What causes the double bronchial wall sign on the chest radiograph?
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Answer:
The double bronchial wall sign is the appearance of both the inner and outer surfaces of a bronchial wall as separate parallel lines, and is a specific radiographic marker of pneumomediastinum. Normally only the luminal surface of a central bronchus is visible, because the outer surface abuts mediastinal soft tissue of identical attenuation. When gas dissects into the mediastinum it surrounds the bronchus, outlining its adventitial surface, while the air within the lumen continues to outline the mucosal surface; the wall itself is then displayed as a thin soft-tissue stripe with air on both sides. It is best appreciated along the main and lobar bronchi and is usually accompanied by other signs of mediastinal gas such as the continuous diaphragm sign, the ring-around-the-artery sign and lucent streaks along the mediastinal contours. Causes include barotrauma, asthma, vomiting (Boerhaave), oesophageal or airway injury, and spontaneous alveolar rupture.
Why is it called so?
Air on both the inside and the outside of the bronchus renders the wall twice over, so a single bronchus appears to have two walls.
Pathophysiology
Alveolar rupture allows gas to dissect centrally along the bronchovascular sheaths (the Macklin effect) into the mediastinum, where it collects around the central airways and great vessels. Because that gas is of the same attenuation as the air already inside the bronchial lumen, the intervening bronchial wall is rendered as a discrete soft-tissue line silhouetted on both sides, converting the normally one-sided bronchial outline into a doubled one.
Alternative names: Double wall sign
Other associated named signs: Continuous diaphragm sign, ring-around-the-artery sign, spinnaker sail sign, and the V sign of Naclerio
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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