What causes the figure 3 sign on the chest radiograph?
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Answer:
The figure 3 sign is the contour of the descending aortic silhouette in aortic coarctation, formed by three components in sequence: prestenotic dilatation of the distal arch and left subclavian artery above, a focal indentation at the coarctation itself (the tuck), and poststenotic dilatation of the descending aorta below. Together these produce the reversed-3 outline seen at the left upper mediastinal border on a frontal chest radiograph. It is frequently accompanied by inferior rib notching, erosion of the undersurfaces of typically the third to eighth ribs by dilated, tortuous intercostal collaterals; rib notching is a separate sign, not part of the figure 3 itself, and generally develops only after several years. The figure 3 sign should not be confused with the barium-oesophagram reverse 3 sign (E sign), in which the same three segments indent the contrast-filled oesophagus from the other side.
Why is it called so?
The three-part outline of dilatation, indentation and dilatation traces the numeral 3 along the left mediastinal border.
Pathophysiology
A discrete narrowing of the aorta near the ligamentum arteriosum obstructs flow. Above it, chronically raised pressure dilates the distal arch and left subclavian artery. At the stenosis the wall is indented. Beyond it, the jet emerging through the narrow segment strikes the aortic wall and causes turbulent poststenotic dilatation. The combination of these three adjacent calibre changes produces the numeral-3 contour. Blood reaching the lower body must travel through the intercostal arteries, which enlarge and become tortuous and erode the inferior rib margins over years.
Alternative names: None (the figure 3 is the chest radiograph sign; the oesophagram counterpart is a distinct sign, below)
Other associated named signs: Reverse 3 sign or E sign on barium swallow (the same aortic contour indenting the oesophagus), rib notching (Rösler sign), and the tulip bulb sign of associated aortopathy
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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