The aorta is the largest blood vessel in the body that 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 most probably plays a secondary role.
An
uncomplicated aneurysm usually produces no symptoms.
The most common
complication is rupture, which represents a medical emergency and is often
fatal.
When necessary and appropriate, aortic aneurysms
are surgically repaired.
What is an abdominal aortic 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 into various branches that eventually
supply all the tissues and organs of the body. One of the main branches of the
aorta travels down the chest cavity and through the abdomen, before dividing
into separate vessels which descend into the legs.
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. Aneurysms arise 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, so that
a bulge arises in the artery.
There are two main types of aneurysms, which are distinguished by
the different ways in which they are caused. These different types are referred
to as true aneurysms and false aneurysms. In the case of true aneurysms the
weakening of the arterial wall is caused by some disease process. False
aneurysms occur when the damage that weakens the arterial wall is caused by an
acute traumatic event, such as may occur as a result of blunt trauma (injury) to
the chest, a stab wound or when a large needle is inserted into an artery during
a medical procedure. False aneurysms occur less frequently than true aneurysms.
True aneurysms can occur in any large artery but are particularly
common in the aorta and the arteries branching off the aorta. Approximately 75%
of aortic aneurysms occur in the abdominal aorta. More than one aneurysm may
occur simultaneously in a single artery 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 or
arterial dissection. 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” and creating a flap
inside the artery.
Dissection occurs more commonly in the section of the aorta which
is in the chest than they do in the abdominal aorta. However, those that occur
in the region of the chest often spread downwards toward the abdomen.
What causes abdominal aortic aneurysms?
By far the most
common disease that leads to 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 can now briefly describe.
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 arterial 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
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,
and these slowly enlarge over time.
Degenerative processes of the arterial wall and inflammation are
the primary aetiological factors.
Risk factors for abdominal aortic aneurysm
The main risk
factor is the presence of atherosclerosis, which will be compounded by
coexisting high blood pressure and cigarette smoking.
This disorder tends to run in families, so a family history of
abdominal aortic aneurysms will also increase the risk if atherosclerosis is
present.
Symptoms and signs of abdominal aortic aneurysm
True aortic
aneurysms often do not produce symptoms, although rapidly enlarging aneurysms
can become tender. Some people with this condition may complain of low back pain
or a pulsation in the abdomen.
Most 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, and this 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.
A further complication that can arise from an abdominal aortic
aneurysm is when a small part of a blood clot on the internal wall of the
aneurysm becomes dislodged. Such a clot will travel 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 a
medical 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, absent pulses or a tender mass in
the aorta. A mass can usually be felt in the abdomen.
How is an abdominal aortic aneurysm diagnosed?
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.
Aortograpy is an x-ray examination of the aorta which 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.
Can an abdominal aortic aneurysm be prevented?
If a person is
known to have atherosclerosis, then controlling high blood pressure and giving
up smoking will help to prevent aneurysm formation.
Once an aneurysm has formed, however, nothing can be done to
prevent it from progressing, although progression is faster in people who
continue to smoke.
How is an abdominal aortic aneurysm treated?
Rupture and
dissection are emergency situations and are always treated with surgical repair
of the aorta if possible. Following acute rupture, the mortality associated with
an emergency operation is generally greater than 50%.
When an aneurysm is detected prior to rupture or other
complications, a decision must be made as to whether or not surgery should be
carried out.
Two types of surgical repair are available. The traditional
procedure involves replacing the dilated section of the artery with a synthetic
arterial graft. More recently, a procedure has been developed in which a device,
inserted via the groin, is placed inside the damaged artery. This device, called
a stent, prevents the atherosclerotic plaques and clots from obstructing the
blood flow through the artery, as well as preventing the arterial wall from
rupturing. A great advantage of this latter technique is that it is a relatively
minor surgical procedure and poses less risk to the patient than the insertion
of a graft. The technique is still new but the results have been encouraging.
However, this procedure is appropriate only for limited types of aneurysms.
Surgery is unnecessary in the case of asymptomatic aneurysms that
are not at risk of rupturing. In other cases an assessment needs to be made of
the comparative risks posed by surgery and by rupture of the aneurysm. The risk
of rupture depends on two primary factors: the size of the aneurysm and the rate
of enlargement of the aneurysm. A rapidly expanding aneurysm, regardless of its
size, is likely to rupture and should be surgically repaired. When considering
more stable aneurysms, the risk of rupture is proportional to the size of the
aneurysm. Aneurysms less than 5 cm do not pose a great risk of rupture and are
usually followed up regularly with ultrasound and other non-invasive
investigations. In patients with aneurysms between 5 and 6 cm, surgery is
seriously considered because there is a high risk of rupture. The likelihood of
rupture in aneurysms greater than 6 cm is extremely high and surgery is
indicated in all patients except those with an excessively high risk of
complications from surgery.
Some aneurysms may be of considerable length and may therefore
affect a section of the aorta at which smaller arteries, such as those supplying
the kidneys, branch off from the main vessel. Surgery to repair such aneurysms
is obviously more involved than in the less complicated cases, but is
nevertheless performed routinely and successfully.
With careful evaluation of cardiac status and other risk factors,
and following good postoperative care, the mortality associated with surgery for
abdominal aortic aneurysms is around one to five percent.
When to see your doctor
If you notice a
pulsating mass in your abdomen. If you suffer from signs of atherosclerosis such
as heart disease and high blood pressure and you experience low back pain,
particularly if this is a new symptom.
If you are
living with someone with a known abdominal aortic aneurysm and they complain of
sudden severe pain, become sweaty and pale, and collapse, you should call for
emergency medical attention immediately
