For an adult with an iliac artery aneurysm and no connective‑tissue disease or severe hypertension, how often should imaging be performed to monitor its size based on the aneurysm diameter?

Surveillance Intervals for Iliac Artery Aneurysms

For iliac artery aneurysms <3.5 cm in diameter, annual imaging surveillance with duplex ultrasound is recommended, while aneurysms ≥3.5 cm should proceed to elective repair rather than continued surveillance.

Size-Based Surveillance Strategy

Aneurysms <3.0 cm

• Annual duplex ultrasound surveillance is appropriate for common iliac artery aneurysms measuring 2.0-3.0 cm, as rupture at these sizes is exceedingly rare and growth rates average 2.9 mm/year 1, 2.
• In one large series of 438 patients, no iliac aneurysm ≤3.8 cm ruptured during an average 3.7-year follow-up period 1, 2.
• Some centers have safely observed aneurysms of 2.0-2.5 cm without any documented enlargement over 57 months of follow-up 3.

Aneurysms 3.0-3.5 cm

• Surveillance every 6-12 months with duplex ultrasound is reasonable for this size range, as these aneurysms approach the repair threshold 1, 2.
• The growth rate accelerates as diameter increases, with hypertensive patients showing faster expansion (0.32 vs 0.14 cm/year) 2.
• This size range requires closer monitoring because the 3.5 cm threshold for repair is evidence-based on rupture risk 1.

Aneurysms ≥3.5 cm

• Elective repair is recommended rather than continued surveillance once diameter reaches 3.5 cm, as this threshold balances procedural risk against rupture risk 1.
• The median diameter of ruptured iliac aneurysms is 6.8 cm, but ruptures have occurred at diameters as small as 3.8 cm 1.
• The repair threshold of 3.5 cm is supported by large registry data showing rare ruptures below 4 cm diameter 1.

Imaging Modality Selection

First-Line Imaging

• Duplex ultrasound is the preferred modality for routine surveillance due to its non-invasive nature, lack of radiation exposure, and cost-effectiveness 1, 4.
• Ultrasound has demonstrated consistent measurement accuracy that approximates CT and MRI for iliac artery assessment 1.

Alternative Imaging

• CT angiography should be used when ultrasound visualization is inadequate or when planning intervention, as it provides superior anatomic detail of the iliac arteries and their branches 1, 4.
• MRI is a reasonable alternative to CT in selected patients to reduce cumulative radiation exposure, particularly in younger patients requiring long-term surveillance 4.

Special Surveillance Considerations

Rapid Growth

• Shorten surveillance intervals to 3-6 months if the aneurysm demonstrates rapid expansion (≥5 mm per 6 months or ≥10 mm per year), as intervention may be warranted regardless of absolute diameter 4.
• Hypertensive patients require particular attention as they demonstrate significantly faster expansion rates 2.

Concomitant AAA

• When iliac aneurysms coexist with abdominal aortic aneurysms (present in 20-40% of AAA patients), surveillance must monitor both segments 1.
• Common iliac arteries >16 mm in AAA patients are more likely to expand (81% vs 53%) and warrant closer monitoring 5.
• In 86% of iliac aneurysm patients, a concomitant AAA is present or has been previously treated 2.

Critical Pitfalls to Avoid

• Do not delay repair once the 3.5 cm threshold is reached, as rupture risk increases substantially and median rupture diameter is 6.8 cm 1.
• Do not rely solely on diameter - symptomatic iliac aneurysms (abdominal/back pain, pulsatile mass) require immediate surgical evaluation regardless of size 1.
• Do not overlook bilateral disease - 65% of patients with iliac aneurysms have bilateral involvement requiring surveillance of both sides 2.
• Avoid fluoroquinolones in patients with iliac aneurysms unless absolutely necessary, as these antibiotics may accelerate aneurysm growth 4, 6.