Abdominal Aortic Aneurysm

An abdominal aortic aneurysm is a dilation (ballooning) of part of the aorta that is within the abdomen. An abdominal aortic aneurysm (AAA) usually causes no symptoms unless it ruptures (bursts). A ruptured AAA is often fatal. An AAA less than 55 mm wide has a low chance of rupture. An operation to repair the aneurysm may be advised if it is larger than 55 mm, as above this size the risk of rupture increases significantly. Men aged 65 and over are to be offered a routine scan to screen for AAA.

The aorta is the largest artery (blood vessel) in the body. It carries blood from the heart and descends through the chest and the abdomen. Many arteries come off the aorta to supply blood to all parts of the body. At about the level of the pelvis the aorta divides into two arteries, one going to each leg.

An aneurysm is where a section of an artery widens (balloons out). The wall of an aneurysm is weaker than a normal artery wall. The pressure of the blood inside the artery causes the weaker section of wall to balloon.

Arteries
Diagram showing the main arteries of the body and details of an aortic aneurysm

Aneurysms can occur in any artery, but they most commonly occur in the aorta. Most aortic aneurysms occur in the section of the aorta that passes through the abdomen. These are known as abdominal aortic aneurysms (AAAs). Sometimes they occur in the section going through the chest. These are known as thoracic aortic aneurysms.

The rest of this leaflet is only about AAAs.

The normal diameter of the aorta in the abdomen is about 20 mm. An AAA is said to be present if a section of the aorta within the abdomen is 30 mm or more in diameter.

AAAs vary in size. As a rule, once you develop an AAA, it tends gradually to get larger. The speed at which it gets larger varies from person to person. However, on average, an AAA tends to get larger by about 10% per year.

In most cases

The exact reason why an aneurysm forms in the aorta in most cases is not clear. Most instances occur in older people. An AAA is rare in people under the age of 60. So, ageing has a major role to play.

The wall of the aorta normally has layers of smooth muscle, and layers made from tissues called elastin and collagen. Elastin and collagen are strong supporting tissues. What seems to happen is that a part of the aorta loses its normal strength and elasticity in some people as they become older. Research suggests that this is due to changes in the elastin, collagen and smooth muscle tissues. There seem to be complicated biochemical processes that cause these changes. Some people are more prone than others to these changes.

Your genetic make-up plays a part, as you have a much higher chance of developing an AAA if one of your parents has, or had, one.

Atheroma may also play a part. Atheroma is a fatty substance that deposits within the inside lining of arteries. Atheroma is sometimes called furring of the arteries. Most AAAs are lined with some atheroma. Anyone can develop atheroma, but it develops more commonly with increasing age. Certain risk factors also increase the chance of atheroma forming. They include: smoking, high blood pressure, diabetes, raised cholesterol level, taking little exercise, and obesity. These are the same risk factors that increase the chance of atheroma forming in the heart (coronary) arteries, which can cause angina and heart attacks.

In a minority of cases

Rare causes of AAAs include injury or infection of the aorta. Also, certain uncommon hereditary conditions can affect the artery structure. In these uncommon situations an aneurysm may develop at a relatively young age.

About 4 in 100 men and about 1 in 100 women over the age of 65 have an AAA. It becomes more common with increasing age. However, most people with an AAA are not aware that they have one (see below in the section on symptoms). An AAA is rare in people under the age of 60.

The main concern is that the aneurysm might rupture (burst). The wall of the aneurysm is weaker than a normal artery wall and may not be able to withstand the pressure of blood inside. If it ruptures then severe internal bleeding occurs which is often fatal. Most AAAs do not rupture - only a certain proportion (see below).

Often there are no symptoms. At the time of diagnosis, 7 in 10 people with an AAA will not have had any symptoms due to the aneurysm. The ballooning of the aneurysm does not cause any symptoms unless it becomes large enough to put pressure on nearby structures. If symptoms do occur, they are likely to be mild abdominal or back pains. There are many causes of mild abdominal and back pain. Therefore, the diagnosis may be delayed unless the aneurysm is large enough to be felt by a doctor when he or she examines your abdomen.

Sometimes small blood clots form on the inside lining of an AAA. These may break off and be carried down the aorta and block a smaller artery further on. These blood clots are called emboli and can be dangerous. For instance, complete blockage of an artery that supplies a foot may lead to loss of blood to part of the foot, which can cause pain in the foot and gangrene if left untreated.

If the aneurysm does rupture then you are likely to have sudden severe abdominal and/or back pain. This is commonly soon followed by collapse as the internal bleeding causes a sharp drop in blood pressure.

  • Sometimes a doctor feels the bulge of an aneurysm during a routine examination of the abdomen. However, many AAAs are too small to feel.
  • An X-ray of the abdomen (often done for other reasons) will show calcium deposits lining the wall of an AAA in some, but not all, cases.
  • An ultrasound scan is the easiest way to detect an AAA. This is a painless test. It is the same type of scan that pregnant women have to look at the baby in the womb. The size of the aneurysm can also be measured by ultrasound. As discussed later, it is important to know the size.
  • A more detailed scan, such as a CT scan, is sometimes done. This may be done if your doctor needs to know whether the aneurysm is affecting any of the arteries that come off the aorta. For instance, if the aneurysm involves the section of the aorta where the arteries to the kidneys branch off, surgeons need to know this information if they plan to operate.

