What causes Hilum overlay sign in the chest on chest radiography?
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Answer:
The hilum overlay sign occurs when a mass or opacity overlies the pulmonary hilum but the sharp margins of the hilar vessels remain visible, distinguishing it from a true hilar mass. It is caused by anterior or posterior mediastinal masses or hilar adenopathy that do not obscure the vessels, unlike primary hilar lesions such as bronchogenic carcinoma or lymphoma involving the hilum.
Why is it called so?:
The sign is named for the visual superposition (overlay) of a mass over the hilar structures on chest radiographs, where the normal hilar vascular margins are preserved and “seen through” the opacity due to their distinct anatomical position.
Pathophysiology:
Hilar vessels adjacent to aerated lung maintain sharp margins on frontal chest radiographs. A true hilar mass encases or abuts these vessels, obliterating their margins via the silhouette sign. In contrast, an anterior or posterior mass overlies the hilum in a different plane, preserving vessel-lung interfaces and allowing clear delineation of vascular borders despite the opacity.
Alternative names: None
Other associated named signs: Hilar convergence sign
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