Key Facts

Abdominal aortic aneurysm (AAA) is a permanent dilatation of the abdominal aorta to a diameter of ≥3 cm. It most commonly affects the infrarenal aorta. The major risk is rupture, which is often fatal.

Epidemiology

• Prevalence: ~4-8% of men >65 years; ~1-2% of women >65 years
• Male:female ratio ~6:1
• UK NHS screening has reduced AAA-related mortality
• Declining prevalence due to reduced smoking rates

Aetiology

• Atherosclerosis: degenerative process; primary risk factor
• Smoking: strongest modifiable risk factor (odds ratio ~5-6)
• Hypertension: contributes to wall stress
• Family history: ~20% have first-degree relative with AAA
• Connective tissue disorders: Marfan, Ehlers-Danlos (more thoracic)
• Inflammatory: inflammatory AAA (~5%); periaortic fibrosis
• Infectious (mycotic): Salmonella, Staphylococcus, TB (rare)

Pathophysiology

• Degradation of aortic wall: loss of elastin and collagen by matrix metalloproteinases (MMPs)
• Chronic inflammation and oxidative stress in arterial wall
• Loss of smooth muscle cells
• Progressive dilatation → increased wall tension (Laplace's law: wall tension ∝ pressure × radius)
• Rupture occurs when wall stress exceeds wall strength
• Risk of rupture increases exponentially with diameter: ~1%/year at 4-5 cm; ~30-40%/year at >8 cm

Unruptured AAA

• Usually asymptomatic: discovered incidentally or on screening
• Pulsatile abdominal mass: expansile (hands pushed apart, not just pulsatile)
• Vague abdominal or back pain (suggests expansion or impending rupture)

Ruptured AAA

• Classic triad: sudden severe abdominal/back pain, hypotension/shock, pulsatile abdominal mass
• Often presents with cardiovascular collapse
• May mimic renal colic, MI, mesenteric ischaemia
• Retroperitoneal rupture: may be temporarily contained (patient may be transiently stable)
• Free intraperitoneal rupture: rapid exsanguination, usually fatal before reaching hospital

Red Flags

• New-onset abdominal/back pain in patient with known AAA
• Hypotension with pulsatile mass
• Rapid growth (>1 cm/year)
• Symptomatic AAA of any size

Bedside

• Abdominal examination: pulsatile, expansile mass above umbilicus
• Observations: HR, BP, peripheral pulses

Bloods

• FBC, U&Es, coagulation, crossmatch: essential for ruptured AAA (emergency surgery preparation)
• Lactate: elevated in ruptured AAA with hypoperfusion
• Group and save: all AAA patients

Imaging

• Abdominal ultrasound: first-line for screening and monitoring; highly accurate for diameter measurement
• CT angiography: gold standard for detailed anatomy, pre-operative planning, and ruptured AAA diagnosis
• MR angiography: alternative if CT contraindicated

NHS AAA Screening Programme

Non-pharmacological

• Smoking cessation: most important modifiable intervention (slows growth)
• Cardiovascular risk management: exercise, healthy diet
• Surveillance USS: as per screening programme intervals

Pharmacological

• Cardiovascular risk optimisation:
  • Statin (atorvastatin 80mg OD)
  • Antihypertensive therapy (target BP <140/90)
  • Antiplatelet (aspirin 75mg) if concurrent cardiovascular disease
• No pharmacological therapy has been proven to slow AAA growth (trials of doxycycline, ACEi, statins have been inconclusive)
• Ruptured AAA: activate massive transfusion protocol, permissive hypotension (SBP ~80-100 mmHg) to reduce further bleeding while maintaining perfusion

Surgical/Interventional

Elective repair indications:

• Diameter ≥5.5 cm (men) or ≥5.0 cm (women)
• Growth ≥1 cm/year
• Symptomatic AAA of any size

Repair options:

• EVAR (endovascular aneurysm repair): endograft placed via femoral arteries
  • Perioperative mortality: ~1-2%
  • Suitable anatomy required (~60-70% of elective cases)
  • Higher re-intervention rate (~10-20% over 5 years)
  • EVAR-1 trial: lower perioperative mortality vs open repair; no long-term survival advantage
  • EVAR-2 trial: EVAR in patients unfit for open repair; no survival benefit vs conservative management
• Open surgical repair: aortic replacement with synthetic graft (Dacron)
  • Perioperative mortality: ~5%
  • More durable; lower re-intervention rate
  • Higher perioperative morbidity

Ruptured AAA:

• Emergency surgery (open or EVAR if anatomy suitable and haemodynamically stable)
• Mortality: ~40-50% for those reaching hospital alive; ~80% overall

Referral Criteria

• AAA ≥5.5 cm: vascular surgery referral
• Rapid growth or symptomatic AAA: urgent referral
• Ruptured AAA: emergency transfer to vascular centre

• Elective repair mortality: EVAR ~1-2%; open ~5%
• Ruptured AAA: ~80% overall mortality (including pre-hospital deaths); ~40-50% for those undergoing emergency surgery
• 5-year survival post-elective repair: ~65-70% (limited by comorbidities)
• Small AAA (<5.5 cm): annual rupture risk ~1% for 4-5 cm; surveillance is safe
• Large AAA (>7 cm): annual rupture risk ~20-40%
• EVAR: higher re-intervention rate but lower perioperative risk
• Open repair: more durable long-term repair
• NHS screening programme: estimated to reduce AAA-related mortality by ~50% in screened men

Annual Rupture Risk by Diameter

EVAR vs Open Repair Comparison