Pneumothorax

Differential diagnosis with skin folds

Sometimes contours appear on the radiograph that an inexperienced eye mistakes for the edge of a collapsed lung. Most often, this effect is caused by skin folds. To avoid errors, it is important to evaluate three radiographic criteria.

• Vascular pattern: if it is a skin fold, blood vessels will be clearly visible lateral to this line.
• Edge thickness: normally, the visceral pleura line is very thin and delicate, whereas the edge of a skin fold always appears wide.
• Orientation: the direction of the fold line typically does not match the anatomical contour of even a slightly collapsed lung.

If doubts remain after analyzing the image, confirmatory tactics are applied. The patient is ordered a repeat radiograph, an expiratory view, or chest imaging in the lateral decubitus position (decubitus). In the most complex cases (especially for differentiation from bullae), CT or ultrasound comes to the aid.

Atypical presentation: supine patient

The classical presentation of a pneumothorax changes radically if the patient is in the supine position (supine). In this position, free air migrates to the anterior chest wall, accumulating predominantly medially and basally. Due to this redistribution, the gas may not separate the lung parenchyma from the chest wall at the apices or laterally, as occurs in standing patients.

Specific radiographic signs in supine patients:

1. Ipsilateral (on the affected side) increased radiolucency, which may be generalized or limited to the hypochondriac region.
2. The deep sulcus sign: the lateral costophrenic angle becomes abnormally deep and acquires a characteristic finger-like shape.
3. The double diaphragm sign: the anterior costophrenic angle is visualized as an oblique line or boundary in the hypochondrium, closely mimicking the contour of the adjacent diaphragm.
4. A radiolucent band running parallel to the diaphragm or mediastinum, accompanied by an excessively sharp border of the central structures.
5. Visibility of the inferior surface of the heart and epicardial fat pads (they may appear as rounded opacities mimicking pathological masses).
6. Flattening (depression) of the diaphragmatic dome.

To confirm the diagnosis in critically ill patients unable to stand, radiography in the lateral decubitus position (lateral decubitus) is used. An alternative is a supine view where the cassette is placed dorsolaterally at a 45-degree angle, and the X-ray tube is directed strictly perpendicular to the cassette.

Loculated pneumothorax

If there is partial obliteration (adhesions) in the pleural cavity, a loculated pneumothorax (loculated) is formed. In this case, air is trapped within a confined space.

Identifying such localized collections requires differential diagnosis with a number of conditions that produce localized radiolucencies: cysts, bullae, pneumatoceles, localized emphysema, and pneumomediastinum. Standard radiography in such conditions is often uninformative, and a CT scan is required for an accurate diagnosis.

Ultrasound examination (ultrasound)

Ultrasound is actively used for rapid diagnosis in intensive care settings. Normally, on US, the pleura is visualized as an echogenic line (the pleural stripe). Distal reverberation artifacts in the form of vertical echogenic bands, known as comet tails, form posterior to it. During respiration, the pleural surface shimmers characteristically—this is the normal lung sliding phenomenon.

When a pneumothorax develops, the US picture changes:

• Signs of normal lung sliding and comet tails disappear without a trace.
• A-lines (A lines) appear—horizontal, equidistant reverberation artifacts caused by multiple reflections of the signal from the separated pleura.
• The lung point sign: a dynamic boundary between the sliding lung and the pneumothorax area is visualized. This sign helps estimate the size of the pneumothorax. It is important to note that in a massive air collection, the lung point is absent because the lung tissue does not contact the chest wall at all.

Clinically, US is used for prompt detection of pneumothorax after central venous catheter placement or lung biopsy, as well as for monitoring resolution during chest tube drainage.

Caution, false-positive results! The absence of sliding and comet tails can also be observed without a pneumothorax: in hyperinflation, severe bullous lung disease, air trapping, or as a result of prior pleurodesis.

Computed tomography (CT)

Computed tomography is the most sensitive imaging modality for pneumothorax, especially if the volume of free air is small and the patient is examined strictly in the supine position.

Indications for CT:

• The presence of extensive subcutaneous emphysema or massive lung consolidation, when the pneumothorax is reliably obscured on a chest radiograph.
• Searching for concomitant injuries and pathologies (e.g., identifying areas of pulmonary contusion in polytrauma).
• Navigational support during chest tube insertion. This is especially critical in patients with severe emphysema to prevent accidental damage to bullae by the drainage tube.

An impressive fact proving the importance of the method: 25% to 40% of pneumothoraces occurring as a complication after lung biopsy are completely invisible on a standard radiograph and are detected only thanks to CT.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Technique of expiratory radiography and its physical rationale
• Patient positioning rules for lateral decubitus
• Differential diagnosis algorithm for localized radiolucencies (cysts, bullae, pneumatoceles)
• Physics of horizontal A-lines and vertical comet tails on ultrasound
• Management tactics for patients with pneumothorax after percutaneous needle biopsy

Other topics in the «Chest Radiology» section

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