See also:

Ruptured Aortic Aneurysm
Aortic Dissection.

Definition

An aneurysm is a permanent and irreversible dilatation of a blood vessel by at least 50% of the normal expected diameter.¹ Aortic aneurysms are classified as abdominal (the majority) or thoracic.

The “normal” diameter of the abdominal aorta is approximately 2 cm; it increases with age.² An abdominal aneurysm is usually defined as an aortic diameter exceeding 3 cm.²

Theoretically, an abdominal aortic aneurysm (AAA) means one which is below the diaphragm, but there is no consensus on the definition:³

• Common practice uses the term AAA to refer to aneurysms below the level of the renal arteries.
• However, the term AAA can include aneurysms involving the renal ostia and supra-renal aneurysms.

Thoracoabdominal aneurysms:⁴

• These extend to a variable degree from the thoracic aorta into the abdominal aorta.
• They typically affect the origins of the visceral and renal arteries, which must be reimplanted into the graft during repair.

Epidemiology²

• AAAs are present in 5-7.5% of men >65 years and 1.5-3.0% of women >65 years.
• Ruptured AAAs cause 12,000 deaths per year. 8,000 of these are infra-renal.
• The incidence of AAA is increasing.

Aetiology^2,3

Most have no specific identifiable cause, but risk factors are:

• Severe atherosclerotic damage of the aortic wall - but new evidence suggests this is not the only factor, and aneurysmal disease is probably a distinct arterial pathology.
• Family history - there are probably strong genetic factors. About 15% of first-degree relatives of a patient with an AAA, mainly men, will develop an aneurysm.⁵
• Tobacco smoking is an important factor.
• Male sex.
• Increasing age.
• Hypertension.
• Chronic obstructive pulmonary disease.
• Hyperlipidaemia.
• In population-based studies, people with diabetes have a lower incidence of aneurysms than non-diabetics.

For a minority, there may be a specific cause, e.g:

• Trauma.
• Infection - brucellosis, salmonellosis, tuberculosis.
• Inflammatory diseases, e.g. Behçet's disease, Takayasu's disease. Inflammatory aneurysms may have multifactorial pathology.^6,7
• Connective tissue disorders - Marfan's syndrome, Ehlers-Danlos syndrome type IV.

Pathology:

• There is degradation of the elastic lamellae, a leukocytic infiltrate, enhanced proteolysis and smooth muscle cell loss.
• The dilatation affects all 3 layers of the vascular tunic (if not, it is a pseudoaneurysm).

Presentation^3,4,8

Unruptured AAA

• Most patients with unruptured AAA have no symptoms.
• AAA may be an incidental finding on clinical examination, or on scans - ultrasound, CT or MRI. It may sometimes be visible on a plain X-ray film.
• Possible symptoms and signs are:
  • Pain in the back, abdomen, loin or groin
    • This may be due to pressure on nearby structures. Back pain may be due to erosion of the vertebral bodies.
    • Note: Severe lumbar pain of recent onset may indicate impending rupture.
  • The patient or doctor may find a pulsatile abdominal swelling
  • Distal embolisation may produce features of limb ischaemia. The appearance of microembolic lower limb infarcts in a patient with easily palpable pedal pulses suggests a popliteal or abdominal aneurysm
  • Ureterohydronephrosis can also occur with AAA
  • Inflammation or retroperitoneal fibrosis can complicate AAA, and may cause symptoms e.g. back pain, weight loss and symptoms related to entrapment of adjacent structures^6,7

Ruptured AAA

• Note: The diagnosis may not be obvious. Ruptured abdominal aortic aneurysm should be considered in any patient with hypotension and atypical abdominal symptoms. Similarly, abdominal pain in a patient with a known aneurysm or pulsatile mass must be considered as a possible ruptured or rapidly-expanding aneurysm and treated accordingly.
• Ruptured AAA may present with:
  • Pain in the abdomen, back or loin - the pain may be sudden and severe
  • Syncope, shock or collapse
    • The degree of shock varies according to the site of rupture and whether it is contained, e.g. rupture into the peritoneal cavity is usually dramatic, with death before reaching hospital; whereas rupture into the retroperitoneal space may be contained initially by a temporary seal forming.
  • A pulsatile abdominal mass
  • Can mimic renal colic, testicular pain or other types of acute abdominal pain, e.g. diverticulitis, bowel ischaemia, acute pancreatitis
  • Rare presentations are
    • Severe haematemesis from an aortoduodenal fistula.
    • A fistula into the inferior vena cava produces lower limb oedema and high-output cardiac failure.

Examination:

• Bimanual palpation of the supra-umbilical region can detect 61% of aneurysms >3 cm and 82% of those >5 cm.
• Sensitivity is reduced with increasing waist size.
• There may be an abdominal bruit.
• Clinical signs vary according to the whether the aneurysm compresses other structures or ruptures - and if ruptured, the site and extent of bleeding.
• Retroperitoneal haemorrhage may cause Grey Turner's sign, i.e. flank bruising.

Investigations³

This covers the non-urgent scenario. Investigations aim to evaluate the detailed anatomy of the aneurysm, any treatable causes and the patient's fitness for surgery:

• Blood tests:
  • Full blood count, clotting screen, renal function and liver function
  • Cross-match if surgery planned
  • ESR and/or CRP if inflammatory cause suspected
• ECG, chest X-ray and possibly lung function tests.
• Scans:
  • Ultrasound is simple and cheap; it can assess the aorta to an accuracy of 3 mm. It is used for initial assessment and follow-up
  • CT provides more anatomical details, e.g. can show the visceral arteries, mural thrombus, the 'crescent sign' (blood within the thrombus, which may predict imminent rupture), and para-aortic inflammation. CT with contrast can show rupture of the aneurysm
  • MRI angiography may be used. This is safer than conventional angiography as it does not use nephrotoxic contrast medium

Management of uncomplicated AAA²

For treatment of ruptured aortic aneurysm click here and for aortic dissection see here.

