Mediastinal displacement

Hilar displacement

Overinflation of remaining normal lung

Approximation of pulmonary vessels

Approximation of ribs


*Some sources consider increased radiopacity an indirect sign of atelectasis.

TABLE 22-1

RADIOGRAPHIC APPEARANCE OF ATELECTASIS

Atelectasis Imaging Findings
Lung Opacification of the entire hemithorax, compensatory shift of the mediastinum with overinflation, and herniation of opposite lung toward collapsed lung side
Lobe*
Right upper lobe Radiodense lobe displaced superomedially, superiorly displaced diaphragm, oblique and horizontal fissures; can see the characteristic reversed “S shape of Golden sign”10 when a central mass is found in combination with the displaced horizontal fissure
Right middle lobe Best demonstrated on the lateral projection, where the minor and major fissure approximate one another, bordering a thin, oblique-to-horizontal density; loss of silhouette of the left heart border on the frontal projection; on frontal projection, lordotic projection best for visualizing a wedge-shaped density with base adjacent to heart border
Right lower lobe Radiodense lobe displaced posteromedially with posterior displacement of oblique fissure noted on lateral projection
Left upper lobe Radiodense lobe displaced anterolaterally with overinflation of lower lobe occasionally located between collapsed upper lobe and mediastinum, “Luftsichel” sign35
Left lower lobe Radiodense lobe displaced posteromedially, often obstructed by heart shadow; posterior displacement of the oblique fissure on the lateral projection
Segment Increased radiodensity and volume loss that correspond to region collapsed and appear less marked than lobe involvement
Subsegment Also known as plate-like, discoid, or linear atelectasis; represents peripheral atelectasis of small areas of pulmonary tissue appearing as 2 to 10 cm horizontal linear density in the lower lung field
Round (folded lung) Rare form of atelectasis associated with asbestosis-related pleural disease, in which the involved lung appears as a round mass (2–7 cm) in the lower lung

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*Multiple lobes may be collapsed, yielding a combination of findings.

Clinical Comments

Whenever atelectasis is found, it should prompt a vigorous search for a cause. Although several mechanisms of collapse have been identified, obstruction of the airway resulting from neoplasm is another common cause of lobar or segmental atelectasis that should direct patient management.

Key Concepts

• Atelectasis is a sign of underlying disease process, defined as incomplete inflation of the lung.

• The five categories of atelectasis are obstructive, compressive, passive, adhesive, and cicatrization. Obstructive is the most common.

• The radiographic appearance depends on the location and extent of collapse; findings include loss of pulmonary volume, increased radiopacity, and distorted anatomic structures.

Bronchial Asthma

Background

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Feb 16, 2016 | Posted by in RESPIRATORY IMAGING | Comments Off on Diseases of the Airways
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