Introduction
Background
The term atelectasis, which is defined as diminished lung volume, is derived from the Greek words ateles and ektasis, which mean incomplete expansion (see Image 1). Atelectasis may affect all or part of a lung, and it is one of the most common radiographic abnormalities. Recognizing atelectasis on a chest radiograph is important because a sinister underlying pathology may be present.1,2,3,4,5 Several types of atelectasis have been described; each has a unique radiographic pattern. Atelectasis can be categorized as obstructive or nonobstructive.
Obstructive atelectasis
An obstruction between the alveoli and trachea causes reabsorption of alveolar gas, leading to an obstructive atelectasis. The obstruction can occur at the level of the larger or smaller bronchus, and it may be secondary to a foreign body, benign or malignant tumor, mucus plug, and blood clot, as well as bronchial transection, fibrotic stenosis from granulomas or inflammation, polychondritis, post brachytherapy or radiotherapy stenosis, and other obstructive lesions.
The development of atelectasis depends on several factors, including the extent of collateral ventilation and the composition of inspired gas. Obstruction of a larger bronchus is likely to produce lobar atelectasis, whereas the obstruction of a smaller bronchus causes segmental atelectasis. The pattern of atelectasis often depends on collateral ventilation, which is provided by the pores of Kohn and the canals of Lambert.
Right middle lobe (RML) syndrome, a form of chronic atelectasis, usually results from bronchial compression and obstruction by surrounding lymph nodes or bronchial scarring. Partial bronchial obstruction and recurrent infection may also lead to chronic atelectasis and acute or chronic pneumonitis.
Nonobstructive atelectasis
Loss of contact between the parietal and visceral pleurae causes nonobstructive atelectasis. The etiologies may be lung compression, the loss of surfactant, and scarring or infiltrative disease of the lung. Several types of nonobstructive atelectasis are known to occur from a variety of causes.
A pleural effusion or a pneumothorax eliminates contact between the parietal and visceral pleurae, and relaxation or passive atelectasis results. The uniform elasticity of a normal lung preserves the shape, even after atelectasis is present. The middle and lower lobes collapse more than the upper lobes in the presence of a pleural effusion, whereas the upper lobes are more affected by a pneumothorax.
Compression atelectasis occurs when any space-occupying lesion of the thorax compresses the lung and forces air out of the alveoli. The mechanism is similar to relaxation atelectasis.
Adhesive atelectasis results from surfactant deficiency.Surfactant lowers the surface tension of the alveoli and therefore plays an important role in preventing the alveoli from collapsing. Decreased production or inactivation of surfactant, as observed in acute respiratory distress syndrome (ARDS) and similar disorders, leads to alveolar instability and atelectasis.
Cicatrization atelectasis results as a sequela of severe parenchymal scarring and is usually caused by granulomatous disease or necrotizing pneumonia. The lobar collapse from cicatrization may be either obstructive if the bronchi are involved or nonobstructive because of the fibrotic process in the lung parenchyma. Replacement atelectasis occurs when the alveoli of an entire lobe are filled by tumor (eg, bronchioalveolar cell carcinoma), resulting in a loss of volume.
Rounded atelectasis, also called folded-lung syndrome or Blesovsky syndrome, occurs as the lung collapses and folds secondary to fibrous bands and adhesions to the visceral pleura.6 The incidence is high in asbestos workers (65-70% of cases). Patients are typically asymptomatic, and the mean age at presentation is 60 years. Rounded atelectasis is a benign disorder.
For excellent patient education resources, visit eMedicine's Lung and Airway Center and Cancer and Tumors Center. Also, see eMedicine's patient education articles Collapsed Lung and Lung Cancer.
Related eMedicine topics:
Acute Respiratory Distress Syndrome
Atelectasis, Pulmonary
Pneumothorax
Related Medscape topics:
Resource Center Lung Cancer
Resource Center Pneumonia
CME/CE Positive End-Expiratory Pressure in Acute Lung Injury
CME Radiological Imaging in Pneumonia: Recent Innovations
Pathophysiology
The mechanisms of obstructive, nonobstructive, platelike, and postoperative atelectasis are different and determined by several factors.
Obstructive atelectasis
After the obstruction of a bronchus, the absorption of gas in the peripheral alveoli leads to retraction of the lung and an airless state within a few hours. A filling of the alveolar spaces with secretions and cells may occur, thereby preventing complete collapse of the atelectatic lung. The uninvolved surrounding lung tissue becomes distended and displaces the surrounding structures, shifting the heart and mediastinum toward the atelectatic area, elevating the diaphragm, and flattening the chest wall. If the obstruction is removed, any complicating postobstructive infection subsides, and the lung returns to its normal state.
