Pneumatocoele

Pneumatocoeles associated with pneumonia infection are usually seen in children especially with Staphylococcus aureus or Gram-negative bacteria (see staphylococcus pneumonia). The mechanism is generally a check-valve type transient airway obstruction caused by inflammatory exudate that leads to distal air trapping and pulmonary overinflation. Development of communication between an abscess and small airways as a result of necrosis of intervening tissue can also cause air trapping within the abscess due to check-valve obstruction by oedematous airway mucosa or intraluminal material. Pneumatocoeles in association with an infection or hydrocarbon ingestion typically form during the resolution of pneumonia and resolve spontaneously without a sequela.

Post-traumatic pneumatocoeles result from air trapping in a lung laceration and appear immediately after or within a few hours after a blunt chest trauma. They most commonly develop in children and young adults because the flexible chest wall injures the lung upon impact. They typically present as thin-walled air cysts with or without an airfluid level on chest radiograph (Fig.1). In some cases, surrounding lung contusion may obscure the pneumatocoeles. Other signs of injury such as pneumothorax, haemothorax, or pneumomediastinum may be seen. CT is much more sensitive to pneumatocoeles than chest radiograph.

In the case of barotrauma, high pressure from mechanical ventilation or Valsalva manoeuvre results in alveolar rupture. Air enters the interstitium, with subsequent development of pneumatocoeles. As a general rule, they progressively decrease in size and disappear, but occasionally they may continue to grow and lead to respiratory failure. Subpleural pneumatocoeles can rupture into the pleural space and cause pneumothorax.

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