Abdominal Aortic Aneurysm: Pathophysiology, Diagnosis, and Management
Pathophysiology
AAA results from chronic degenerative and atherosclerotic processes that cause structural deterioration of the aortic wall, leading to progressive dilation of the infrarenal aorta to ≥3.0 cm diameter. 1
- The disease process involves thinning of the media and adventitia due to loss of vascular smooth muscle cells and degradation of the extracellular matrix, which weakens the aortic wall structure 2
- Most AAAs are fusiform in morphology and located in the infrarenal abdominal aorta, representing advanced atherosclerotic changes occurring over years 1
- The weakened wall cannot withstand normal blood pressure mechanical stress, leading to progressive expansion and eventual rupture risk when wall strength is exceeded 2
- Smoking is the most strongly associated modifiable risk factor, along with male sex, age >60-65 years, Caucasian race, hypertension, and positive family history 3, 2
Diagnosis
Ultrasound is the primary screening and surveillance modality for AAA, while CT angiography is the gold standard for preoperative planning when repair thresholds are reached. 4, 5
Initial Diagnostic Approach
- Physical examination detects only 50% of AAAs, but can identify aneurysms in thin patients (abdominal girth <100 cm) when specifically directed toward AAA detection 6
- Most AAAs are asymptomatic and discovered incidentally during imaging for other indications 1, 3
- When AAA is suspected clinically (pulsatile abdominal mass), ultrasound is the appropriate initial imaging study with 100% specificity and positive predictive value 4, 6
Imaging Modalities
- Ultrasound: Standard for screening and surveillance, measuring maximum aortic diameter with high accuracy 4
- CT angiography: Gold standard for preoperative planning, providing superior visualization of aortic anatomy, branch vessels, and precise measurements using 3D multiplanar reformations perpendicular to the vessel centerline 4, 5
- MR angiography: Reasonable alternative to CT when contrast is contraindicated or to reduce radiation exposure, with non-contrast techniques allowing accurate diameter determination 4, 5
Measurement Standards
- Maximum aortic diameter should be measured perpendicular to the longitudinal axis of the aorta using 3D multiplanar reformatted images to avoid overestimating diameter in tortuous vessels 4, 7
- AAA is defined as infrarenal aortic diameter ≥3.0 cm or >1.5 times the adjacent normal segment 4, 1, 3
Management
Surveillance Strategy
Surveillance intervals are determined by aneurysm size and sex, with more frequent imaging as diameter approaches surgical thresholds. 4
Size-Based Surveillance Protocol:
- 3.0-3.9 cm: Ultrasound every 3 years 4
- 4.0-4.9 cm (men) or 4.0-4.4 cm (women): Ultrasound annually 4
- ≥5.0 cm (men) or ≥4.5 cm (women): Ultrasound every 6 months 4
- Patients who smoke or have diabetes: Consider shorter intervals due to increased growth risk 4
When Ultrasound is Inadequate:
- CT surveillance is recommended when ultrasound cannot adequately define the aneurysm 4
- MRI is reasonable when CT is contraindicated or to reduce cumulative radiation exposure 4
Indications for Repair
Elective repair should be performed when AAA diameter reaches ≥5.5 cm in men or ≥5.0 cm in women to prevent rupture. 1
Additional repair indications include:
- Symptomatic AAA (abdominal or back pain suggesting impending rupture) 3
- Rapid growth >5 mm in 6 months 3
- Ruptured AAA (surgical emergency with 65-85% mortality) 2
Repair Modalities
- Endovascular aortic repair (EVAR): Less invasive but requires specific anatomic criteria including proximal neck length ≥10-15 mm, neck diameter <30 mm, and adequate iliac access 7
- Open surgical repair: More invasive initially but more durable long-term 8
Post-Repair Surveillance
After EVAR, surveillance imaging at 1 month and 12 months is recommended, with the 6-month interval eliminated if 1-month imaging shows no concerning findings. 4
- Duplex ultrasound is 95% accurate for measuring sac diameter and 100% specific for detecting type I and III endoleaks 4
- CT or MRI every 5 years is reasonable after EVAR when duplex ultrasound is used for routine surveillance 4
- Additional cross-sectional imaging is reasonable when duplex shows abnormal findings (endoleak, sac enlargement, stent migration) 4
- After open repair: CT or MRI within 1 year postoperatively, then every 5 years to detect para-anastomotic aneurysms (incidence 1% at 5 years, 27-35% at 15 years) 4
Critical Considerations
- Up to 27% of AAA patients may have concomitant thoracic aneurysms, requiring comprehensive aortic evaluation 1
- The 10-year risk of death from other cardiovascular causes may be 15 times higher than aorta-related death, emphasizing the importance of managing cardiovascular risk factors 1
- CT is required for preoperative planning when repair threshold is reached to confirm diameters and detail anatomy 4