Atelectasis may be classified based on etiology into obstructive and non-obstructive types. Obstructive atelectasis, is the most common type of atelectasis which may develop due to obstruction by foreign bodies, tumors and mucus plugs. Atelectasis may also be classified based on duration of symptoms into acute and chronic types. Acute atelectasis is associated with airlessness due to recent lung collapse while chronic atelectasis involves a combination of infection, bronchial destruction, and fibrosis, in adition to airlessness.

Atelectasis may be classified based on etiology into obstructive and non-obstructive types.

• Obstructive atelectasis, which is the most common type of atelectasis may develop due to obstruction by foreign bodies, tumors and mucus plugs.
• In case of obstruction from the trachea to the alveoli at any level, alveolar gas reabsorption may occur leading to subsequent atelectasis.^[1]
• Middle lobe syndrome (fixed or recurrent atelectasis of the lingula/ right middle lobe) may occur due to Sjogren’s syndrome. Intraluminal or extraluminal obstruction (compression of the bronchi by adjacent structures) may result in middle lobe syndrome.^[2][3]

• Non obstructive atelectasis may occur due to the following reasons:^{[1]_Causes,_Symptoms,_Treatment_&_Prevention-4|[4]}
  • Severe lung scarring caused by necrotizing pneumonia or granulomatous diseases: Cicatrisation atelectasis
  • Lung infiltration: Replacement atelectasis
  • Extrinsic lung compression: Due to thoracic space occupying lesions
  • Diminished levels of surfactant: Adhesive atelectasis presenting as ARDS
  • Absence of contact between the parietal and visceral pleurae due to fluid (pleural effusion), air (pneumothorax), blood (hemothorax): Passive atelectasis
  • Formation of fibrous bands which adhere the lung to the pleura in patients with asbestosis: Rounded atelectasis
  • Complication of surgery or anaesthesia leading to decreased surfactant activity and dysfunction of the diaphragm: Postoperative atelectasis

Atelectasis may also be classified based on duration of symptoms into acute and chronic types.

• Acute atelectasis is associated with airlessness due to recent lung collapse.
• Acute atelectasis includes postoperative atelectasis, after thoracic or abdominal surgery, chest trauma, and rib fractures. Surfactant deficiency, excessive oxygen therapy and mechanical ventilation may lead to acute atelectasis.

• Chronic atelectasis is not only associated with airlessness, but a combination of infection, bronchial destruction, widening and fibrosis leading to scarring.
• Middle lobe syndrome and rounded atelectasis are causes of chronic atelectasis.
• Middle lobe syndrome (fixed or recurrent atelectasis of the lingula/ right middle lobe) may occur due to Sjogren’s syndrome. Intraluminal or extraluminal obstruction (compression of the bronchi by adjacent structures) may result in middle lobe syndrome.^[5][6]
• Rounded atelectasis is characterized by the formation of fibrous bands which adhere the lung to the pleura in patients with asbestosis.