Introduction

An abdominal aortic aneurysm (AAA) is a localized, irreversible abnormal dilatation of the abdominal aorta. The average aortic diameter is 2cm and the vessel is considered aneurysmal when the diameter is 1.5 times its normal diameter. The vast majority of AAAs are asymptomatic; however, a high proportion of them are found incidentally as part of imaging studies for other co-morbidities. An AAA that has never been found can expand to reach a size where its wall is weakened significantly and eventually ruptures. An AAA rupture is a full blown surgical emergency that needs immediate management.

Epidemiology

    • AAA incidence is higher in males than females and in white men than African-American men.
    • Incidence of AAA increases with age reaching its peak at the age group of 70-79years old (Figure 1)
    • Incidence of AAA is 8.2% in UK.
    • Incidence of rupture is 13 cases per 100,000 in the UK.
    • 50% of the AAA rupture cases do not survive to the Emergency Department
      Figure 1 Age as a risk factor of AAA adapted from medscape.com

      Pathophysiology

      • Current indications for surgical intervention, suggest that AAA diameter is an important risk factor for AAA rupture.
      • Research shows that other factors such as aneurysm volume are a risk factor for rupture.
      • AAA rupture occurs when the mechanical stress acting on the wall exceeds the tensilestrength of the wall. The equation for wall tension is given by Laplace’s Law: Wall tension= P x R/W where P is Mean Arterial Pressure, R is radius of aneurysm and W is the wall thickness of the vessel. 
      • Computerized generated models suggest that aneurysm volume is a better predictor of AAA rupture than aneurysm diameter, as it is more sensitive in identifying areas of peak wall tension where a rupture will occur.
      • AAAs are expanding with a rate of 0.2-0.8mm/y. This represents a dynamic situation which evolves as time passes, increasing the rupture risk as the AAA expands and increases in size.
      • The bigger the diameter the higher the rupture risk is. (Table 1)
      Infrarenal AAA rupture adapted from aorticstents.com

      Table 1 - Aneurysm Size (cm) vs Rupture Risk

      3.0-3.9

      4.0-4.9

      5.0-5.9

      6.0-6.9

      >7.0

      0%

      0.5-5.0%

      3.0-15.0%

      10.0-20.0%

      20.0-50.0%

      Complications of AAA rupture

      • Hemoperitoneum and exsanguination
      • Ischemic Myocardial Infarction
      • Ischaemic Stroke
      • Kidney Failure
      • Hypovolaemic shock
      • Distal thromboembolism

      Factors that affect risk stratification

      Morphological Factors

      • Diameter
      • Size
        • Expansion Rate
          • Wall Stress
            • Shape

            Related Factors

            • COPD
            • Smoking
            • Hypertension
            • family History of ruptured AAA
            • Lifestyle factors (e.g. diet)

             

            Why the shape?

            The shape of the aneurysm has also been associated with risk of rupture. Saccular aneurysms are associated with a low risk for rupture whereas fusiform aneurysms have a moderate rupture risk. Other more eccentric shapes with variation in neck anatomy and tortuosity (anteversion and twisting) are characterised as hostile aneurysms with a very high risk of rupture.

            Diameter and aneurysm rupture riskSaccular and fusiform aneurysms adapted from. neuro.wehealney.org

            Presentations of AAA rupture

            Typical AAA rupture presentation

            • Abdominal or back pain
            • Pulsatile abdominal mass around the umbilical area (where most infrarenal AAA occur just before the aortic bifurcation)
            • Hypotension
            • Tachycardia
            • Loss of consciousness, syncope
            • Cyanosis

            Atypical AAA rupture presentations:

            • Peripheral embolism with associated claudication
            • Aortocaval fistulae when the weakened wall ruptures in the vena cava producing congestive heart failure, tachycardia, fluid thrill, renal failure and lower limb swelling.
            • Aortoduodenal fistulae: Aorta ruptures in the duodenum leading to an upper GI bleed presentation and a following exsanguinating hemorrhage.

            The presence of an AAA pulsatile abdominal mass might be missed even by experts when the abdominal girth exceeds 102cm. This complicates the differential diagnosis as you are left with abdominal or back pain, a symptom that has a vast differential diagnosis.

            Differential Diagnosis

            • Appendicitis
            • Cholelithiasis
            • Diverticular Disease
            • Gastritis and Peptic Ulcer Disease
            • Urinary Tract Infection in women
              • Large Bowel Obstruction
              • Myocardial Infarction
              • Pancreatitis in Emergency Medicine
              • Small Bowel Obstruction in Emergency Medicine

              Pre-operative assessment

              Acute emergency scenario - Haemodynamically unstable

              • Confirmation of rupture by portable ultrasonography
              • Haemodynamic stabilization by surgical intervention
              • Prophylactic antibiotics (cephalosporin)
              • Blood counts sent for analysis
              • If blood type is identified, blood transfusions should be considered, especially if the patient has lost copious amounts of blood.

