Pneumothorax on chest X‑ray
A pneumothorax is air in the pleural space, between the lung and the chest wall. It may be primary spontaneous (in people without known lung disease, classically tall young adults), secondary spontaneous (with underlying lung disease such as COPD), traumatic or iatrogenic (after a central line, lung biopsy or ventilation). Most are seen easily on an erect chest film once you know where to look, but small apical, supine and post-procedure pneumothoraces are some of the most commonly missed findings in chest radiology. A tension pneumothorax is a life-threatening clinical emergency that should be treated without waiting for a film.
1. How to look
- Check the position first. On an erect film air rises to the apex and lateral chest. On a supine film it rises to the front of the chest and collects anteriorly and at the lung base. See X-ray projections.
- Trace the lung edge all the way round on both sides: from the apex, down the lateral chest wall, to the costophrenic angle. Small pneumothoraces hide behind the ribs and clavicle at the apex.
- Use the workstation. Magnify the apices, adjust window and level, and try inverting the image if available.
- Check the soft tissues for surgical emphysema, and the ribs for fractures, which make pneumothorax more likely.
- Compare with previous films, especially after a procedure.
An expiratory film is not routinely recommended to look for a pneumothorax (BTS); a standard inspiratory PA film is the usual first test.
2. Key signs on the erect film
- Visceral pleural line: a thin, sharp, white line, the edge of the lung, usually running parallel to the chest wall and convex outwards.
- No lung markings beyond the line: the space between the line and the chest wall is black and featureless.
- Increased lucency of the affected hemithorax, sometimes subtle.
- The collapsed lung may look slightly denser than the other lung, because the same tissue occupies a smaller volume.
- Associated findings: a small effusion blunting the costophrenic angle; surgical emphysema; rib fractures.
Hydropneumothorax
Air and fluid together in the pleural space produce a perfectly straight, horizontal air–fluid level on an erect film, with a pleural line above it. A simple effusion, by contrast, has a curved meniscus. Causes include trauma (haemopneumothorax), a recent drain or procedure, and bronchopleural fistula. See pleural effusion.
Pearl
Look for the white line, not just for blackness. Lack of lung markings alone can be caused by over-exposure, emphysema or a bulla. The pleural line is what confirms a pneumothorax.
3. The supine pneumothorax
In supine trauma and ICU patients, air collects in the anterior and inferior pleural space, which is the least dependent part of the chest when lying flat. The classic apical pleural line is often absent. Signs include:
- Deep sulcus sign: an abnormally deep, lucent, sharply outlined lateral costophrenic angle on the affected side.
- An unusually sharp heart border or sharp hemidiaphragm (the "too-crisp" outline).
- Hyperlucency of the lower chest or upper abdomen on the affected side.
- The double diaphragm sign: the dome and the anterior part of the diaphragm both outlined by air.
Plain films are insensitive in this setting. Up to about 30% of pneumothoraces in supine trauma patients are not seen on the chest film (figures vary between studies) [VARIES]. CT and ultrasound are more sensitive. A pneumothorax that is small on a supine film may enlarge rapidly once the patient is ventilated.
4. Size: what the guidelines say
Key measurements [VARIES between guidelines]
- BTS 2010: a visible rim of ≥2 cm between the lung margin and the chest wall, measured at the level of the hilum, is "large". A 2 cm rim approximates a pneumothorax of about 50% by volume.
- BTS 2023 (Roberts et al., Thorax 2023): management is driven mainly by symptoms, physiological compromise and patient preference rather than size. A rim of about 2 cm, at the hilum or the apex, is still used as a guide to whether there is enough air to aspirate or drain safely.
- ACCP (2001, US): an apex-to-cupola distance of ≥3 cm defines a large pneumothorax.
- Formula methods (Light index, Collins method) estimate percentage volume from interpleural distances; they are mainly used in research and advanced practice.
Because definitions differ, always say how you measured: "a 2.5 cm rim at the level of the hilum".
