Etiology and mechanism of development
Subcutaneous emphysema rarely occurs as an independent condition. Usually, gas enters the soft tissues due to a compromise of the integrity of the airways, lungs, or pleural cavity.
The main causes of air in the subcutaneous tissue include:
- Iatrogenic interventions: status post thoracic surgery and chest tube insertion.
- Trauma: penetrating and blunt chest injuries.
- Pneumomediastinum: air can migrate into the subcutaneous tissue in spontaneous or acquired gas collections in the mediastinum.
Radiological presentation and diagnostic difficulties
Upon entering the soft tissues, air spreads along the path of least resistance — along tissue fascias and between muscle bundles. On radiographs, this creates a specific pattern: linear areas of increased lucency (linear transradiancies) are visualized. Because gas absorbs X-rays less than soft tissues do, it appears darker on the image.
The main diagnostic problem is that pronounced subcutaneous emphysema visually overlaps the underlying structures of the lungs and chest wall. This overlay artifact can greatly complicate the diagnosis of pneumothorax on a standard chest radiograph.
Management: if the radiograph does not reliably rule out pneumothorax due to overlapping emphysema, the patient is indicated for computed tomography (CT). Axial CT slices allow clear separation of layers and flawlessly visualize both pneumothorax and pneumomediastinum against the background of soft-tissue gas accumulation.
Soft tissues and bone structures
When evaluating a radiograph, it is important to consider not only the presence of air, but also the volume of soft tissue, which affects the overall density of the image. For example, left-sided mastectomy causes the lower part of the left hemithorax to appear more lucent than the right, solely due to the absence of the soft-tissue mass (see Fig. 3.1).
Although assessing bone abnormalities is not the primary goal of a chest X-ray, partially visible bone structures (especially ribs) require careful study. Normally, 12 pairs of ribs are visualized. For a detailed characterization of any bone lesions, detection of congenital or acquired remodeling, and complicated fractures, CT with multidetector 2D and 3D reconstructions is indicated.
Rib pathologies on radiographs
Changes in the osseous framework can be congenital or acquired:
- Cervical ribs (cervical ribs): found in 1–2% of the population. Usually bilateral, but often asymmetric. May cause thoracic outlet syndrome.
- Congenital anomalies: mostly affect the upper ribs. They can be bifid, splayed, fused, or hypoplastic. They occur isolated or as part of syndromes (nevoid basal cell carcinoma syndrome, Sprengel deformity).
- Acquired remodelling: is focal in nature. It results from fractures, surgical interventions, osteomyelitis, empyema drainage, or external pressure. An important symptom is rib notching (rib notching), which most commonly accompanies aortic coarctation and type I neurofibromatosis.
- Fractures and tumors: a bone callus forming after a fracture can mimic an intrapulmonary opacity on an image. Destructive lesions are most often associated with neoplastic processes (e.g., Pancoast tumor often invades the rib from the lung; MRI is indicated to assess extent) or rare osteomyelitis (staphylococcal, tuberculous, or actinomycosis).
Briefly: answers to topical questions
How to differentiate pneumothorax and subcutaneous emphysema on a standard radiograph?
On a standard radiograph, differentiating these conditions is extremely difficult because prominent subcutaneous emphysema superimposes on underlying structures. In doubtful cases, computed tomography (CT) is indicated. Axial CT slices better visualize pneumothorax and pneumomediastinum in the presence of emphysema.
What radiological signs characterize pneumomediastinum?
Pneumomediastinum is characterized by signs arising from anatomical structures being outlined by accumulated air. The following radiological signs are distinguished:
- Continuous diaphragm sign (continuous diaphragm sign) — continuous contour of the diaphragm due to air accumulation behind the pericardium.
- Double bronchial wall sign (double bronchial wall sign) — visualization of the bronchial wall due to air outside and inside the lumen.
- Ring around the artery sign (ring around the artery sign) — accumulation of air around the pulmonary artery.
- Sail sign (sail sign) — elevation of the thymus.
- Tubular artery sign (tubular artery sign).
Which structures are evaluated on MRI when diagnosing a Pancoast tumor?
When diagnosing a Pancoast tumour (Pancoast tumour) on MRI, the extent of the neoplasm's spread into adjacent anatomical regions is assessed. The study allows for reliable diagnosis of tumour invasion into the following structures:
- Chest wall and ribs — detection of direct extension from the lung.
- Diaphragm.
- Mediastinum.
- Root of the neck.
- Neurovascular bundles.
What are the causes of thoracic outlet syndrome?
Thoracic outlet syndrome is caused by congenital or acquired compression of the brachial plexus or subclavian vessels as they pass through the thoracic outlet. Sources cite cervical ribs as a cause, which occur in 1–2% of the population and can cause this syndrome.
Class and exam questions
What does subcutaneous emphysema look like on a radiograph?
It manifests as linear areas of increased lucency (darker bands on the image). This occurs because air distributes along tissue fascias and between muscle bundles.
Which imaging modality should be chosen if the X-ray is uninformative due to emphysema?
In doubtful cases, computed tomography (CT) is the gold standard. Its axial slices easily differentiate pneumothorax and pneumomediastinum from gas in the subcutaneous tissue.
Can a rib fracture mimic pulmonary pathology?
Yes, during fracture healing, a bone callus forms. On a radiograph, this density can erroneously mimic an intrapulmonary opacity.
What else to study on this topic
- Pathophysiology of pneumomediastinum and its relationship with subcutaneous emphysema
- Rib notching: differential diagnosis between aortic coarctation and neurofibromatosis
- Diagnostic imaging of Pancoast tumor (Pancoast tumour) and involvement of bone structures
- Thoracic outlet syndrome in the presence of cervical ribs
- Use of MRI in evaluating destructive chest wall lesions
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