For uncomplicated AAA, small aneurysms (<5 cm) are monitored, and larger ones (>5 cm) should be considered for surgery.

Medical management

• Regular ultrasound monitoring - the frequency dictated by the diameter of the aneurysm at the time of detection, e.g:
  • 3.5-4.0 cm - annual ultrasound
  • 4.0-5.0 cm - 6-monthly ultrasound
  • >5.0 cm - consider surgery (see below), 3-monthly ultrasound
• Treat underlying causes where feasible, e.g. infection. Some cases of peri-aortitis or retroperitoneal fibrosis may merit treatment with prednisolone or immunosuppressants.⁷
• Attention to risk factors:
  • Smoking cessation
  • Rigorous blood pressure control
  • Statins - the role of statin therapy in AAA is unproven, but statins are advised because AAA patients have increased cardiovascular disease risk
  • Anti-platelet therapy where appropriate
• Other medical treatment:
  • There is some evidence that the following may reduce the rate of expansion of small aneurysms, but their role is not yet clear:
    • Doxycycline or roxithromycin⁹
    • ACE inhibitors or losartan⁹
    • Statins⁹
    • Low dose aspirin¹⁰

Driving¹¹

A summary of the DVLA rules is:

• Group 1 (own driving license):
  • Notify DVLA of aneurysm >6 cm
  • Aortic diameter > 6.5 cm disqualifies from driving
• Group 2 (LGV/PCV license):
  • Notify DVLA
  • Aortic diameter >5.5 cm disqualifies from driving

Surgery

Indications:

• For all patients with aneurysms of ≥5.0 cm diameter, consider elective surgical repair (if fit enough for surgery).
• The decision about surgical intervention is based on the risk of surgery versus rupture for each patient. In general the risk of rupture is mainly determined by the aneurysm diameter, but rupture rates are higher in patients who smoke, in females, in those with hypertension and in those with a strong family history.
• Other indications for surgery are rupture, rapid expansion, or onset of sinister symptoms/signs such as back or abdominal pain or tenderness.

Types of repair for AAA:

Surgical (open) repair

• This is the traditional operation. It involves exposure of the abdominal aorta, aortic and iliac clamping and replacement of the aneurysmal segment with a prosthetic graft. The graft is effective and durable.

Endovascular repair of AAA

• In the last decade, endovascular aneurysm repair (EVAR) has become available. Its role is still debated and the technology is evolving rapidly.
• EVAR involves introducing a stent-graft system through the femoral arteries, which relines the aneurysm, diverts blood flow through the endograft and allows the aneurysm to thrombose. This requires that the aneurysm has an adequate (1.2 cm) “neck” below the renal arteries for stent fixation.
• Currently, about 65% of patients with AAA are suitable for endovascular repair. Many clinicians believe that endovascular techniques should be the first line therapy for all suitable AAAs.

Pros and cons of endovascular repair

Advantages:

• Avoids open abdominal surgery.
• Avoids aortic cross clamping.
• Recent RCTs suggest reduced mortality in the first 4 years of follow-up; longer term results are unknown.²

Disadvantages:

• Follow-up with ultrasound or CT scans is essential to monitor the endograft.
• Failure of the endograft can occur - this was a common problem with early grafts, but the newer designs are more durable.

Prognosis²

Natural history:

• The risk of rupture is mainly determined by aneurysm diameter.
• The natural history of small AAA is gradual expansion at an annual rate of approximately 10% of the initial arterial diameter.

Elective repair:

• Overall mortality in the UK is 7.8% for elective AAA repair. There is an inverse relationship between operative mortality and the number of cases performed in individual hospitals, with many specialist centres reporting mortality rates well below 5%.
• The mortality rate of aneurysm repair depends on the patient's fitness for surgery and the morphology of the aneurysm.
• Repair of thoracoabdominal aneurysms carries a higher mortality than for infra-renal surgery.⁴
• Patients with severe cardio-respiratory or renal disease may have high peri-operative mortality rates, and for them the threshold for elective repair may be set at a larger aneurysm diameter.

Ruptured AAA:

• The outcome is poor, with 80% overall mortality from ruptured AAA.
• Most patients die before reaching hospital. Surgical repair of ruptured AAA has a 50% mortality.

Screening for AAA

Screening by ultrasound is feasible to allow early diagnosis. The idea is to offer a single scan to men aged 65. If negative, this effectively rules out AAA for life. In the UK, the National Screening Committee decided that AAA screening could be considered by UK health departments. A screening programme in Scotland is due to start in 2011. AAA screening is also planned for England. ¹²

The pros and cons of screening are debated:^13,14

• Possible benefits are a reduction in mortality, because the vast majority of deaths from aortic aneurysms are due to rupture of an undiagnosed aneurysm, which might be prevented by elective treatment.
  • Trials of screening for AAA have been conducted. Results so far suggest that screening programmes may be worthwhile for men >65 years^15,16
  • The benefits of screening women have not been established
• Disadvantages of screening are:
  • Anxiety caused by diagnosis of a serious condition which may cause sudden death
  • Intervention (repair) carries significant risk of death or complications (see Prognosis section)
  • Screening with repair reduces aneurysm-related mortality, but has less effect on all-cause mortality (i.e. patients still die from other causes)^15,17
  • Sufficient resources are needed to implement both the screening and the interventions (monitoring and repair)
• The cost-effectiveness of screening has been reviewed.¹⁸