The chance of rupture is low if an AAA is small. As a rule, the risk of rupture increases with increasing size. This is much like a balloon - the larger you blow it up, the greater the pressure, and the greater the chance it will burst. The diameter of an AAA can be measured by an ultrasound scan. The following gives overall risk figures for the size (diameter) of the aneurysm:

  • 40 mm-55 mm: about a 1 in 100 chance of rupture per year.
  • 55 mm-60 mm: about a 10 in 100 chance of rupture per year.
  • 60 mm-69 mm: about a 15 in 100 chance of rupture per year.
  • 70 mm-79 mm: about a 35 in 100 chance of rupture per year.
  • 80 mm or more: about a 50 in 100 chance of rupture per year.

As a rule, for any given size, the risk of rupture is increased in smokers, females, those with high blood pressure, and those with a family history of an AAA.

The short answer is no. Surgical repair of an AAA is a major operation and carries risks. A small number of people will die during, or shortly after, the operation. If you have a small AAA, the risk of death caused by surgery is higher than the risk of rupture. Therefore, surgery is usually not advised if you have an AAA less than 55 mm wide. However, regular ultrasound scans will normally be advised to see if it gets larger over time.

Surgery is commonly advised if you develop an AAA larger than 55 mm. For these larger aneurysms the risk of rupture is usually higher than the risk of surgery. However, if your general state of health is poor, or if you have certain other medical conditions, this may increase the risk if you have surgery. So, in some cases the decision to operate may be a difficult one.

Emergency surgery is needed if an AAA ruptures. On average, about 8 in 10 people who have a ruptured aortic aneurysm will die due to the sudden severe bleeding. However, emergency surgery is life-saving in some cases.

There are two types of surgical operation to repair an AAA.

The traditional operation is to cut out the bad piece of aorta and replace it with an artificial piece of artery (a graft). This is a major operation and, as mentioned, carries some risk. Some people die during this operation. However, it is successful in most cases and the aneurysm is totally fixed. The long-term outlook is good. The graft usually works well for the rest of your life.

A newer technique allows the aorta to be repaired by a method called endovascular repair. This has become a popular option in recent years. In this method a tube is passed up from inside one of the leg arteries into the area of the aneurysm. This tube is then passed across the widened aneurysm and fixed to the good aorta wall using metal clips. The advantage to this type of repair is that there is no abdominal surgery. This technique is therefore safer than the traditional operation, and you need to spend less time in hospital. A disadvantage is that some people have to undergo a further operation at a later stage to refine the initial procedure.

Surgical techniques continue to develop and improve. Your surgeon will advise about the pros and cons of surgery, the different types of operation, and the best option for you.

If you have an AAA, you are likely to have a significant amount of atheroma that lines the artery. Therefore, you are at risk of having significant atheroma formation in other arteries, such as the coronary (heart) arteries and brain arteries. Therefore, you are likely to be at increased risk of developing heart disease (angina, heart attack, etc) and stroke.

In fact, most people who develop an aortic AAA do not die of the aneurysm but die from other vascular conditions, such as a heart attack or stroke.

Therefore, you should consider doing what you can to reduce the risk of these conditions by other means. For example:

  • Eat a healthy diet which includes keeping a low salt intake.
  • If you are able, exercise regularly.
  • Lose weight if you are overweight.
  • Do not smoke.
  • If you drink alcohol, do so in moderation.
  • If you have high blood pressure, diabetes, or a high cholesterol level, they should be well controlled on treatment.
  • You may be prescribed a statin drug to lower your cholesterol level and low-dose aspirin to help to prevent blood clots from forming.

See separate leaflet called Preventing Cardiovascular Diseases for more details.

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Research studies suggest that a routine ultrasound scan is worthwhile for all men aged 65. This is because most people with an AAA do not have symptoms. Following a routine scan, surgery can be offered to men found to have an aneurysm over 55 mm wide. Follow-up scans can be offered to monitor those with smaller aneurysms between 30 mm and 54 mm wide.

In early 2008, the Government announced that a national screening programme should be rolled out for men aged 65, while men aged over 65 should be able to self-refer. Implementation of the NHS AAA Screening Programme in England began in Spring 2009 and screening covered the whole of England by March 2013. Screening was also introduced in Scotland, Wales and Northern Ireland in 2013. Screening for AAA is offered only to men, as the condition is much more common in men than in women.

However, there are some people who have concerns about screening for AAA; for example, see the paper by Johnson cited under 'References', below.

The Circulation Foundation

Web: www.circulationfoundation.org.uk
Publishes a number of patient information leaflets to help identify and treat vascular illness. It also funds research into the prevention and causes of vascular disease.

NHS Abdominal Aortic Aneurysm Screening Programme

Web: http://aaa.screening.nhs.uk
Includes information for the public and health professionals about the national screening programme.

Original Author:
Dr Tim Kenny
Current Version:
Peer Reviewer:
Dr Hannah Gronow
Last Checked:
19/07/2012
Document ID:
4560 (v43)
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