The outcome of prognosis of abdominal aortic aneurysm depends on the size and location of the aneurysm and on various other patient related factors. One of the most dreaded outcomes of the condition is the risk of rupture of the aneurysm that more often than not may lead to death unless the bleeding is stopped urgently.
What determines risk of rupture?
The risk of rupture is mainly determined by aneurysm diameter. Normally the aneurysm gradually expands each year by approximately 10% of the initial arterial diameter.
|AAA Size (cm)
||Growth rate (cm/yr)
||Annual rupture risk (%)
Without surgical repair of an aneurysm that is over 5cm in diameter, the annual survival rate is only about 20%. The risk of rupture of the abdominal aorta aneurysm increases with aneurysm size and aneurysms over 6 cm have a 25% yearly risk of rupture.
After a rupture of an abdominal aorta aneurysm the risk of death is approximately 80%. Most patients die before reaching hospital. In most patients outcome is poor and surgical repair is successful in case of a ruptured aneurysm in only about 50% of cases.
Complications of elective surgery
Overall risk of death due to complications of elective surgery for repair of an abdominal aortic aneurysm in UK is 2.47 percent. Many specialist centres report mortality rates below 52 percent. The rate of success of elective surgery for aneurysm correction depends on patient’s fitness for surgery and the type and location of the aneurysm. Those patients who have other medical disorders like heart disease, kidney disorders etc. have a high risk of failed surgery and complications due to surgery.
Risk of rupture and diameter of the aneurysm
The risk of rupture can be predicted as per the diameter of the aneurysm. The actual inner diameter of the aneurysm and risk of rupture is diagnosed using the common engineering technique of the finite element method (FEM) to determine the wall stress distributions. These stress distributions correlate with the overall geometry of the abdominal aorta aneurysm rather than the maximum diameter alone.
The principles show that wall stress is not the only factor that governs rupture because the aneurysm will rupture when the wall stress exceeds the wall strength. Thus both patient-specific wall stress and patient-specific wall strength needs to be calculated to assess the risk of rupture.
Rupture-risk assessment methods
Some of the more recently proposed abdominal aorta aneurysm rupture-risk assessment methods include assessment of:-
- Abdominal aorta aneurysm wall stress
- Abdominal aorta aneurysm expansion rate
- Degree of asymmetry
- Rupture potential index (RPI)
- Finite element analysis rupture index (FEARI)
- Biomechanical factors coupled with computer analysis
- Geometrical parameters of the Abdominal aorta aneurysm
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)