The aorta is the largest blood vessel in the body and carries oxygenated blood away from the heart.
An abdominal aortic aneurysm is the localised dilation of the aorta in the abdomen.
The most common cause of abdominal aortic aneurysm is degeneration of the arterial wall; atherosclerosis being an important contributory factor.
An uncomplicated aneurysm usually produces no symptoms.
The most common complication is rupture, which is a medical emergency and is often fatal.
When necessary and appropriate, aortic aneurysms are surgically repaired.
An aneurysm is the localised dilation (swelling or enlargement) of an artery. When such a dilation occurs within the abdominal aorta it is referred to as an abdominal aortic aneurysm (AAA). The abdominal aorta is the commonest site of arterial aneurysm.
The aorta is the largest artery in the body. Oxygenated blood is pumped directly from the heart into the aorta and is then distributed via various branches to eventually supply all the tissues and organs of the body. From the heart, the aorta continues down the chest cavity and through the abdomen, before dividing into separate vessels which descend into each leg.
There are different types of aneurysms
True aneurysm – by far the commonest type
False aneurysm - usually related to trauma
Dissecting aneurysm – related to hypertension, usually affecting the thoracic aorta
Inflammatory aneurysm – a rare but specific syndrome
Other types – associated with congenital conditions, and infections
Abdominal aortic aneurysms (AAA) are the most common true aneurysms. This article focuses on AAA.
Causes and associated risk factors
Aneurysms develop when the wall of an artery becomes damaged with the result that the internal pressure exerted by the blood within the artery causes the weakened arterial wall to expand, resulting in a bulge in its wall. This can be likened to a blow-out in a weakened part of a car tyre.
True aneurysms can occur in any large artery but are particularly common in the aorta and its direct branches. Approximately 75% of aortic aneurysms occur in the abdominal aorta. More than one aneurysm may occur simultaneously in a single or in separate arteries.
Occasionally the arterial wall of an aneurysm splits longitudinally in such a way that blood begins to flow into the artery wall itself. When this occurs, the aneurysm is referred to as a dissecting aneurysm. The blood flowing in such a false channel within the arterial wall may rejoin the arterial circulation at another damaged site further along the artery. Alternatively, a portion of the internal layer of the split wall may become partially detached, "peeling off" which creates a flap inside the artery.
Dissection occurs more commonly in the section of the aorta that is in the chest than in the abdominal aorta. However, those that occur in the region of the chest often spread downwards toward the abdomen.
By far the most common disease associated with the formation of aneurysms is atherosclerosis, which is a vascular disease (a disease of the blood vessels). Other, much less common, diseases that may lead to aneurysms are arteritis (inflammation of the arteries), syphilis, and congenital connective tissue disorders such as Marfan’s syndrome. This article, though, is primarily concerned with aneurysms caused by atherosclerosis, the effects of which we will now describe briefly.
The wall of an artery consists of several layers. Atherosclerosis begins with the deposition of certain types of lipids (fats) on the internal wall of an artery as well as between the layers of the wall. These deposits, called plaques, weaken and damage the arterial wall as well as causing the wall to thicken. This process continues slowly over many years and is usually compounded by an additional "hardening" of the arterial wall - referred to as calcification - which occurs once the plaques have become established. These degenerative processes leave the atherosclerotic artery vulnerable to the development of an aneurysm.
The calcification of the arterial wall causes the wall to lose its elasticity, so that should the wall be stretched it will suffer damage to its internal structures. Small blood clots begin to form at the sites of damage. These enlarge gradually over time.
Degenerative processes of the arterial wall and inflammation are the primary aetiological factors.
The main risk factor is the presence of atherosclerosis, which will be compounded by:
Coexisting high blood pressure
Age – AAA is rare under the age of 60
Gender – males are affected 4-5 times as often as women
Caucasians – the race most commonly affected
Family history – AAA has a strong familial risk
Other medical conditions – coronary heart disease and peripheral vascular disease have a strong association with AAA
Symptoms and signs
Most aortic aneurysms are small and do not produce any symptoms.
Some aneurysms cause a small pulsating mass near the navel. This is often not noticed by the patient, but detected during a routine medical examination. Other small aneurysms are detected co-incidentally during ultrasound or X-ray examination of the abdomen done for other reasons.
Rapidly enlarging aneurysms can become tender, and may cause pain in the lower back or abdomen.
Many of the symptoms experienced by people with aneurysms are caused by complications of the aneurysm, rather than by the aneurysm itself. A large aneurysm may apply pressure on the abdominal organs surrounding it and may even cause tissue damage to these organs, which is often painful.
Should an aneurysm leak, blood will build up under pressure in the tissues surrounding the aorta, which can result in acute pain and tenderness in these areas. This is particularly the case if the aneurysm leaks from the back of the aorta. If the leak occurs in the front of the artery, greater blood loss is likely to occur as the blood is able to escape more freely into the abdominal cavity. Should this occur, the person will usually collapse. This is often the first sign that the aneurysm is about to rupture and is regarded as a medical emergency.
The symptoms of a leaking aneurysm can mimic other conditions, such as renal colic, diverticulitis, pancreatitis, mesenteric ischaemia, heart disease, or even liver disease. These symptoms occurring in an elderly person, or one known to have atherosclerosis, should alert the doctor to the possibility of a leaking AAA.
The risk of rupture depends on the size of the aneurysm and the rate at which it is expanding. On average, AAA expand at 0.3 - 0.4cm per year. Statistics show that AAA of more than 5cm diameter have a 25% risk of sudden rupture. Any AAA which expands rapidly (more than 0.5cm over six months) also has a high risk of rupture. Ruptured AAA has a high mortality risk. Of those surviving long enough to reach the operating theatre, less than 50% survive.
