Abdominal Aortic Aneurysm (AAA)

Definition

• Most common type of true arterial aneurysm

• Segmental, full-thickness dilation of the aorta exceeding 50% of its normal diameter

• Typically considered aneurysmal if the diameter is >3.0 cm

Aetiology/Causes

• Degenerative changes: Due to atherosclerosis

• Genetic predisposition: Family history of AAA

• Connective tissue disorders: Marfan syndrome, Ehlers-Danlos syndrome

• Infectious causes (rare): Mycotic aneurysms (e.g., bacterial infections)

Risk Factors

Modifiable:

• Smoking (current or past)

• Hypertension

• Hypercholesterolaemia

Non-modifiable:

• Age >65 years

• Male sex

• Family history of AAA or other arterial aneurysms (e.g., iliac, femoral)

• Caucasian ethnicity

Pathophysiology

• Progressive aortic wall weakening due to the degradation of elastin and collagen, often accelerated by risk factors like smoking and hypertension. This leads to vessel dilation and potential rupture.

Symptoms

Asymptomatic:

• Most AAAs are detected incidentally via imaging for other reasons.

Symptomatic but not ruptured:

• Vague abdominal, back, or flank pain

• Pulsatile abdominal mass

• Limb ischaemia (acute or chronic)

Ruptured:

• Severe abdominal or back pain, radiating to the groin or flank

• Hypotension or shock

• Pulsatile abdominal mass

• High mortality if not promptly treated (80–90%)

Differential Diagnosis

• Renal colic

• Diverticulitis

• Lumbar spine disease

• Ischaemic bowel disease

• Gastrointestinal haemorrhage

• Acute cholecystitis

• Myocardial infarction

Investigations

Imaging:

• Abdominal ultrasound: First-line for asymptomatic patients and surveillance.

• CT angiography: Preferred for symptomatic or suspected ruptured AAA to assess extent and planning for surgery.

• MRI: Used if CT is contraindicated or for further assessment of surrounding structures.

Surveillance intervals:

• Based on the size of the aneurysm:

  • 3.0–3.9 cm: Ultrasound every 24 months

  • 4.0–4.9 cm: Ultrasound every 12 months

  • 5.0–5.4 cm: Ultrasound every 6 months

  • >5.5 cm: Consider surgical repair

Management

Conservative:

• Surveillance: Regular ultrasound for smaller AAAs.

• Risk factor modification: Smoking cessation, blood pressure control, statins.

Surgical Intervention:

• Indications for surgical repair:

  • Male with AAA >5.5 cm

  • Female with AAA >5.0 cm

  • Rapid growth >1.0 cm/year

  • Symptomatic AAA (abdominal/back pain/tenderness, distal embolisation)

• Urgent/Emergency Surgery:

  • Indicated in ruptured AAA or symptomatic non-ruptured AAA.

• Surgical Approaches:

  • Open AAA repair: A graft is sewn into place through a large incision in the abdomen.

  • Endovascular Aneurysm Repair (EVAR): A minimally invasive technique using a graft delivered via the femoral arteries.

Complications

• Rupture: Catastrophic event with a high mortality rate (80-90%). 

• Thromboembolism: Can lead to limb ischaemia; Risk of distal embolisation.

• Post-repair complications: Endoleak, graft infection.

• Aortoenteric fistula: Rare but serious complication, especially after open surgical repair.

Prognosis

• Untreated rupture: Mortality approaches 90%.

• Elective repair: Lower mortality risk (<5%).

• Long-term outcomes: Dependent on comorbid conditions and repair success.

Notes:

• Screening recommendations vary; in Australia, targeted screening may apply to high-risk groups (e.g., males >65 years, smokers).

• Surveillance strategies differ between regions but are often based on diameter and growth rate.

• Lifestyle Modifications: Smoking cessation, managing hypertension and hyperlipidaemia are crucial in slowing the expansion of AAA.

• Regular Follow-Up: Essential for patients with known AAA, particularly as the aneurysm approaches the surgical threshold size.

• There is a high rate of misdiagnosis with ruptured AAA; symptoms may mimic renal colic, diverticulitis, or lumbar spine disease.

• No benefit in AAA screening for those at low risk (<10% on the Framingham Risk Score).