2 Clinical need and practice
2.1 Aortic aneurysms develop when the wall of the aorta weakens, causing it to bulge and form a balloon-like projection. This leads to further stretching of the wall of the aorta and an increase in tension. Eventually the wall may rupture, leading to massive internal bleeding. Aneurysms are often a result of atherosclerosis and most occur in the abdominal section of the aorta. An abdominal aortic aneurysm (AAA) is defined as an enlargement of the aorta of at least 1.5 times its normal diameter or greater than 3 cm diameter in total. Most AAAs occur in the lower part of the abdominal aorta, below the kidney (infra-renal). The main risk factors for AAA include increasing age, high blood pressure, smoking and family history of AAA. AAAs are about three times more common in men than in women.
2.2 Most AAAs are detected by chance during clinical investigation (for example, ultrasound or X-ray) for other conditions. Because most AAAs are asymptomatic, it is difficult to estimate their prevalence, but screening studies in the UK have estimated a prevalence of 1.3–12.7% depending on the age group studied and the definition of AAA. The incidence of symptomatic AAA in men is approximately 25 per 100,000 at age 50, increasing to 78 per 100,000 in those older than 70 years. The implementation of a national screening programme for AAA is under way with the first centres expected to start screening by March 2009. The remaining centres will be managed in a phased roll-out over the next 5 years.
2.3 Symptoms that can occur as an aneurysm enlarges include a pulsating sensation in the abdomen, back pain and abdominal pain that may spread to the back. Patients with a symptomatic AAA need rapid medical attention. Among patients with a ruptured AAA the mortality rate is about 80%; even when they undergo emergency surgery, only about half survive beyond 30 days. The risk of rupture increases with the size of the aneurysm, and those aneurysms larger than 6 cm in diameter have an annual risk of rupture of 25%. Several studies indicate that without surgery the 5-year survival rate for patients with aneurysms larger than 5 cm is about 20%.
2.4 Patients with an AAA can be treated by surgical repair to prevent rupture. Conventional (open) surgical repair (OSR) involves making a large incision in the abdomen and inserting a prosthetic graft to replace the damaged section of the aorta. OSR can also be performed laparoscopically, either by hand-assisted laparoscopic surgery or totally laparoscopic surgery. Endovascular aneurysm repair (EVAR) is a minimally invasive technique that involves a stent–graft being inserted through a small incision in the femoral artery in the groin. It is carried to the site of the aneurysm using catheters and guide wires and placed in position under X-ray guidance. Once in position, the stent–graft is anchored to the wall of the aorta using a variety of fixing mechanisms.
2.5 Potential advantages of EVAR over OSR include reduced time under general anaesthesia, elimination of the pain and trauma associated with major abdominal surgery, reduced length of stay in the hospital and intensive care unit (ICU), and reduced blood loss. Potential disadvantages include the development of endovascular leaks (endoleaks), which occur when blood continues to flow through the aneurysm because the graft does not seal completely (type I endoleak) or because of backfilling of the aneurysm from other small vessels in the aneurysm wall (type II endoleak). Patients who have had OSR do not require any special follow-up, but patients who have undergone EVAR may require computed tomography (CT) or ultrasound scans to check for the presence of late-occurring endoleaks. In addition, if EVAR is unsuccessful or complications arise during the procedure, conversion to OSR may be necessary even in patients initially considered unfit for open surgery.
2.6 In current UK clinical practice, elective surgery is generally recommended for patients with aneurysms larger than 5.5 cm in diameter and with aneurysms larger than 4.5 cm in diameter that have increased by more than 0.5 cm in the past 6 months. Current guidelines from the Vascular Society and the National Screening Committee recommend that patients with symptomatic aneurysms of less than 4.5 cm in diameter should be followed up with ultrasonography every 6 months, and aneurysms of 4.5–5.5 cm in diameter should be followed up every 3 or 6 months.