Lung atelectasis

Lung atelectasis refers to collapse or incomplete expansion of pulmonary parenchyma. Note that the term "atelectasis" is typically used when there is partial collapse, whereas the term "collapsed lung" is typically reserved for when the entire lung is totally collapsed.

Classification

Atelectasis is a radio-pathological sign which can be classified in many ways. The aim of each classification approach is to help identify possible underlying causes together with other accompanying radiological and clinical findings.

Atelectasis can be sub-categorised based on underlying mechanism, as follows:

• resorptive (obstructive) atelectasis
  • occurs as a result of complete obstruction of an airway
  • no new air can enter the portion of the lung distal to the obstruction and any air that is already there is eventually absorbed into the pulmonary capillary system, leaving a collapsed section of the affected lung
  • because the visceral and parietal pleura do not separate in resorptive atelectasis, traction is created, and if the loss of volume is considerable, mobile thoracic structures may be pulled toward the side of volume loss ("mediastinal shift")
  • potential causes of resorptive atelectasis include obstructing neoplasms, mucous plugging in asthmatics or critically ill patients and foreign body aspiration
  • resorptive atelectasis of an entire lung ("collapsed lung") can result from complete obstruction of the right or left main bronchus
• passive (relaxation) atelectasis
  • occurs when contact between the parietal and visceral pleura is disrupted
  • the two most common specific aetiologies of passive atelectasis are pleural effusion and pneumothorax
• compressive atelectasis
  • occurs as a result of any thoracic space-occupying lesion compressing the lung and forcing air out of the alveoli
• cicatrisation atelectasis
  • occurs as a result of scarring or fibrosis that reduces lung expansion
  • common aetiologies include granulomatous disease, necrotising pneumonia and radiation fibrosis
• adhesive atelectasis
  • ​occurs from surfactant deficiency ²
• gravity dependant atelectasis ((dependent atelectasis)
  • in the most dependent atelectasis
  )portions of the lungs due to the weight of the lungs

Atelectasis can also be sub-categorised by morphology as follows: 

• linear (plate, band, discoid, subsegmental) atelectasis: a minimal degree of collapse as seen in patients who are not taking deep breaths ("splinting"), such as postoperative patients or patients with rib fracture or pleuritic chest pain; this is very common
• lobar atelectasis: collapse of one or more lobes of a lung.
• segmental atelectasis: collapse of one or more individual pulmonary segments
  • subsegmental atelectasis
• round atelectasis: classically associated with asbestos exposure.

Specific types

• osteophyte induced adjacent pulmonary atelectasis and fibrosis

Radiographic features

Vary depending on the underlying mechanism and type of atelectasis

Plain radiograph / CT

Resorptive (obstructive) atelectasis

• increased density (opacity) of the atelectatic portion of lung
• displacement of the fissures toward the area of atelectasis
• upward displacement of hemidiaphragm ipsilateral to the side of atelectasis
• crowding of pulmonary vessels and bronchi in region of atelectasis
• +/- compensatory overinflation of unaffected lung
• +/- displacement of thoracic structures (if atelectasis is substantial)

Linear (plate, discoid, subsegmental) atelectasis

• relatively thin, linear densities in the lung bases oriented parallel to the diaphragm (known as Fleischner's lines) 

History and etymology

Atelectasis in greek translates to 'incomplete stretching or expansion'.