Abdominal Aortic Aneurysm (AAA)

Introduction

Abdominal aortic aneurysm (AAA) may present to the Emergency Department (ED) in a variety of ways, including:

1. An incidental finding.
2. An acute expansion of the aneurysm.
3. A contained leak.
4. A shocked patient as a result of frank rupture:

Ruptured AAA is a catastrophic event associated with high mortality even despite intervention.

Rupture into the peritoneal cavity is usually rapidly fatal, whereas retroperitoneal rupture may transiently stabilise, providing a window of opportunity for lifesaving intervention.

5. An atypical presentation.

In the case of a suspected ruptured AAA, Emergency Departments should have an emergency CODE AAA activation system in place.

Terminology

An aneurysm is defined as a permanent localised or diffuse dilation of an artery at least 1.5 times its normal diameter, involving all three layers of the vessel wall.

The term ectasia is used when the dilation is less than 50%.

Aneurysm types

• Fusiform — entire circumference of the vessel wall is dilated.
• Saccular — only part of the circumference is involved.

Pseudoaneurysms (false aneurysms)

These are a localised collection of flowing blood that communicates with the arterial lumen but is contained only by the adventitia.

Epidemiology

AAA is rare before the age of 55 years. Thereafter, the incidence rises sharply with increasing age.

AAA affects approximately 4–7% of men and 1–2% of women over the age of 65 years.³

Pathophysiology

Causes

AAA can result from:

1. Degenerative changes in collagen and elastin with age.
2. Atherosclerotic vascular disease.

Rare causes:

3. Infective (mycotic, syphilitic).
4. Post-trauma.
5. Post-aortic dissection.
6. Inflammatory arteritis (e.g. PAN).
7. Genetic factors:
   • ~15% incidence among first-degree relatives of patients with AAA.
   • Inherited genetic disorders (Marfan’s syndrome, Ehlers-Danlos syndrome).

Risk Factors

Risk factors for AAA:

1. Advancing age.
2. Male gender.
3. Smoking.
4. Family history.
5. Atherosclerosis.
6. Hypertension.
7. Hypercholesterolaemia.
8. Presence of other vascular aneurysms.

Risk Factors for Rupture

The likelihood of rupture increases with:

1. Aneurysm diameter > 5.5 cm.
2. Rapid expansion (>0.5 cm over six months):

● Average rate of expansion: 0.3–0.4 cm per year.

3. Smoking.
4. Uncontrolled hypertension.

Absolute Risk of Rupture

AAA diameter (cm)	Risk of rupture (% per year)
3.0–3.9	Zero
4.0–4.9	1%
5.0–5.9	1–10%
6.0–6.9	10–22%
>7.0	30–50%

(Reprinted from Australian Family Physician Vol. 42, No. 6, June 2013.)

Location

• 90–95% of AAAs occur in the infra-renal segment.
• Extension into iliac arteries is common.
• Extension above the renal arteries is uncommon.
• Thoraco-abdominal aneurysms may involve visceral or renal vessels.

Clinical Features

AAA typically does not cause symptoms unless:

1. Expanding rapidly.
2. Large enough to compress surrounding structures.
3. Associated with inflammation (inflammatory or infected aneurysm).
4. Ruptured.

Clinical Scenarios in the ED

Incidentally Detected Aneurysms

AAA may be detected incidentally on clinical exam, ultrasound, or other imaging.

Clinically Detectable Aneurysm

• In thin patients, normal abdominal aortic pulsation may be felt in the epigastrium.
• Expansile pulsation suggests aneurysm:
  • Place fingers on either side of the mass; separation with systole indicates expansile nature.
• Clinical detection generally occurs at 4–5 cm, depending on body habitus and examiner experience.

Acute Expansion of the Aneurysm

• Presentation: acute back pain or epigastric pain.
• No overt rupture, but pain may indicate acute expansion or small retroperitoneal bleed.

Contained Leak

• Rupture contained within retroperitoneal space.
• Patient may appear relatively stable or may deteriorate.
• Pain often severe, typically in the back.

Shocked Patient with Frank Rupture

• Most dramatic presentation.
• Bleed may be retroperitoneal or intraperitoneal.
• Classic clinical triad:
  1. Pain — acute, severe, typically back pain.
  2. Hypovolaemic shock.
  3. Pulsatile abdominal mass — may be difficult to elicit if:
     • Hypotensive.
     • Obese.
     • Large retroperitoneal haematoma.
     • Voluntary guarding.

