Introduction

An abdominal aortic aneurysm (AAA) is a dilatation of the abdominal aorta greater than 3cm in diameter. At diameters greater than 5cm there is a significant risk of rupture and this event is life threatening and treated as a time critical medical emergency. Despite significant developments in screening and treatment, ruptured AAAs still result in many deaths each year.

Epidemiology

  • Incidence: 10.00 cases per 100,000 person-years
  • Peak incidence: 70+ years
  • Sex ratio: more common in males 2:1
Condition Relative
incidence
Renal stones15.00
Acute pyelonephritis10.00
Ruptured abdominal aortic aneurysm1
<1 1-5 6+ 16+ 30+ 40+ 50+ 60+ 70+ 80+

Aetiology

There are a number of risk factors which are linked with a ruptured AAA:
  • Female gender
    • Around 75% of AAA patients are male and the current NHS screening programme is only for men.
    • However the risk of rupture is much higher in women diagnosed with a AAA and can occur in aneurysms of a smaller diameter.
  • Age
    • Current NICE guidance is that all men over 55 be offered screening and all women over 70 with risk factors be offered screening for AAAs.
  • Smoking
    • Being a current smoker or having a significant smoking history are both the greatest risks for developing a AAA.
  • Chronic obstructive pulmonary disease (COPD)
    • There is an association between COPD and development of a AAA, especially in older women. It is likely this is simply a reflection of a significant smoking history.
  • Hypertension
    • Poorly controlled hypertension is a significant risk for AAA rupture.
  • Existing vascular disease
    • The presence of peripheral vascular disease, coronary heart disease or cerebrovascular disease indicates a higher risk of developing a AAA.
  • Family history
    • There appears to be a genetic link in AAAs and having a first degree relative with a AAA confers a higher risk, especially to women.

It is important here to note that diabetes mellitus does not confer a higher risk of developing a AAA or of it rupturing when diagnosed. Diabetics therefore do not need to undergo screening any earlier than suggested by the national screening programme.

Pathophysiology

The underlying cause of a AAA is usually atherosclerotic disease and there is a clear pathophysiological process from aneurysm formation to rupture:
  • Deposition of lipids within the wall of the abdominal aorta and atheroma formation causes the development of an inflammatory environment.
  • The inflammation subsequently causes release and activation of matrix metalloprotease enzymes from inflammatory cells which result in damage of the internal and external elastic laminae of the aortic wall.
  • The loss of the elastic laminae reduce the ability of the vessel wall to cope with the variance in pressure between systole and diastole and over time the aorta dilates.
  • The ongoing dilation causes a fibrotic response in the wall which results in both further loss of elasticity and thinning and loss of tensile strength of the vessel wall.
  • The eventual result of this process is that the aortic wall is significantly weaker than is required to counteract the systolic pressure.
  • Coupled with pre-existing hypertension the result is spontaneous AAA rupture.

Clinical features

The symptoms of a ruptured AAA depend on the severity and time since onset:
  • Pain
    • This is the most common symptom present in approximately 80-90% of cases and is often described as back or loin pain. Some people describe it as abdominal pain which radiates through into the back.
  • Cardiovascular failure
    • Even small ruptures cause significant haemorrhage and although posteriorly located ruptures can tamponade and temporarily self-contain, the volume can easily be multiple litres. The result is rapidly progressive tachycardia and hypotension (shock) which is poorly responsive to volume resuscitation. All AAA ruptures will eventually develop a state of shock with anteriorly located ruptures often developing severe hypovolaemia within minutes.
  • Distal ischaemia
    • If there is a haematoma within the aneurysm cavity, small quantities of this can embolise when the AAA ruptures and cause distal arterial occlusion. This can present anywhere on the spectrum of acute lower limb ischaemia and may rarely (<10%) be the presenting feature of a AAA.
  • Death
    • Approximately 33% of patients will die at the time of rupture and it should always be kept in mind as a differential for sudden death in middle age. Without treatment all AAA ruptures will eventually lead to death and it can be considered a terminal event.

Investigations

The screening test for a AAA is an abdominal ultrasound to visualise the abdominal aorta and perform a two-dimensional measurement of the aortic diameter. Ultrasound is also a quick diagnostic test in the emergency department to rule out a AAA as a cause of abdominal or back pain as it can be performed by the bedside and gives an instant objective measurement of aortic diameter.