Detection of AAA and prevention of rupture is thus the goal of treatment.
A further complication which may arise from an abdominal aortic aneurysm is an embolism, when a small part of a blood clot on the internal wall of the aneurysm becomes dislodged and travels down into one of the legs, where it may block an artery. This may result in the circulation of the leg becoming sufficiently reduced to cause the tissue of the leg to die. This is an emergency requiring immediate surgery. In a less serious case, the clot may simply reduce blood flow enough to cause pain on walking.
A doctor examining a patient with an abdominal aortic aneurysm may find either high or low blood pressure (usually high), absent pulses or a tender mass in the aorta. A mass can usually be felt in the abdomen. Often, a murmur or bruit can be heard over the aneurysm mass.
Since most aneurysms show no symptoms, they are usually detected on routine examination of the patient’s abdomen, when the doctor will notice a pulsating mass. It can also be an incidental finding on plain X-rays of the abdomen, where calcification of the expanded wall of the artery is seen. Routine ultrasound of the abdomen will also reveal the aneurysm.
Once the aneurysm is detected, further investigations are carried out to determine its size (width) and extent (length). Cross-sectional ultrasound is the most cost-effective and least invasive method of evaluation. It usually gives a clear picture of the extent and size of an aneurysm.
CT (computerised tomography) scanning is equally accurate in determining the size of the aneurysm and will also give information about the rest of the abdomen.
Aortography is an X-ray examination of the aorta that involves injecting a contrast medium into the aorta, after which a series of X-rays are taken. This reveals the site and dimensions of the aneurysm and is very helpful in planning treatment. It can also show whether other aneurysms are present. This investigation does, however, have certain shortcomings. It shows the lumen (inside) of the aorta and aneurysm, but gives little information about atheroma or the condition of the aortic wall.
The best investigation at present is the 3-dimensional CT angiogram, as this gives a clear picture of the aneurysm, as well as the state of the inside of the aorta, e.g. the presence of atheroma and/or clots. This information is often not obtainable by using other investigations, and can drastically affect the proposed treatment plan.
The goal is to prevent rupture, which carries such a high mortality rate.
The risk of rupture is directly related to the size of the aneurysm and the rate at which it expands. Therefore, once an aneurysm is diagnosed, no matter how small or lacking in symptoms, it must be carefully monitored. This is best done by regular six-monthly abdominal ultrasounds. Beta-blockers are prescribed unless there is a specific contra-indication in a patient, as they reduce blood pressure and have been shown to reduce the rate of expansion of AAA. If the patient remains free of symptoms, all that is required is continued use of beta-blockers and regular monitoring. Underlying associated risk factors must continue to be managed, such as to give up smoking.
Once an AAA reaches 5.5cm in diameter, or expands faster than 0.5cm over six months, or becomes tender/painful, specific treatment is required. The type of treatment will be individualised, taking into account other medical problems the patient may have.
This is generally the treatment of choice, though the risks of surgery increase with advancing age, and the presence of other health problems (e.g. coronary artery disease).
The commonest form of surgery involves removing the diseased part of the aorta, and replacing it with a synthetic graft. This may be technically very difficult (sometimes impossible) if the AAA extends over a long segment, and involves other arteries, e.g. the arteries to the kidneys. In the hands of an experienced vascular surgeon, the outcome is good. The potential benefits of early surgery must, however, be weighed against the pre-operative mortality risk.
Leaking or dissecting aneurysms are emergencies, best treated surgically wherever possible. These patients have a high mortality rate, exceeding 50%.
For patients who are considered not suitable for surgery (e.g. advanced age, unable to tolerate an anaesthetic) an endovascular stent can be used. This procedure involves placing an expandable stent inside the AAA via the groin artery. Once correctly positioned, the stent is expanded to form a tube within the aorta. Blood now flows through the stent instead of through the diseased part of the aorta. This means that the risk of clot formation is reduced, and, because the stent is able to withstand the blood pressure, the original aortic wall no longer expands or risk a rupture.
AAA repair is a major surgical procedure. In the hands of skilled and experienced vascular surgeons, the in-hospital and short-term mortality has been found to be 2 – 5%. Should patients have associated medical problems, they will contribute to the overall mortality. Complications related to the surgery are those common to all major procedures, e.g. bleeding, stroke and emboli.
Patients treated with endovascular stents have a one-year survival rate similar to those treated surgically. However, stented patients generally require many more interventions after the initial procedure than do surgical patients. The overall cost of stenting is thus much higher than with surgery. Stenting may be the only option for certain compromised patients who might otherwise be at risk of rupture and death.
Some risk factors such as age, gender, race and family history cannot be changed, but the management of hypertension, high cholesterol and smoking can minimise the risk of aneurysm formation.
Once an AAA has developed, its progression cannot be halted, but can be drastically slowed down by careful management (e.g. the use of beta-blockers and stopping smoking). Rupture can be prevented by careful monitoring and well-timed surgery. For patients who cannot tolerate surgery, stenting is an option.
When to see a doctor
If a pulsating mass in your abdomen is noted; if there are signs of atherosclerosis such as heart disease and high blood pressure; and if you experience lower back pain, particularly if this is a new symptom.
Emergency medical help should be sought when a patient with a known abdominal aortic aneurysm complains of a sudden severe pain, becomes sweaty and pale, or collapses.
Reviewed by Dr A.G. Hall (B.Soc.Sc.(SW), MB,Ch.B), December 2008