Clin Risk 2008;14:208-210
doi:10.1258/cr.2008.080076
© 2008 Royal Society of Medicine Press
Abdominal aortic aneurysm disease: health risks, management and screening
Hany Hafez
Email: mail{at}hanyhafez.net
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Abstract
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Abdominal aortic aneurysm (AAA) disease is a condition that
affects mainly men over the age of 65 years. The majority of
AAAs will remain asymptomatic and therefore undetected. These
undetected aneurysms will invariably grow enough for their walls
to rupture leading to death in nearly 75% of those affected.
Ultrasound-based AAA screening has been shown to reduce the
risk of dying of an AAA by half. This significant reduction
of mortality offered by a simple and sensitive non-invasive
test has prompted the Department of Health to introduce a national
AAA screening programme for England and Wales. In this article,
the public health risks of AAA disease together with details
of the impact of the anticipated national AAA screening programme
are discussed.
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Introduction
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It has been long known that certain arteries can show localized
dilations either spontaneously or as a result of trauma or infection.
If such a dilation exceeds 50% of the normal diameter of the
affected artery, then it is described as an aneurysm. The most
common of these aneurysms is that of the aorta below the origin
of the arteries to the kidneys. These aneurysms are widely,
but rather inaccurately, known as abdominal aortic aneurysms
(AAA). A more anatomically accurate description would be infrarenal
aortic aneurysms. In men, the maximum normal aortic diameter
at this level should not exceed 2.5 cm. An aorta that is 3 cm
or more in diameter at this level qualifies as being aneurysmal.
The prevalence of AAA varies according to ethnicity, age and gender. Men are six times more likely to be affected by this condition. At the age of 65 years, 3% of men will have an AAA. The prevalence then increases with age to reach nearly 8% at the age of 80. AAAs represent nearly 98% of aneurysms of the whole aorta. Rarer aneurysms of the aorta, such as thoracic and thoracoabdominal, fall outside the scope of this article.
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Nature of AAA
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Although the exact causation of AAA disease remains largely
unknown, evidence exists that most aneurysms are associated
with specific connective tissue degenerative changes in the
form of elastin breakdown and collagen build-up. Elastin and
collagen are the two main proteins responsible for maintaining
the elasticity and strength of arteries. It is possible that,
in affected individuals, genetically predetermined connective
tissue weakness is potentiated by environmental factors such
as smoking and high blood pressure leading to the formation
of AAA.
Once formed, the natural history of an AAA is to grow with time. The rate of growth varies considerably and is subject to many factors including smoking, high blood pressure and medications. One of the most significant predictors of AAA growth is the size of the aneurysm itself. Larger aneurysms will tend to grow faster. This is probably a reflection of the fact that the underlying degenerative process in these aneurysms is more aggressive.
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AAA and risk of rupture
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AAA disease is potentially lethal. Allowed time, some aneurysms
will grow to an extent where the wall of the affected aorta
undergoes a catastrophic structural failure and ruptures. In
few individuals, rupture will be preceded by some symptoms such
as abdominal or back pain. However, most AAA ruptures occur
suddenly and without any warning. It is estimated that 50% of
those who suffer an AAA rupture will die instantly from massive
blood loss. The remaining 50% will make it to hospital but only
half of them will survive surgery. The overall chance of surviving
a ruptured AAA is between 25% and 35%, thus constituting some
of the lowest survival figures of any surgical emergency. This
low survival rate has made this disease the 10th cause of death
in England and the 13th in the USA.
In 2005, nearly 3000 men in England and Wales1 and 8000 men in the USA2 died from a ruptured AAA. The most significant risk of rupture these individuals would have had is their AAA size. While aneurysms smaller than 5.5 cm in diameter have less than 1% annual risk of rupture, similar risk for larger aneurysms is rather difficult to predict. It is generally accepted that aneurysms between 5.5 cm and 6 cm diameter will have a 5–10% annual risk of rupture. Aneurysms between 6 cm and 7 cm in diameter will have a 10–20% annual risk of rupture and for aneurysms larger than 7 cm, the annual risk of rupture will be between 20% and 50%.3 These figures have not changed much over the last three decades despite better understanding of the disease. This understanding, however, falls short of identifying effective preventive medical measures similar to those that have been very successful with cerebrovascular and coronary arterial disease.