              Haemodynamically stable patient

              • Full blood count to assess transfusion requirements and possibility of infection
              • Assessment of pulmonary function by blood gas measurements or by questioning patient.
              • Portable ultrasonography to confirm rupture
              • Spiral CT scan with intravascular contrast (to achieve better imaging of the relevant anatomy and consider Endovascular Repair), only if patient is stable and Intravenous (IV) access has already been established
              • Assess Cardiac status of patient
              • Open or Endovascular (EVAR) surgical intervention

              Management

              The management of a ruptured AAA (RAAA) is only by surgical intervention in order to regain control of blood circulation. Once the blood loss is under control, the circulation needs to be re-established by an aortic stent or a bifurcating stent in an open repair procedure. Endovascular repair (EVAR) is possible in patients that are haemodynamically stable and have had a spiral CT with contrast to assess the size of the graft needed. Cost is its main disadvantage as well as the availability of the appropriate graft in an emergency situation.

              Surgical Repair Indications

              • AAA aneurysm diameter >5.5cm as identified by CT
              • AAA expansion rate of more than 1cm/year
              • Symptomatic Presentation (pain along the femoral nerve distribution, satiety, vein thrombosis, urinary obstruction)

              Quick Summary of Open Repair Procedure

              1. Identify level of lession
              2. Proximal clamp in place; in juxtarenal RAAA renal arteries are not to be spared
              3. Distal clamp in place
              4. Clean thrombi and/or emboli
              5. Graft is sewn into place while being heparinized
              6. Attached distally to aorta (straight graft) or common iliac arteries (bifurcated)
              7. Remove clamps
              8. Check for leakage
              9. Sac is closed around the graft

              Open repair

              The patient is anaesthetized and the surgeon carries out an open laparotomy. The aim is to quickly identify the level at which the leakage occurs and obtain control of the bleeding by clamping proximally and distally. Two clamps are placed proximal and distal to the level of the lesion, the later one placed to prevent atheroemboli.

               The graft is sewn into place proximally (aorta) and distally either in the aorta prior to the bifurcation or in the common iliac arteries. Heparin is rinsed on the graft for the entire duration before the final stitch.

              Quick Summary of Endovascular Procedure

              1. Exposure of femoral arteries
              2. Femoral arteries incised for approximately 1cm
              3. 3/4 of graft delivered from one side
              4. 1/4 of graft from contralateral side and fixed on the other graft
              5. Alignment checked by angiography during and after deployment via radioopaque markers located on the graft
              6. Angiography done to check for patency of renal arteries as occluding them would prove fatal
              7. Close the femoral artery incision site

              Endovascular Repair EVAR

              This method is used in cases where the patient is haemodynamically stable and a spiral CT with contrast has been taken in order to identify the suitable EVAR graft to be used. Moreover, EVAR is a good solution for RAAAs that are infrarenal and have an aneurysm neck 1-2cm below the renal arteries so that the graft can anchor safely and no leakage will occur.

              However, in a study done by Robert J. Hinchliffe et al. (Vascular. 2007;15(4):191-196) there was no difference in mortality between the group that underwent EVAR when compared with the open repair group, therefore there is no benefit for RAAA patients undergoing EVAR over open repair in this acute setting.



              Complications of AAA aneurysm open and EVAR repair

              OPEN REPAIR

              • Anastomotic Aneurysm
              • Graft occlusion
              • Aortoenteric fistula
              • Incisional hernia
              • Sexual dysfunction
              • Small bowel obstruction
              • Buttock claudication
              • Infection

              EVAR

              • Migration of the graft
              • Endoleak (leak in the aneurysm sac outside the stent graft which can still rupture)
              • Graft thrombosis
              • Pseudoaneurysm due to graft infection
              • Colon ischaemia and necrosis
              • Aortic dissection

              Conservative treatment

              Conservative treatment should be considered in patients who are not suitable for surgery either because they do not satisfy the indications or due to other contraindications (e.g. concomitant pathologies (CKD), old age etc) in order to allow for a succesful palliation scheme.

              Blood Pressure control schemes

              ACE inhibitor (captopril, enalapril)

              Calcium channel blocker (amlodipine, nifedipine)

              Diuretics (thiazide, spironolactone)

              If blood pressure is still high β-blockers should be considered along with other more advanced Imidazoline receptor antagonists etc.

              Lipid control

              Statins

              Thrombus prevention

              Aspirin

              Warfarin

              (low molecular weight heparin)

               

              References

              1. http://emedicine.medscape.com/article/1979501
              2. http://www.uptodate.com/contents/patient-information-abdominal-aortic-aneurysm
              3. Ruptured Abdominal Aortic Aneurysm: Endovascular Repair Does Not Confer Any Long-term Survival Advantage Over Open Repai. Robert J. Hinchliffe;  Bruce D. Braithwaite, Vascular.2007;15(4):191-196.
              4. http://www.geraldlawriemd.com/sections/aaa/JVS AAA Treatment Guidelines.pdf
              5. Brewster DC. et al. "Guidelines for the treatment of Abdominal Aortic Aneurysms. Report of the Subcommittee of the Joint Council of the American Association for Vascular Surgery and Society for Vascular Surgery. J. Vasc. Surg. 37 (5):1106-1117.

               

              Images

              1) www.aorticstents.com Accessed on 14/08/2012

              2) www.moondragon.org

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