Occult Pneumothorax

Occult pneumothorax is a pneumothorax that is not seen on the initial supine chest radiograph but is shown by CT or ultrasound. It is usually small, and most are discovered during trauma imaging.

Epidemiology. It is found in about 2 to 10% of blunt trauma patients scanned by CT, and supine films miss over half of all pneumothoraces after trauma. Ultrasound has higher sensitivity than supine radiography.

Causes and associations. Blunt chest trauma, rib fractures, penetrating injury, central line insertion, transbronchial biopsy, barotrauma and following chest drain removal.

Pathophysiology. In the supine patient air collects anteriorly and basally and spreads over a wide area, giving little pleural line on film. Positive pressure ventilation or air transport can enlarge a small collection quickly and convert it into tension pneumothorax.

Presentation is often none, or mild pleuritic chest pain and a small fall in saturation. Subcutaneous emphysema suggests an air leak. Features of tension (hypotension, tracheal shift, raised venous pressure) appear if the collection enlarges, particularly after intubation.

Red flags.

• Planned positive pressure ventilation, general anaesthesia or air transport.
• Rib fractures or subcutaneous emphysema with new hypoxia.
• Enlargement on repeat imaging.

Extended ultrasound examination (absent lung sliding, absent comet tails, lung point), CT chest, and erect or decubitus films where possible. Measure the largest depth of air on CT. Classify as small when the depth is under 2 cm.

Differential diagnosis. Skin fold, bulla, pleural line artefact, haemothorax and surgical emphysema.

Decisions depend on size, ventilation and transport.

Observation (small, stable, breathing spontaneously).

• Repeat chest radiograph at 4 to 6 hours and at 24 hours, with pulse oximetry.
• Give oxygen 2 to 4 L/min to speed absorption.
• Admit, avoid air transport and avoid positive pressure ventilation without a drain.

Chest drain.

• Insert a 14 to 20 Fr drain before positive pressure ventilation, general anaesthesia or air transport.
• Insert a drain for enlargement, symptoms, hypoxia, haemothorax, or when the patient cannot be observed.
• Prefer a small bore catheter (14 Fr) when no blood is present, and 28 to 32 Fr when haemothorax is possible.

Antibiotic. Not routinely needed.

Monitoring. Respiratory rate, saturation, subcutaneous emphysema and repeat imaging. In ventilated patients, watch airway pressures and haemodynamics.

Referral and escalation. Refer to the surgical or trauma team if the air leak persists over 48 hours, the lung fails to expand, or the pneumothorax enlarges.

Special groups. In patients on mechanical ventilation, a drain is advised for all except the smallest pneumothoraces. In patients needing air travel or hyperbaric treatment, drain first.