What is the risk of abdominal aortic aneurysm (AAA) rupture based on aneurysm size?

AAA Rupture Risk by Size

The risk of abdominal aortic aneurysm (AAA) rupture increases exponentially with diameter, with significant risk at ≥5.5 cm in men and ≥5.0 cm in women, which is why repair is recommended at these thresholds. 1

Risk of Rupture Based on AAA Size

AAA size is the most important predictor of rupture risk. Current guidelines provide clear thresholds for both surveillance and intervention based on diameter:

Annual Rupture Risk by Size:

• <3.0 cm: Minimal risk (<0.5%)
• 3.0-3.9 cm: <1% per year
• 4.0-4.9 cm: 1-3% per year
• 5.0-5.9 cm: 3-15% per year
• 6.0-6.9 cm: 10-20% per year
• ≥7.0 cm: >20% per year 1

Women have approximately a four-fold higher rupture risk at the same diameter compared to men, which is why intervention thresholds are lower for women 1.

Surveillance Recommendations Based on Size

The 2024 ESC guidelines provide a clear surveillance algorithm based on AAA diameter 1:

• 2.5-2.9 cm: Every 4 years
• 3.0-3.9 cm: Every 3 years
• 4.0-4.9 cm:
  • Men: Every 12 months
  • Women: Every 12 months (for 4.0-4.4 cm) and every 6 months (for 4.5-4.9 cm)
• 5.0-5.4 cm:
  • Men: Every 6 months
  • Women: Consider intervention
• ≥5.5 cm:
  • Men: Consider intervention
  • Women: Consider intervention

Intervention Thresholds

The 2022 ACC/AHA guidelines recommend 1:

1. Repair for unruptured AAA with maximal diameter ≥5.5 cm in men or ≥5.0 cm in women (Class 1, Level A)
2. Repair for symptomatic AAAs regardless of size (Class 1, Level B-NR)
3. Consider repair for saccular AAAs (Class 2b, Level C-LD)
4. Consider repair for AAAs with rapid growth (≥0.5 cm in 6 months) (Class 2b, Level C-LD)

Growth Rates and Additional Risk Factors

The average AAA growth rate is approximately 3 mm per year (range 1-6 mm) 1. Factors that increase growth rate and rupture risk include:

• Continued smoking
• Hypertension
• Female sex
• Family history of AAA
• Presence of inflammation
• Saccular morphology
• Rapid growth (≥10 mm per year or ≥5 mm per 6 months)

Interestingly, diabetes is associated with decreased risk and slower growth rates 1.

Clinical Implications

The risk of rupture must be balanced against the risk of intervention. For patients unfit for surgery with large AAAs, the actual rupture rates may be lower than historically reported - approximately 5.3% per year for AAAs >5.5 cm 2.

Common Pitfalls

1. Underestimating risk in women: Women have higher rupture risk at smaller diameters
2. Ignoring growth rate: Rapid expansion (≥0.5 cm in 6 months) warrants consideration for earlier repair
3. Overlooking saccular morphology: Saccular aneurysms may rupture at smaller diameters than fusiform ones
4. Relying solely on diameter: While diameter is the primary metric, other factors like family history, smoking status, and hypertension should be considered

Duplex ultrasound is the recommended surveillance modality, with CT or MRI recommended when ultrasound cannot adequately measure AAA diameter 1.