Although the risk of AAA rupture is closely related to aneurysm size, other factors such as aneurysm growth rate, smoking and gender are also important. While AAAs in women are 4.5 times more likely to rupture, the incidence of AAA in women is relatively low and the age of onset is approximately 10 years more than in men thus making this condition less of a health risk in this gender.
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AAA treatment
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To date, surgery remains to be the only definitive treatment
for this condition. The traditional (open) surgical approach
involves direct exposure of the aneurysm followed by replacing
the aneurysmal part of the aorta with a synthetic graft. Endovascular
aneurysm repair (EVAR) is a more modern and less invasive technique
which is becoming widely used. In EVAR, the aneurysm is accessed
through both femoral arteries under radiological guidance. A
specially designed graft is then deployed from within the aneurysm.
Once in position, this graft acts as a barrier between the blood
stream and the aneurysm wall thus reducing the risk of rupture
to a great extent.
The decision to treat an AAA is not as straightforward as one might hope. Elective open surgical repair carries a 5–7% risk of mortality. This is two to three times as much as the risk of mortality following coronary artery bypass surgery. For EVAR, this risk is reduced to 2%, however the technique itself has its own limitations and remains cost-ineffective.
For what is essentially a prophylactic procedure, AAA treatment-associated risks have to be carefully weighed against the benefits of such treatment. It is on this basis that small (<5.5 cm) asymptomatic aneurysms are best left untreated and kept under surveillance. It is also on this basis that any AAA screening programme will have two tasks, identification and surveillance.
Although surgery is likely to remain the main method for AAA treatment in the next decade, the increasing understanding of the disease may lead to new medically-oriented treatment options. Observational studies have already demonstrated an association between certain drugs and lower risk of AAA rupture4 and growth rate.5 Early predictions suggest that a time may come when AAA growth rate is medically controlled to the extent that surgery may not be necessary at all.
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AAA screening
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The concept of screening for AAA disease as a means of reducing
its mortality was introduced over two decades ago. The theory
was that screening for AAA, with a view to timely intervention,
has the potential of reducing the incidence of AAA-related mortality
in the screened populations. Since the infrarenal abdominal
aorta is readily amenable to ultrasonographic visualization
capable of correctly identifying an AAA 98% of the time, aortic
ultrasound was the obvious ideal screening method. The examination
itself is relatively easy, non-invasive and economical.
A number of large trials were designed to test the potential benefit of ultrasound-based screening for AAA disease.6–9 The most notable of these trials was the Multicentre Aneurysm Screening Study (MASS). In the MASS trial, 67,800 men aged between 65 and 74 years were randomized to either receive a screening scan or not. In the screening arm, 80% of those who were invited for a scan attended. The trial showed that screening reduced the risk of dying from AAA by 42% in the whole cohort and by 53% in those who attended a scan.5,6 These remarkable results were instrumental in the subsequent introduction of national AAA screening in the USA, England and Wales and, most recently, Scotland. Other nations continue to study the feasibility of introducing this service and more national AAA screening programmes are likely to follow.
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National AAA screening for England and Wales
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In January 2008, the Department of Health announced the introduction
of a national AAA screening programme for England and Wales.
The intention is to introduce the programme in phases, aiming
at achieving complete coverage of England and Wales by 2013.
Since this announcement, the National Screening Committee has
set plans for the establishment of five to six early implementation
programmes by April 2009. As well as acting as first wave screening
service providers, these early implementation programmes will
also act as training centres for the subsequent waves.
Once fully operational, the AAA screening programme for England and Wales will offer its services to approximately 270,000 men reaching the age of 65 years each year. Assuming 80% compliance, 216,000 of these men will actually attend for a scan. Added to this will be a yet undetermined number of male screenees who are older than 65 years and may wish to be included. These potential additional screenees will not be actively invited but will be accepted into the programme upon their request. With time, the number of these self-referred screenees will reduce as more and more men go through the invitation at the age of 65 years.
Of the expected 216,000 attendees, 6480 will be diagnosed with an aneurysm (based on 3% prevalence). The majority of these individuals will have small aneurysms and only an estimated 432 will need surgery straight away. The remaining positive screenees will be offered repeat scanning. The frequency of these repeat surveillance scans will depend on the size of the aneurysm. Figure 1 shows the expected protocol for the national screening programme as well as the frequency of scanning for each AAA size category.
