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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
(1)
- most AAAs occur in the lower part of the abdominal aorta, below the arteries
to the kidney (infra-renal)
- main risk factors for AAA include being male, increasing age, high blood
pressure, smoking and family history of AAA
AAA is the most common form of aortic aneurysm. It is known that 4% of men
aged 65 have an AAA. AAA is uncommon in women: the male to female ratio is in
the order of 6:1.
Most AAA are small, and not immediately dangerous. As time passes they can
grow, and large AAA, especially above 6 cm in diameter, are liable to rupture.
Without prompt intervention, ruptured AAA is often fatal. Fernando Santiago
called AAA "the U-boat in the belly - silent, deep and lethal". Rupture of an
AAA is the 13th most common cause of death in the USA; in England and Wales
there are around 6,000 deaths each year from ruptured AAA. Most AAA are entirely
symptom free and are not detected before they rupture.
The estimated annual rupture rate of aneurysms larger than 6cm is 10%, rising
to more than 30% for aneurysms larger than 8 cm. The five year survival for
patients with aneurysms larger than 5cm who are not operated on is about 20%
(3).
The slow growth rate of AAA means that they are an ideal target for ultrasound
screening. A national programme started earlier this year (2009), which by 2013
aims to offer screening to all men in England in their 65th year. Most men offered
a scan in their 65th year (96%) will be reassured and discharged. Men with a
small AAA will enter a surveillance programme to monitor AAA growth (2). There
is evidence (3) that elective repair of an aneurysm less than 5.5 cm cannot
be justified since the risks of treatment are greater than the risk of rupture.
Above this size, AAA can be treated by surgery or endovascular stenting. The
risks of intervention vary from 2-7% depending on the type of procedure and
the fitness of the patient. Research has demonstrated that offering men ultrasound
screening in their 65th year could reduce the rate of premature death from ruptured
AAA by up to 50% (4).
In animal models various drugs have shown promise in reducing aneurysm growth;
especially propranolol. However, three randomised controlled trials showed no
benefit from propranolol in the treatment of small AAAs (5).
Surgical treatment is associated with a 5-7% mortality. There is evidence (4)
that elective repair of an aneurysm less than 5.5 cm cannot be justified when
considering the risk of rupture with that of surgery.
NHS AAA Screening Programme:
Notes:
- the 'normal' diameter of the abdominal aorta is approximately 2cm, a dimension
that increases with age (6)
Reference:
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