Lung atelectasis

Lung atelectasis (plural: atelectases) 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 radiopathological 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 subcategorised 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, mucus 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 three most common specific aetiologies of passive atelectasis are pleural effusion, pneumothorax and diaphragmatic abnormality
• 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
  • depending on aetiology, this deficiency may either be diffuse throughout the lungs or localised 
• gravity dependant atelectasis (dependent atelectasis)
  • in the most dependent portions of the lungs due to the weight of the lungs

Atelectasis can also be subcategorised 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.
• osteophyte induced adjacent pulmonary atelectasis and fibrosis

Radiographic features

Vary depending on the underlying mechanism and type of atelectasis

Plain radiograph / CT

Direct signs of atelectasis

• crowded air bronchograms
• displacement of interlobar fissures
• crowding together of pulmonary vessels
• crowded air bronchograms (does not apply to all types of atelectasis; can be seen in subsegmental atelectasis due to small peripheral bronchi obstruction, usually by secretions; if the cause of the atelectasis is central bronchial obstruction, there will NOT be air bronchograms.)

Indirect signs of atelectasis

• pulmonary opacification
• shifting granuloma (or any other previously documented lesion, used as a reference for comparison).
• compensatory hyperexpansion of the surrounding or contralateral lung
• displacement of the heart, mediastinum, trachea, hilum
• elevation of the diaphragm
• propinquity of the ribs

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 lines) 

History and etymology

Atelectasis comes from the greek words ateles and ektasis translating to 'incomplete stretching or expansion'.