The definition of atelectasis is loss of air in
the alveoli; alveoli devoid of air (not replaced).
A diagnosis of atelectasis requires the following:
- A density, representing lung devoid of air
- Signs indicating loss of lung volume - classification
Types of Atelectasis:
When airways are obstructed there is no further ventilation to the lungs and beyond. In the early stages, blood flow continues and gradually the oxygen and
nitrogen get absorbed, resulting in atelectasis.
The lung is held close to the chest wall because of the negative pressure in the pleural space. Once the negative pressure is lost the lung tends to recoil due to elastic properties and becomes atelectatic. This occurs in patients with pneumothorax and pleural effusion. In this instance, the loss of negative pressure in the pleura permits
the lung to relax, due to elastic recoil. There is common misconception that atelectasis is due to compression.
Surfactant reduces surface tension and keeps the alveoli open. In conditions where there is loss of surfactant, the alveoli collapse and become atelectatic. In ARDS this occurs diffusely to both lungs. In pulmonary embolism due to loss of blood flow and lack of CO2, the integrity of surfactant gets impaired.
Alveoli gets trapped in scar and becomes atelectatic in fibrotic disorders.
- An instance where the lung gets trapped by pleural disease and is devoid of air. Classically
encountered in asbestosis.
Signs of Loss of Lung Volume:
- It is difficult to ascertain the etiology of atelectasis unless there are other clues to suggest an etiology.
- Mass or nodes in hilum , foreign body
(coin in trachea-bronchial tree), lytic lesion in bone.
- Clinical history and physical findings should help in determining the most probable etiology for a given atelectasis.
A word of caution
Progressive decrease in size of density should be carefully evaluated to make sure that it is not progressive
More complete the atelectasis, the smaller the shadow!
Special Radiological Signs:
- Bowing Sign
- In LUL atelectasis or following resection, the oblique fissure bows forwards in the lateral view. Bowing sign refers to this feature.
- Double Lesion Sign
- If you encounter atelectasis of RUL and RLL sparing RML, it is difficult to comprehend a single endobronchial lesion to account for both lesions. There has to be two independent endobronchial lesions, hence it was proposed that it is unlikely to be due to primary bronchogenic cancer. The concept is based on knowledge of
- Caution: Do not rule out lung cancer when you see this phenomenon, you just have to find an alternate explanation for the second lesion.
- S Curve of Golden
- The transverse fissure is "S" shaped. The proximal portion of the fissure is convex because the tumor mass prevents the fissure from moving towards
the hilum. Whenever you see this shape to a fissure, consider a mass causing atelectasis.
- Open Bronchus Sign
- When air bronchogram is visible in an atelectatic lung, it implies that there is no airway obstruction. It is more a trapped lung with patent airways.
- Inverted Comma Sign
- Do not mistake an azygous lobe to movement of transverse fissure. Azygous lobe gives the appearance of an inverted comma.