What causes the fallen lung sign on CT of the chest?
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Answer:
The fallen lung sign is the finding of a collapsed lung that falls away from the hilum, hanging posteriorly and laterally in the dependent hemithorax, instead of collapsing toward it. It indicates complete transection of a main or lobar bronchus, which detaches the lung from its central tethering so that it is held only by its vascular pedicle and drops under gravity. A lung collapsed for any other reason, including a large pneumothorax, remains anchored by the bronchus and retracts centrally toward the hilum, so the direction of collapse, not its degree, is what carries the diagnosis. The clinical setting is blunt deceleration or crush injury, and the corroborating findings are a persistent large pneumothorax and continuing air leak despite a correctly positioned chest drain, pneumomediastinum, and discontinuity of the bronchial wall on thin-section CT with multiplanar and virtual bronchoscopic reformats. Most tracheobronchial injuries occur within 2.5 cm of the carina and the right main bronchus is involved more often than the left. The sign is uncommon but important because unrecognised transection leads to bronchial stenosis, distal atelectasis and recurrent infection weeks later; bronchoscopy confirms the injury and surgical repair with primary anastomosis preserves the lung if performed early.
Why is it called so?
The lung is described as having fallen, because on the image it lies slumped in the dependent hemithorax like a detached organ rather than being drawn toward the hilum.
Pathophysiology
Sudden anteroposterior compression of the chest against a closed glottis, rapid deceleration shearing the relatively mobile lung against the fixed carina, and traction on the mainstem bronchi as the lungs move laterally all concentrate force at the tracheobronchial junction. When the bronchial wall ruptures completely, the airway retracts and the lung loses its only rigid attachment. Air escapes freely into the pleural space, producing a pneumothorax that cannot be evacuated because the leak is proximal, and the deflated lung, suspended solely by pulmonary vessels, sinks dependently.
Alternative names: Ptotic lung sign; fallen lung
Other associated named signs: Deep sulcus sign and the continuous diaphragm sign of pneumomediastinum in the same trauma setting
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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