Abdominal aortic aneurysm (AAA)

Epidemiology and aetiology:

• Present in 5% and responsible for the death of 1% of men over 60 years of age
• The principle cause of death is rupture, but distal embolism from thrombus in the aneurysm sac and, rarely, a thrombotic occlusion can also set the scene for death

Pathological features:

• An aortic aneurysm inevitably expands, a process made much more likely by hypertension
• Eventually, rupture takes place through all the attenuated layers, with either the:
• Initial formation of a retroperitoneal clot, or
• Immediate free bleeding into the peritoneal cavity sufficient to cause death

• The risk of rupture is related to the size of the aneurysm – the normal aorta is 1.5-2.5cm in diameter and is defined as aneurysmal when it is >4cm
• The reported annual risk of rupture is probably in the region of:
• 4cm 1-2%
• 5cm 5-10%
• 6cm 10-15%
• 7cm >20%

Only 30% of patients with rupture live long enough to reach hospital , and of those operated upon, only 50% survive

Thus, the overall (community) mortality for rupture is as high as 80-90%

Clinical features:

• In thin patients, the aneurysm itself as well as its transmitted pulsation may be visible on inspection
• On palpation, there will be a pulsatile, expansile swelling in the midline of the abdomen, usually extending towards the left-hand side
• However, clinical examination alone is very unreliable at confirming the presence or absence of an AAA
• Any suspicion of AAA should therefore prompt an US investigation
• There may be a bruit on auscultation in association with origin stenoses of the branches of the abdominal aorta (coeliac axis, superior mesenteric, renal arteries)
• There is an association between AAA and aneurysms elsewhere, so one should specifically exclude the presence of femoral and popliteal aneurysms

Management:

• The unpredictable nature of this process means that vascular surgeons usually organise repeat US examination at 3-6 monthly intervals for those who do not require urgent management

Indications for elective repair:

The decision to operate involves weighing the known risk of leaving the AAA in place against that of the operation. The first depends on:

• Size
• Presence of symptoms
• Age and physiological state

The risk of operation depends primarily upon cardiorespiratory status. There is no advantage to be gained in repairing a small aneurysm at low risk of rupture in an elderly patient with severe myocardial disease whose cardiac prognosis is poor