What causes tram-tracking in the lungs on chest X-ray?
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Answer:
Parallel linear opacities representing thickened bronchial walls due to bronchiectasis, a condition characterized by irreversible bronchial dilatation from recurrent infection, inflammation, or obstruction.
Why is it called so?
Named for the resemblance to parallel tramway tracks formed by the two thickened walls of a dilated bronchus seen in tangential/longitudinal projection.
Pathophysiology
Chronic inflammation and infection lead to bronchial wall fibrosis and smooth muscle hypertrophy, causing parallel wall thickening visible as tram-lines when the bronchus is viewed longitudinally on CXR or CT.
Note: “Tram-track sign” is a purely descriptive term for two parallel lines and is also used, in unrelated contexts, for the parallel gyriform cortical calcifications of Sturge-Weber syndrome on neuroimaging and for the optic nerve sheath outline in optic nerve sheath meningioma – these are distinct entities from the bronchiectasis usage on chest imaging.
Alternative names: Tram track sign; tram-line sign.
Other associated named signs: Signet-ring sign, finger-in-glove sign (both seen in bronchiectasis).
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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