Expansion
Usually infrarenal, palpable above the umbilicus, expansile rather than pulsatile
Expansion
- About 90 per cent are infrarenal - below the renal arteries and above the bifurcation at L4
- Therefore palpable in the epigastrium and above the umbilicus
- Defined as a diameter greater than 3 cm
The infrarenal aorta is prone to aneurysm because it has fewer elastic lamellae, no vasa vasorum in the media, and is exposed to reflected pressure waves from the bifurcation.
On examination, distinguish expansile pulsation (hands pushed apart, indicating an aneurysm) from transmitted pulsation (hands pushed in the same direction, as with a mass overlying a normal aorta).
Rupture is usually posterolateral into the retroperitoneum, which can tamponade temporarily and gives the classic triad of abdominal or back pain, hypotension and a pulsatile mass. Anterior rupture into the peritoneal cavity is rapidly fatal.