What is an infrarenal abdominal aortic aneurysm?

Infrarenal Abdominal Aortic Aneurysm

An infrarenal abdominal aortic aneurysm is a localized dilatation of the abdominal aorta below the renal arteries, defined as having a diameter of 3 cm or greater, which represents at least a 50% increase from the normal aortic diameter. 1, 2

Definition and Anatomy

• An infrarenal abdominal aortic aneurysm (AAA) specifically occurs in the segment of the abdominal aorta that is distal to (below) the renal arteries 2, 3
• The normal dimension of the infrarenal abdominal aorta is up to 2 cm in anteroposterior diameter 1
• The aorta is considered aneurysmal when it reaches ≥3 cm in diameter, or ectatic if between 2 and 3 cm in diameter 1
• The threshold for defining an aneurysm is approximately 50% larger than the expected normal diameter of that arterial segment 1
• The threshold for aneurysm is about 10% smaller in women than in men 1

Epidemiology and Risk Factors

• AAAs are most common in men over 65 years of age 2
• Major risk factors include:
  • Smoking (strongest modifiable risk factor) 2, 4
  • Male gender 2, 4
  • Advanced age 2
  • Positive family history 2
  • Hypertension 4
• Interestingly, despite higher prevalence of hypertension, African-American men have 39% lower prevalence of AAA compared to white men 4

Pathophysiology

• AAA results from structural changes in the aortic wall including:
  • Thinning of the media and adventitia 2
  • Loss of vascular smooth muscle cells 2
  • Degradation of extracellular matrix 2
• Inflammation, rather than atherosclerosis alone, may be essential to AAA development 4
• The mechanical stress of blood pressure on the weakened wall can eventually lead to rupture if wall strength is exceeded 2

Clinical Presentation

• Most AAAs are asymptomatic until rupture occurs 2, 3
• A pulsatile abdominal mass may be detected on physical examination, alerting clinicians to the possible presence of an AAA 1
• However, pulsatile abdominal masses can also be caused by:
  • Tortuous abdominal aorta 1
  • Transmitted pulsations from the aorta to a nonvascular mass 1

Diagnosis

• Ultrasound is the first-line imaging modality for suspected AAA (rated 9/9 for appropriateness) 1
• CT angiography (CTA) with IV contrast is highly appropriate (rated 8/9) for comprehensive evaluation 1
• MR angiography without and with IV contrast is also highly appropriate (rated 8/9) 1
• Imaging studies help determine:
  • Size of the aneurysm 1
  • Involvement of abdominal branches (visceral and parietal) 1
  • Extent of aneurysm (infrarenal aorta; infrarenal aorta and iliac artery; isolated iliac artery; or juxtarenal, suprarenal, or thoracoabdominal aorta) 1

Natural History and Risk of Rupture

• The natural history of AAA consists of progressive expansion and potential rupture 1
• The strongest predictor of AAA rupture is the diameter 4
• Risk of rupture is independently associated with:
  • Female gender 4
  • Large initial aneurysm diameter 4
  • Current smoking 4
  • Higher mean blood pressure 4
• Rupture results in life-threatening intra-abdominal hemorrhage with mortality rates of 65-85% 2

Management and Surveillance

• For smaller AAAs, periodic surveillance is recommended at intervals based on maximum size 1:
  • Every 6 months for 4.5 to 5.4 cm diameter
  • Every 12 months for 3.5 to 4.4 cm diameter
  • Every 3 years for 3.0 to 3.4 cm diameter
  • Every 5 years for 2.6 to 2.9 cm diameter
• Elective repair is considered for AAAs ≥5.5 cm in diameter in men 1, 4
• For women, a lower threshold between 4.5 cm and 5.0 cm is recommended for elective repair 4
• Treatment options include:
  • Open surgical repair (OSR) 5
  • Endovascular aneurysm repair (EVAR) - currently the primary treatment method due to improved short-term morbidity and mortality outcomes 5

Importance in Endovascular Planning

• For infrarenal AAAs, the proximal neck (segment of aorta between the most caudal renal artery and the proximal boundary of the aneurysm) is crucial for EVAR planning 1
• Conventional EVAR requires a neck size of >10 to 15 mm in length and <30 mm in diameter to provide adequate proximal graft seal 1
• Anatomical variations, such as an infrarenal origin of the superior mesenteric artery, can significantly impact EVAR planning 6

Screening

• Population-based ultrasound screening has proven cost-effective for men >65 years of age 1
• Screening is particularly important for those with risk factors including history of hypertension, smoking, three-vessel coronary artery disease, and a first-degree male relative with AAA 1