Once a AAA has been diagnosed or known AAA patient has been admitted with a suspected rupture, the required investigations are used to plan treatment. The gold-standard imaging is a CT angiogram as this allows for a three-dimensional picture of the aneurysm to be created and this can be used to plan for surgery to repair it.

Along with imaging, a patient with a ruptured AAA should have the following blood tests taken:
  • Full blood count to ascertain whether there is a low platelet count which may require transfusion and affect surgical bleeding risk.
  • Renal function (urea and electrolytes) as if the aneurysm is treated endovascularly the patient will be exposed to large volumes of contrast and pre-existing renal failure may contraindicate this.
  • A coagulation screen should be performed to ensure there is no underlying bleeding risk as during any vascular procedure intravenous heparin is used and the dose may need adjustment if there is a bleeding disorder.
  • Valid blood grouping is essential and ideally once the patient is in a hospital with vascular surgery an a procedure is planned, several units should be crossmatched in preparation for theatre.

Differential diagnosis

Ruptured AAAs can present in a similar way to many other conditions and should always be kept in mind when considering these common problems:
  • Back pain
    • In patients over 50 presenting with acute and severe back pain you should always consider an abdominal ultrasound to measure the aortic dimension to rule out a AAA as the cause of the pain.
  • Acute pancreatitis
    • Measure the serum amylase or lipase in all patients presenting with acute abdominal or upper back pain.
    • If this is normal or only mildly raised, consider ultrasound imaging of the abdomen urgently to exclude a AAA as a cause of the pain.
  • Renal colic
    • In patients with symptoms of renal colic, even with a previous history of stones and a urine dip positive for blood, consider an abdominal ultrasound as this will both indicate ureteric or renal obstruction and give an aortic diameter measurement to rule out or diagnose a AAA.
  • Lower limb ischaemia
    • Although an acutely ischaemic lower limb is a medical emergency, when looking for a cause, or indeed when performing angiographic imaging to look for the location of the occlusion, consider abdominal imaging to look at the aorta to rule out a AAA as the cause.

Management

The management of a ruptured AAA is dependent on the following factors:
  • Anatomy of the aneurysm
  • Baseline health of the patient
  • Clinical state of the patient on admission

Depending on these factors, there are three main treatment options available for ruptured AAAs:
  • Open surgical repair
    • This involves a general anaesthetic and laparotomy then cross clamping the aorta, opening the aneurysm sac and placing a sutured synthetic graft in place to bridge the diseased area of the aorta.
    • Open repair is suitable for any shape of aneurysm as involvement of the renal arteries and iliac arteries can be dealt with using open surgical techniques.
    • A general anaesthetic and laparotomy are significant insults and if the patient has a very poor baseline function then they may not be able to survive the procedure.
    • Additionally the process of cross clamping the aorta generates a huge increase in afterload and the cardiac stress this causes can result in on-table cardiac arrest. This makes open surgery dangerous for patients with a significant cardiac disease history.
  • Endovascular aneurysm repair (EVAR)
    • A wire is passed under fluoroscopic guidance through the aneurysm sac and a stent-graft is inserted to occlude the aneurysm from the inside.
    • The procedure can be performed under local anaesthetic and requires only small incisions in the groin to access the femoral arteries meaning that even very frail patients are able to tolerate the procedure.
    • Application of the stent-graft from the inside of the vessel means additionally there is no need to occlude the aorta as there is in open surgery and so this method is more suitable for patients with significant cardiac disease.
    • The complexities of sizing a graft to ensure there is adequate isolation of the aneurysm sac means it is only suitable for AAAs not involving the renal arteries and whose three-dimensional anatomy allows for insertion of a wire via the femoral arteries.
    • The procedure requires large quantities of radiological contrast and is therefore unsuitable for patients with significant renal impairment.
  • Palliative care
    • Understanding that a ruptured AAA is a terminal event without treatment is important and it is valid to accept this and treat some patients palliatively with best supportive care.
    • Certain patients may simply have such a poor physiological reserve that the likelihood of surviving any procedure is very low. In these cases it is most appropriate to make the patient comfortable and manage their symptoms in the last hours of their life.
    • In some cases by the time the patient presents to a centre with vascular surgery, the AAA rupture and subsequent shock have progressed so far that intervention is no longer an option.
    • In all cases judged to be palliative the treatment should focus on preventing discomfort in the last hours of life with medications such as midazolam for agitation, haloperidol for nausea and morphine for pain. A syringe driver is an option to help control symptoms at this point.
    • It is important to have adequate discussions with family members about the prognosis when patients present with a ruptured AAA, even if surgery is initially intended, as the condition can progress very quickly and become inoperable in a short period of time.