Atypical Presentations

1. Elderly with back pain:
   • Consider AAA; renal colic is uncommon in elderly.
2. Elderly with back pain radiating to legs:
   • Possible nerve root compression from expanding AAA.
3. Chronic back pain:
   • May be due to chronic contained rupture.
4. Known AAA with tender aneurysm:
   • May indicate acute expansion or contained leak.
5. Massive GIT bleeding in AAA patient:
   • Consider aorto-enteric fistula (especially with previous AAA graft).

Investigations

Blood Tests

1. FBE.
2. U&Es / glucose.
3. LFTs.
4. Coagulation profile.
5. VBGs / lactate.
6. Group and hold or crossmatch (if rupture suspected, activate massive transfusion protocol).

Plain Radiography

• Cannot definitively diagnose AAA rupture.
• May show mural calcification.
• Lateral projection more useful than AP.

ECG

• Routine in unwell patients.
• Should not delay definitive investigations in suspected rupture.

Ultrasound

• Excellent first-line investigation, especially in unstable patients.
• Can confirm AAA and detect free fluid.
• Cannot definitively confirm rupture but can inform surgical decision-making.
• Measurement: maximum AP diameter in transverse plane.

CT Angiogram

• Best investigation for stable patients.
• Defines size, extent, and presence of leakage.
• Requires thoracic inlet to mid-thigh scan.
• Do not delay in unstable patients if surgeon is prepared to operate based on clinical and ultrasound findings.

MRI

• Highly accurate for elective assessment.
• Not suitable in suspected rupture (scan time, availability, patient cooperation).

Management

Incidental Findings of AAA

• Refer all incidentally detected AAAs to vascular surgery (if suitable for surgery).
• Follow-up depends on size and symptoms:

Aortic Diameter	Action
<2.5 cm	None
2.5–2.9 cm	Repeat US in 5 years
3.0–3.9 cm	Repeat US in 12 months
4.0–4.4 cm	Repeat US in 6 months
4.5–4.9 cm	Repeat US in 3 months
≥5.0 cm	Urgent referral to vascular surgeon

Indications for Repair

• Size:
  • >5.5 cm in males.
  • >5.0 cm in females.
• Rapid growth:
  • >1.0 cm/year.
• Symptoms:
  • Abdominal/back pain or tenderness.
  • Distal embolisation.
• Patient suitability and preference.

Acute Expansion

• Any symptomatic aneurysm is an emergency.
• Urgent referral to vascular surgery.

Contained Leak

• Requires urgent repair.
• Timeframe may vary based on stability.

Shocked Patient (Frank Rupture)

ABC Measures

• 2 large-bore IV cannulae with pump sets.
• Activate massive transfusion protocol.
• Maintain consciousness and cerebral perfusion, not arbitrary BP targets.

Monitoring

• Continuous ECG, pulse oximetry, non-invasive BP.
• Invasive lines can be placed in theatre.

Analgesia

• IV opioids:
  • Fentanyl 25 mcg IV boluses preferred for cardiovascular stability.

Surgical Repair

Ruptured AAA is fatal unless treated surgically.

Endovascular Stent Grafting (EVAR)

• Preferred where feasible.
• Less invasive, lower mortality than open repair.
• Performed under local/regional anaesthesia.
• Requires lifelong surveillance:
  • Monitor for endoleak.

Open Repair

• For cases not amenable to EVAR.
• Elective mortality: <5%.
• Ruptured AAA mortality: 30–40%.
• Outcomes depend on age, comorbidities, shock severity, time to repair.

Disposition

• All AAA cases → refer to vascular surgery.
• Suspected rupture → activate CODE AAA to alert:
  • Vascular surgery.
  • Radiology.
  • ICU.
  • Anaesthetics/operating theatre.
  • Blood bank.

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References

FOAMed

• Nickson C. Abdominal aortic aneurysm (AAA). LITFL
• Hartung MP. Abdominal CT: aortic aneurysm. LITFL
• Lobo V. POCUS: Abdominal aorta. LITFL
• Lam L. CT Case 084. LITFL
• Nazerian P. Aortic Dissection Detection Risk Score (ADD-RS). MDCalc