Size on a 2D film underestimates the true volume, and a "small" pneumothorax in a breathless patient with COPD may be more dangerous than a large one in a fit young adult. Interpret size alongside the patient.
5. Tension pneumothorax
In tension, air enters the pleural space but cannot leave, pressure rises, and venous return to the heart falls. This is a clinical diagnosis: hypoxia, hypotension, tachycardia and respiratory distress, often with reduced breath sounds. Treatment must not wait for an X-ray.
If a film has been taken, signs include:
- Mediastinal and tracheal shift away from the affected side
- Depression, flattening or inversion of the ipsilateral hemidiaphragm
- Widened rib spaces on the affected side
- A hyperlucent hemithorax with the lung collapsed towards the hilum
Pitfall
Mediastinal shift can be minimal in ventilated patients and in those with stiff lungs (for example ARDS or fibrosis), even when tension physiology is present. Absence of shift on the film does not exclude tension. Treat the patient, not the picture.
6. Pitfalls and mimics
- Skin folds: the commonest mimic on AP and supine films. The line often extends beyond the chest wall, lung markings continue on both sides, and it is usually a broad edge rather than a thin white line.
- Medial border of the scapula: trace it into the rest of the scapula.
- Bullae: a large bulla in emphysema has a concave inner margin facing the chest wall and may contain thin septa. A pneumothorax edge is convex and parallel to the wall. Inserting a drain into a bulla is dangerous; CT helps if unsure. See COPD and airways.
- Clothing, tubing and sheets: lines that continue outside the chest.
- Mach band: an optical dark line next to an edge, with no white pleural line.
- Surgical emphysema can hide an underlying pneumothorax.
See artefacts and mimics.
7. Next steps and other tests
- Management follows the BTS 2023 guideline and local pathways: conservative care, ambulatory devices, needle aspiration or chest drain depending on symptoms, type and patient preference. See chest drain position for drain checks.
- Ultrasound (absent lung sliding, the lung point) is sensitive in experienced hands and widely used in trauma and critical care.
- CT is the reference test for small or occult pneumothoraces, to distinguish bullae from pneumothorax, and in trauma.
- After any line insertion or lung procedure, check the film specifically for a pneumothorax, and remember the supine signs.
Red flags: act now
Escalate immediately
- Clinical signs of tension: treat first. Mediastinal shift away from a lucent hemithorax on any film.
- Haemodynamic compromise or rising oxygen requirement with any pneumothorax.
- Bilateral pneumothoraces.
- Any pneumothorax in a ventilated patient, which can progress to tension quickly.
- Pneumothorax with rib fractures or other trauma, or a hydropneumothorax (possible haemothorax).
What to focus on, by role
| Role | Priorities |
|---|---|
| Medical student | Find the pleural line, tell it from a skin fold, know the BTS 2 cm rule and the signs of tension. |
| Nurse | Recognise the deteriorating patient after a line or procedure, escalate early, and understand drain checks. |
| Junior doctor | Treat tension clinically, check post-procedure films carefully, recognise supine signs, and know when to request CT. |
| Radiographer | Coverage of both apices and costophrenic angles, an inspiratory erect film where possible, and flagging an obvious pneumothorax quickly. |
| Advanced / FRCR | Subtle supine signs, bulla versus pneumothorax, and the differences between BTS and ACCP sizing. |
Practise spotting pneumothoraces →
References
- Roberts ME, et al. British Thoracic Society Guideline for pleural disease. Thorax 2023;78(Suppl 3):s1–s42. thorax.bmj.com
- RCEM Learning. Spontaneous pneumothorax. rcemlearning.co.uk
- Radiopaedia. Pneumothorax. radiopaedia.org
- Radiology Masterclass. Chest X-ray quality and anatomy. radiologymasterclass.co.uk
- Life in the Fast Lane (LITFL). Chest X-ray interpretation. litfl.com
For education only. Not a substitute for a formal report, senior review or your local policy. Never use this page to make decisions about a real patient.