Complications

If a patient survives treatment for a ruptured AAA there are a number of complications which they can develop. Some are generalised to both open and endovascular treatment:
  • Acute limb ischaemia can be caused by either embolus of clot from the aneurysm site or by injury to the lower limb vessels, especially in endovascular repair.
  • Both open and endovascular repair require sacrifice or occlusion of the inferior mesenteric artery and this can lead to bowel ischaemia if the marginal arterial supply to the left colon is inadequate. At open surgery this can be detected and either re-implantation of the inferior mesenteric can be performed or a primary bowel resection can be done depending on the surgeon's preference. Bowel ischaemia detected later confers a poor prognosis.
  • Abdominal compartment syndrome can occur following either form of treatment but is more common in open surgery. The syndrome is of rising intra-abdominal pressure which has the effect of causing a fall in renal perfusion, compression of the inferior vena cava and reducing cardiac preload and hence systemic perfusion and eventually splinting the diaphragm and causing respiratory compromise and type 2 respiratory failure.
  • The most devastating late complication in either form of repair is graft infection and can occur in up to 1% of cases. In the worst case scenario the infection can result in graft dehiscence and catastrophic bleeding.
  • All patients are likely to receive some form of blood transfusion at the time of treatment and indeed massive transfusions are the norm in open surgical repair. There are a number of significant complications resulting from this including coagulopathy, electrolyte disturbances and lung injury.

Complications associated solely with open surgery:
  • Abdominal wound dehiscence in the immediate post-operative period is rare but ruptured AAA surgery has a higher risk than other laparotomies owing to the poor physiological state of patients.
  • A large laparotomy wound has the potential for developing significant incisional hernias in later life and these are not uncommon if patients survive more than 5 years from surgery.

Complications associated with endovascular repair:
  • Haematomas or pseudo-aneurysms can occur in the groin at the site of access to the femoral arteries and these may result in ongoing bleeding in the post-operative phase and may require surgical treatment.
  • The most significant endovascular complication is leakage of blood around the stent-graft into the aneurysm sac resulting in enlargement of the aneurysm around the graft. This is known as endoleak and surveillance for it is required with regular CT angiograms for the rest of a patient's life following endovascular treatment of a AAA.

Prognosis

The prognosis of an untreated ruptured AAA is 100% mortality. Traditionally it was taught that 1/3 of patients would die before reaching hospital and 1/3 of patients would not survive surgery but survival rates have improved slightly:
  • Approximately 50% of patients who reach hospital and receive an intervention now survive, especially with widespread use of endovascular repair.
  • Approximately 33% of patients do still die before reaching hospital due to aneurysm rupture causing haemorrhage into the peritoneal space.
  • A widespread NHS screening programme is now reducing overall numbers of deaths due to ruptured AAAs.
    • The number of patients presenting with undiagnosed aneurysms is decreasing but the patients presenting with ruptures are more often those who have been declared not fit for elective repair.
    • The screening programme has also resulted in older patients who were not screened presenting with ruptured AAAs and more female patients are presenting with ruptures as women are currently not routinely screened.

Screening and prevention

Screening for an abdominal aortic aneurysm consists of a single abdominal ultrasound for males aged 65.


Screening outcome

Aorta widthInterpretationAction
< 3 cmNormal - No further action
3 - 4.4 cmSmall aneurysmRescan every 12 months
4.5 - 5.4 cmMedium aneurysmRescan every 3 months
>= 5.5cmLarge aneurysmReferred to vascular surgery for probable intervention
Only found in 1 per 1,000 screened patients


Further management

Low rupture risk
  • asymptomatic, aortic diameter < 5.5cm (i.e. small and medium aneurysms)
  • abdominal US surveillance (on time-scales outlines above) and optimise cardiovascular risk factors (e.g. stop smoking)

High rupture risk
  • symptomatic, aortic diameter >=5.5cm or rapidly enlarging (>1cm/year)
  • treat with elective endovascular repair (EVAR) or open repair if unsuitable. In EVAR a stent is placed into the abdominal aorta via the femoral artery to prevent blood from collecting in the aneurysm. A complication of EVAR is an endo-leak, where the stent fails to exclude blood from the aneurysm, and usually presents without symptoms on routine follow-up.