What is the significance of the hernia to neck ratio in abdominal aortic aneurysm (AAA) management, specifically for endovascular aneurysm repair (EVAR)?

Significance of Hernia to Neck Ratio in Abdominal Aortic Aneurysm Management for EVAR

The hernia to neck ratio is a critical anatomical measurement for determining EVAR suitability, with unfavorable neck anatomy being the most frequent cause of exclusion from EVAR in over 50% of AAA patients. 1

Anatomical Significance of the Proximal Neck

The proximal neck is defined as the segment of aorta between the most caudal renal artery and the proximal boundary of the aneurysm. Its characteristics are paramount when evaluating AAA for endovascular repair:

• Neck length requirements: >10-15 mm is required for adequate proximal graft seal 1
• Neck diameter limitations: <30 mm is necessary for conventional EVAR 1
• Neck angulation: Severe angulation (>60 degrees) is associated with higher failure rates 2

The hernia to neck ratio specifically refers to the relationship between the diameter of the aneurysm sac (hernia) and the diameter of the proximal neck. A high ratio indicates a significant discrepancy between these measurements, which can affect:

1. Stent graft stability
2. Risk of endoleak development
3. Long-term durability of repair

Anatomical Suitability for EVAR

Research shows that anatomical suitability for EVAR varies significantly:

• Only 49% of patients with ruptured AAAs are candidates for EVAR with conventional devices 2
• Only 10% of patients have arterial anatomy suitable for bifurcated graft implantation 3
• 55% of patients may be suitable for aortouni-iliac graft implantation 3

Clinical Implications of Neck Anatomy

Endoleak Risk

Unfavorable neck anatomy significantly increases the risk of type I endoleak and stent graft migration 1. Endoleaks are the most common complication of EVAR, occurring in up to one-third of procedures 1.

Mortality and Morbidity Considerations

While EVAR offers lower short-term mortality compared to open surgical repair (1.4% vs 4.2%), this benefit does not persist in the intermediate and long-term follow-up 4. The hernia to neck ratio directly impacts:

• Technical success rates
• Need for adjunctive procedures
• Long-term reintervention rates (significantly higher in EVAR than open repair) 4

Surveillance Requirements

Due to the high rate of post-procedural complications (16-30% for AAA EVAR), lifelong imaging surveillance is mandatory 5. CT angiography remains the gold standard for surveillance 1.

Decision Algorithm for EVAR Based on Neck Anatomy

1. Measure key anatomical parameters:

   • Neck length (must be >10-15 mm)
   • Neck diameter (must be <30 mm)
   • Neck angulation (should be <60 degrees)
   • Presence of calcification/thrombus (>90% circumference increases risk)

2. Calculate hernia to neck ratio:

   • Higher ratios indicate greater hemodynamic stress on the proximal seal
   • May require special consideration for device selection

3. Determine EVAR suitability:

   • If all anatomical criteria are favorable → Standard EVAR
   • If neck length inadequate → Consider fenestrated EVAR (FEVAR) 1
   • If neck anatomy unfavorable → Consider open repair or advanced techniques like Transrenal EVAR 6

Pitfalls and Caveats

1. Overestimating neck length: Accurate measurement from the lowest renal artery is critical
2. Underestimating angulation: Severe angulation increases the risk of seal failure
3. Ignoring thrombus/calcification: These factors affect seal quality and increase endoleak risk
4. Focusing only on diameter: The entire neck morphology must be considered
5. Neglecting surveillance: Lifelong monitoring is essential due to the higher reintervention rate with EVAR 1, 5

For patients with unfavorable neck anatomy who are not candidates for open repair, newer techniques like Transrenal EVAR may offer acceptable outcomes comparable to chimney EVAR, particularly in urgent cases 6.