Aortic nipple | Radiology Signs

Answer:

The aortic nipple is a small, convex, nipple-like projection on the lateral or superolateral aspect of the aortic knob, produced by the left superior intercostal vein seen end-on as it arches forward over the aortic arch to drain into the left brachiocephalic vein. It is a normal structure, visible in roughly 1-10% of chest radiographs, and normally measures up to about 4.5 mm. The finding matters because it enlarges when it carries collateral flow: fixed dilatation beyond 4.5 mm, or interval enlargement on serial films, points to superior vena cava or inferior vena cava obstruction, azygos or left brachiocephalic vein occlusion, congenital absence of the azygos vein, portal hypertension, or raised right atrial pressure. It also enlarges transiently with a Valsalva manoeuvre or in the supine position, so a single measurement should be interpreted with the clinical context and, where relevant, confirmed on CT.

Why is it called so?

The small rounded bump projecting from the smooth contour of the aortic knob resembles a nipple.

Pathophysiology

The left superior intercostal vein drains the upper left intercostal spaces and courses anteriorly around the aortic arch, where its short end-on segment is silhouetted by aerated lung and projects as a discrete bump. When central venous return through the superior or inferior vena cava is obstructed, this vein becomes a collateral pathway; the increased flow dilates it, and the end-on projection enlarges correspondingly. The same mechanism explains its enlargement with raised right heart pressures and its transient increase with manoeuvres that raise intrathoracic venous pressure.

Alternative names: Aortic nipple sign

Other associated named signs: Aortic knob contour signs, azygos vein enlargement, and the cervicothoracic sign

References

1. Byerly DW, Schlesinger AE. Acquired aortic nipple: a potential sign of deep venous obstruction. Pediatr Radiol. 2010;40 Suppl 1:S71. PMID: 20614114.