Nonobstructive atelectasis
The loss of contact between the visceral and parietal pleurae is the primary cause of nonobstructive atelectasis. A pleural effusion or pneumothorax causes relaxation or passive atelectasis. Pleural effusions affect the lower lobes more commonly than do pneumothoraces, which affect the upper lobes. A large pleural-based lung mass may cause compression atelectasis by decreasing lung volumes.
Surfactant has phospholipid dipalmitoyl phosphatidylcholine, which prevents lung collapse by reducing the surface tension of the alveoli. The lack of production or the inactivation of surfactant may occur in ARDS, radiation pneumonitis, and blunt trauma to the lung; this change causes adhesive atelectasis. Scarring of the lung parenchyma leads to cicatrization atelectasis. Filling of the entire lobe by a tumor such as bronchoalveolar carcinoma causes replacement atelectasis.
Platelike atelectasis
Also called discoid or subsegmental atelectasis, platelike atelectasis probably occurs because of obstruction of a small bronchus.7 It is observed in states of hypoventilation, pulmonary embolism, or lower respiratory tract infections. Small areas of atelectasis occur because of inadequate regional ventilation and abnormalities in surfactant formation from hypoxia, ischemia, hyperoxia, and exposure to various toxins.
Postoperative atelectasis
Postoperative atelectasis is commonly observed after thoracic and upper abdominal procedures. Diaphragmatic dysfunction and reduced surfactant are consequences of general anesthesia and surgical manipulation that can lead to atelectasis. The atelectasis is typically basilar and segmental in distribution.
Postoperative atelectasis is extremely common. Lobar atelectasis is also common.
Frequency
United States
The incidence or prevalence of this disorder has not been well documented.
International
Data on the international prevalence of this disorder are not available.
Mortality/Morbidity
- Patient mortality depends on the underlying cause of the atelectasis.
- Postoperative atelectasis generally improves, but in lobar atelectasis secondary to endobronchial obstruction, the prognosis depends on treatment of the underlying malignancy or other cause of obstruction.
Race
No data are available regarding the racial predilection of lobar atelectasis; the disorder affects people of all racial origins.
Sex
No sex preference has been documented.
Age
Age as such does not appear to be a risk factor or contributor to atelectasis.
Anatomy
In children and young adults, the trachea passes downward and backward, close to the midline. In older individuals, a slight deviation to the right occurs because of the left-sided aortic arch. The trachea divides into the 2 mainstem bronchi at the level of carina, which is generally at the level of the fifth thoracic vertebra.
The right lung has 3 lobes, and each has its individual bronchus. The right upper lobe (RUL) bronchus takes off from the right mainstem bronchus, close to the carina. The RUL has 3 segments: the apical segment, the posterior segment, and the anterior segment. The right mainstem bronchus continues downward as the bronchus intermedius before dividing into the RML bronchus and right lower lobe (RLL) bronchus. The RML bronchus has a medial segment and a lateral segment. The RLL bronchus has 5 segments: the superior segment, the anterior basal segment, the lateral basal segment, the posterior basal segment, and the medial basal segment.
The left main bronchus is approximately 50 mm long. It divides into the left upper division and the left lower lobe (LLL) bronchus. The right main bronchus is approximately 25 mm in length and has a steeper angle than the left. The left upper division further divides into the left upper lobe (LUL) bronchus, which terminates into 2 segmental bronchi, the left anterior and the apical posterior. The left apical posterior segment bronchus further divides into apical and posterior segments.
The third segmental bronchus is the left anterior segment. The lingular bronchus divides into superior and inferior segments. The LLL bronchus continues downward and has 4 segments: the superior segment, the anteromedial basal segment, the lateral basal segment, and the posterior basal segment. There is considerable variation in bronchial anatomy, particularly of the segmental and subsegmental airways.
Presentation
Atelectasis commonly occurs after thoracic or upper abdominal procedures.
Signs and symptoms
Most symptoms and signs are determined by the rapidity with which the bronchial occlusion occurs, the size of the lung area affected, and the presence of a complicating infection.
Rapid lung collapse causes pain on the affected side, a sudden onset of dyspnea, and cyanosis. Hypotension, tachycardia, fever, and shock may also occur.
Slowly developing atelectasis may be asymptomatic or cause only minor symptoms. Patients with middle-lobe syndrome are often asymptomatic, although they may have a severe, hacking, nonproductive cough.
Physical examination reveals dullness to percussion over the involved area and diminished or absent breath sounds. Chest excursion in the area is reduced or absent. The trachea and the heart are deviated toward the affected side.