It is expected that by five years from its full implementation,
the national AAA screening programme for England and Wales will
refer a total of 2700 patients for treatment each year. The
National Screening Committee predicts that the fully operational
programme will save some 340 lives in its fifth year. This figure
will increase to 880 and 1600 in years 15 and 30, respectively.
These figures take into account lives lost to elective surgery.
In addition to its healthcare role, the national AAA screening programme will have an important regulatory role. To ensure that the screenees referred for treatment by the programme receive the best possible surgical care, the National Screening Committee (NSC) has set a number of conditions that have to be met by vascular surgical providers to the programme. The programme will recognize only units that have regularly submitted their AAA workload data to the National Vascular Database and have met with nationally acceptable volumes of work and mortality figures. Vascular units that do not meet with any of these criteria will not be recognized as providers by the national AAA screening programme. The latter will advise individual strategic health authorities (SHAs) as to eligibility of units within their region. SHAs will then advise primary care trusts on eligibility to receive referrals for the programme for AAA treatment. The performance of these units will be monitored to ensure maintenance of quality.
The regulatory role will be the first of its kind and size so far as vascular surgical services in England and Wales are concerned. To meet the required criteria, some units will need to reconfigure their services to ensure compliance. While the intention is to ensure that screened AAA patients receive the best possible treatment, it is likely that the benefit from any reconfiguration will extend to other vascular conditions, thereby improving the standards of the whole service nationally.
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Conclusion
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AAA disease is one of the main causes of male deaths in the
Western world. The silent nature of this condition makes it
difficult to diagnose in time and before rupture. Elective timely
intervention has a much lower risk of mortality compared with
emergency surgery. Screening for AAA disease in high-risk individuals
followed by elective surgery when indicated has been shown to
reduce the risk of mortality from this condition by half.
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Footnotes
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Hany Hafez MBBS PhD FRCS, Consultant Vascular Surgeon, Director,
Chichester Abdominal Aortic Aneurysm Screening Programme and
Research Unit, St Richard's Hospital, Chichester, West Sussex
PO19 6SE, UK
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References
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- Office of National Statistics. Mortality Statistics. See http://www.statistics.gov.uk/downloads/theme_health/Dh2_32/DH2_No32_2005.pdf
- National Vital Statistics Report. Vol 26, No. 10. See http://www.cdc.gov/nchs/data/nvsr/nvsr56/nvsr56_10.pdf
- Brown LC, Powell JT. Risk factors for aneurysm rupture in patients kept under ultrasound surveillance. UK Small Aneurysm Trial Participants. Ann Surg 1999; 230: 289–96[Medline]
- Hackam DG, Thiruchelvam D, Redelmeier DA. Angiotensin-converting enzyme inhibitors and aortic rupture: a population-based case-control study. Lancet 2006; 368: 659–65[Medline]
- Thompson AR, Cooper JA, Aston HA, Druce S, Humphries SE, Hafez H. The use of ACE inhibitors and angiotensin II receptor antagonists is associated with a significant reduction in AAA growth rate, independent of map. Br J Surg 2008; 95: 265
- Ashton HA, Buxton MJ, Day NE, et al. Multicentre Aneurysm Screening Study Group. The Multicentre Aneurysm Screening Study (MASS) into the effect of abdominal aortic aneurysm screening on mortality in men: a randomised controlled trial. Lancet 2002; 360: 1531–9[Medline]
- Kim LG, Scott RA, Ashton HA, Thompson SG. Multicentre Aneurysm Screening Study Group. A sustained mortality benefit from screening for abdominal aortic aneurysm. Ann Intern Med 2007; 146: 699–706[Abstract/Free Full Text]
- Boll AP, Severens JL, Verbeek AL, van der Vliet JA. Mass screening on abdominal aortic aneurysm in men aged 60 to 65 years in The Netherlands. Impact on life expectancy and cost-effectiveness using a Markov model. Eur J Vasc Endovasc Surg 2003; 26: 74–80[Medline]
- Cosford PA, Leng GC. Screening for abdominal aortic aneurysm. Cochrane Database Syst Rev 2007; 18: CD002945
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Abstract
Introduction
