What are some radiologic signs of atelectasis?
*Increased opacity
*Crowding of the pulmonary vessels
*Displacement of fissures
*Elevation of the hemidiaphragm
*Displacement of the hilus
*Crowding of the ribs
*Compensatory overinflation of normal lung (causing the vascular nodule sign and parallel ray and converging ray signs)
*Shift of the mediastinal contents (e.g. Kattan’s triangle sign)
*Bronchial rearrangement
*Juxtaphrenic peak sign (a pleural peak usually at the site of diaphragmatic insertion of an inferior accessory fissure)
*Shifting granuloma sign

What is the most specific sign of atelectasis?
*Displacement of the fissures.

What are the five mechanisms of atelectasis as per the classification of Fraser and Pare, and what is the definition of each?
1). Obstructive: caused by alveolar air resorption distal to an obstruction
2). Compressive: caused by intrapulmonary abnormalities (e.g., a large mass) that compress surrounding lung
3). Passive (aka Relaxation atelectasis): collapse caused by changes in intrapleural pressure, as in the setting of pneumothorax or pleural effusion
4). Adhesive: caused by adherence of the alveolar wall surfaces in the setting of surfactant deficiency (e.g., hyaline membrane disease of the newborn)
5). Cicatrizing: collapse secondary to scarring/fibrosis (e.g. a late sequela of tuberculosis)

Obstructive atelectasis can be secondary to either large airway obstructive lesions or small-airway obstruction.

What are some causes of large airway obstruction?
1). Bronchogenic carcinoma (always a consideration in patients with histories of persistent atelectasis, recurrent atelectasis, or recurrent pneumonia with failure of complete clearing after treatment)
2). Bronchial carcinoid (above considerations also apply here)
3). Metastases to the bronchi: most commonly renal cell carcinoma, breast carcinoma, melanoma, adenocarcinoma of the colon, sarcomas
4). Lymphoma (usually late stage and accompanied by hilar and mediastinal lymphadenopathy) or other causes of bulky adenopathy
5). Tuberculosis
6). Left atrial enlargement from mitral stenosis (left lower-lobe atelectasis)
7). Foreign body obstruction
8). Mainstem bronchus intubation

What is the most common cause of small-airway obstruction presenting as atelectasis?
*Mucus plugging (esp. in patients on respiratory depressant medications [morphine], in those with central nervous system illnesses producing respiratory depression, as a complication of abdominal and thoracic surgeries, or in the setting of acute exacerbations of chronic obstructive airway diseases like chronic bronchitis, asthma, emphysema, bronchiolitis obliterans, cystic fibrosis, or bronchiectasis)


What are the major causes of compressive atelectasis?
1). Peripheral tumorĐ° compressing normal adjacent lung
2). Extensive air trapping (as seen in bullous emphysema, lobar emphysema, interstitial emphysema, or bronchial foreign body obstruction)

How does passive atelectasis differ from compressive atelectasis?
*In passive atelectasis, the cause of the atelectasis is a pleural space-occupying lesion whereas in compressive atelectasis, the cause of the atelectasis is intrapulmonary. In clinical practice, these entities are commonly considered together.

What are the causes of passive/relaxation atelectasis?
1). Pleural effusion
2). Pneumothorax
3). Hydrothorax, hemothorax
4). Diaphragmatic hernia
5). Pleural masses (including metastases and mesothelioma)

What causes adhesive atelectasis?
*the sticking together of alveolar luminal surfaces presumably secondary to surfactant deficiency

What are the two major causes of adhesive atelectasis?
1). Hyaline membrane disease (aka respiratory distress syndrome of the newborn)
2). Pulmonary embolism

In the appropriate clinical setting, PULMONARY EMBOLISM MUST ALWAYS BE CONSIDERED in the patient with SUBSEGMENTAL ATELECTASIS AND PLEURAL EFFUSION.


What are some classic causes of cicatrizing atelectasis? (any disease processes that result in fibrosis in the interstitial space, either focal or generalized)
1). Tuberculosis, histoplasmosis
2). Pneumoconioses (e.g., silicosis (upper lobe predominance) and asbestosis (lower lobe predominance))
3). Collagen vascular diseases (e.g., scleroderma, rheumatoid lungĐ° both have lower lung predominance)
4). Radiation pneumonitis (late stage): nonanatomic distribution, frequently localized with sharply defined borders at edges of the radiation portals
5). IPF