Causes
The primary cause of acute or chronic atelectasis is bronchial obstruction caused by the following: plugs of tenacious sputum; foreign bodies; endobronchial tumors; or compression of the bronchi and bronchial distortion due to a tumor, lymph node, or aneurysm.
External pulmonary compression caused by pleural fluid or air (eg, pleural effusion, pneumothorax) may also cause atelectasis.
Abnormalities of surfactant production contribute to alveolar instability and may result in atelectasis. These abnormalities commonly occur with oxygen toxicity and ARDS.
Resorptive atelectasis is caused by the following: bronchogenic carcinoma; bronchial obstruction due to a metastatic neoplasm (eg, adenocarcinoma of the breast or thyroid, hypernephroma, melanoma); an inflammatory etiology (eg, tuberculosis, fungal infection); an aspirated foreign body; a mucus plug; a malpositioned endotracheal tube; or extrinsic compression of an airway by a neoplasm, lymphadenopathy, aortic aneurysm, or cardiac enlargement.
Relaxation atelectasis is caused by the following: a pleural effusion, pneumothorax, or large emphysematous bulla.
Compression atelectasis is caused by the following: chest wall, pleural, or intraparenchymal masses or loculated collections of pleural fluid.
Adhesive atelectasis is caused by the following: hyaline membrane disease, ARDS, smoke inhalation, cardiac bypass surgery, uremia, or prolonged shallow breathing.
Cicatrization atelectasis is caused by the following: idiopathic pulmonary fibrosis, chronic tuberculosis, fungal infections, or radiation fibrosis.
Replacement atelectasis is caused by alveoli filled by tumor or fluid.
Rounded atelectasis is caused by asbestos pleural plaques.
Related eMedicine topics:Airway Foreign Body
Effusion, Pleural
Hyaline Membrane Disease
Lung, Postprimary Tuberculosis
Preferred Examination
Chest radiographs are generally sufficient to diagnose lobar atelectasis and to identify the collapsed lobe. Chest radiographs are also useful in diagnosing platelike atelectasis, postoperative atelectasis, and rounded atelectasis, as well as for following the course of the atelectasis. For example, chest radiographs can be used to determine whether an intervention, such as chest physiotherapy, has resulted in improvement.
However, in some situations, chest radiographic findings may not be diagnostic. This generally occurs when a concomitant pleural fluid or large pulmonary masses are present. In such cases, computed tomography (CT) scanning is a useful next imaging study. CT scanning should be used to assess obstructive atelectasis; this modality is also helpful in evaluating the mediastinum, chest wall, hilum, pleura, and adjacent lung.8,9,10,11,12
Magnetic resonance imaging (MRI) has no particular value in the diagnosis of lobar atelectasis, except for distinguishing obstructive from nonobstructive atelectasis.13
Limitations of Techniques
A concomitant pleural effusion, pleural mass, or large lung mass may limit the usefulness of chest radiography in the diagnosis of atelectasis.
When a basal opacity, an opacity of the hemithorax, and other signs of atelectasis are not obvious, determining whether the opacity is a pleural effusion or a lobar collapse may be difficult. In those situations, a CT scan can be of immense help. Intravenous contrast enhancement is often required for appropriate imaging and for differentiating among various causes of atelectasis.
The limitation of CT scanning may be in differentiating between obstructive and nonobstructive causes of atelectasis. Furthermore, a CT scan may not be useful in determining whether the obstructing lesion is a tumor, mucus plug, nonopaque foreign body, or blood clot.
Differential Diagnoses
Other Problems to Be Considered
Bronchogenic carcinoma, which may occur with atelectasis, must be ruled out in all patients older than 35 years.
A spontaneous pneumothorax produces clinical findings that are similar to those of atelectasis, but the percussion note is tympanic, the heart and mediastinum are pushed to the opposite side, and the radiographic results are diagnostic.
A massive pleural effusion may cause absent breath sounds, dyspnea, cyanosis, weakness, and dullness over the hemithorax. However, the heart and mediastinum are deviated away from the involved area.
Related eMedicine topic:
Pneumothorax, Iatrogenic, Spontaneous and Pneumomediastinum
Related Medscape topic:
Resource Center Lung Cancer
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Further Reading
Keywords
obstructive atelectasis, nonobstructive atelectasis, platelike atelectasis, discoid atelectasis, subsegmental atelectasis postoperative atelectasis, cicatrization atelectasis, rounded atelectasis, folded-lung syndrome, Blesovsky syndrome, Blesovsky's syndrome, replacement atelectasis, incomplete expansion, diminished lung volume, pulmonary volume deficiency, lung foreign body, lung tumor, mucus plugging, pleural effusion, right middle lobe syndrome, atelectatic lung tissue, bronchial obstruction, pneumothorax lobar collapse, pneumothorax, airless lung
