What are the Australian guideline recommendations for defining, screening, surveillance intervals, medical management, and surgical thresholds of abdominal aortic aneurysm (AAA) for the AMC clinical exam?

Abdominal Aortic Aneurysm (AAA) for AMC Clinical Exam

Definition

An abdominal aortic aneurysm is defined as a focal dilatation of the abdominal aorta ≥30 mm in diameter, representing at least a 50% increase from the normal aortic diameter. 1, 2

• The normal infrarenal abdominal aorta measures up to 20 mm in anteroposterior diameter 1, 2
• The threshold is approximately 10% smaller in women than in men 1, 2
• Most AAAs are infrarenal and fusiform in shape, often lined with laminated thrombi 1
• AAA extends to the common iliac arteries in 25% of cases 1

Risk Factors

Major risk factors include smoking (strongest modifiable risk), age >65 years, male sex (4:1 male-to-female ratio), and family history of aneurysmal disease. 1, 2

• Other significant risk factors: hypertension, hypercholesterolemia, peripheral artery disease, coronary artery disease 1
• Diabetes is paradoxically associated with decreased AAA risk and slower growth rates 1
• Risk factors for rupture specifically: large diameter, rapid growth, female sex, smoking, hypertension, COPD 1

Screening Recommendations (Australian Context)

One-time ultrasound screening should be offered to men aged 65-75 years who have ever smoked. 3, 4

• Men aged 65-75 years who have never smoked may be offered selective screening if other risk factors present (family history, coronary artery disease, peripheral artery disease) 3, 4
• Women aged 65-75 years with smoking history or family history of AAA: insufficient evidence for routine screening 3
• Women without smoking history and no family history: screening not recommended (harms outweigh benefits) 3
• Ultrasound is the preferred screening modality with 95% sensitivity and near 100% specificity 2, 5

Surveillance Intervals

Surveillance intervals are determined by maximum AAA diameter, with different thresholds for men and women due to women's four-fold higher rupture risk. 1, 2

For Men:

• 25-29 mm: Every 4 years 1
• 30-39 mm: Every 3 years 1, 2
• 40-44 mm: Every 12 months 1, 2
• 45-54 mm: Every 6 months 1, 2
• ≥55 mm: Consider intervention 1, 2

For Women:

• 30-39 mm: Every 3 years 1
• 40-44 mm: Every 12 months 1
• 45-49 mm: Every 6 months 1
• ≥50 mm: Consider intervention 1, 2

Imaging Modality for Surveillance:

• Duplex ultrasound is the standard surveillance technique 1, 2, 5
• CT angiography or cardiovascular MRI should be used if ultrasound does not allow adequate measurement 1, 5
• CT angiography may be helpful before continued surveillance to characterize saccular morphology, which increases rupture risk even below surgical thresholds 1, 2

Indications for Shortened Surveillance Intervals

Increase surveillance frequency if rapid growth is detected: ≥10 mm per year or ≥5 mm per 6 months. 1, 2

• Rapid expansion warrants consideration of earlier surgical intervention regardless of absolute diameter 1, 2, 5
• Growth rates >2 mm per year are associated with increased adverse events 2

Medical Management

All patients with AAA require aggressive cardiovascular risk factor modification, with smoking cessation being the single most important intervention. 2, 5, 6

Essential Medical Interventions:

• Smoking cessation counseling and pharmacotherapy (strongest modifiable risk factor for expansion and rupture) 2, 5
• Optimal blood pressure control (hypertension accelerates aneurysm growth) 2, 5
• Statin therapy for cardiovascular risk reduction (all patients with AAA have atherosclerotic disease) 2, 5
• Screen for other vascular disease (coronary artery disease, peripheral artery disease) as patients with AAA have significantly impaired survival 2, 5
• Avoid fluoroquinolones unless absolutely necessary (may increase AAA growth) 6

Surgical Thresholds

Elective repair is indicated when AAA reaches ≥55 mm in men or ≥50 mm in women. 1, 2, 5

Additional Indications for Intervention:

• Rapid expansion: >10 mm per year or >5 mm per 6 months 1, 2, 5
• Symptomatic AAA (abdominal or back pain) 1, 7
• Saccular morphology (higher rupture risk even at smaller diameters) 1, 2

Rupture Risk by Size:

• 5.5-5.9 cm: 9% annual rupture risk 2, 5
• 6.0-6.9 cm: 10% annual rupture risk 5
• ≥7.0 cm: 33% annual rupture risk 5

Surgical Options

Open surgical repair is the primary treatment for patients who are good or average surgical candidates, while endovascular aneurysm repair (EVAR) is reasonable for high-risk patients with significant cardiopulmonary comorbidities. 2

• Open repair has approximately 4.2% operative mortality 5
• EVAR may be considered in low or average surgical risk patients, though long-term outcomes continue to be evaluated 2
• Pre-intervention imaging with CT angiography or MR angiography is required to plan surgical approach 2, 5

Ruptured AAA Presentation

Ruptured AAA is a medical emergency presenting with hypotension, shooting abdominal or back pain, and a pulsatile abdominal mass, with mortality rates of 75-90%. 1, 4

• Contained rupture may present with atypical low flank or abdominal pain 1
• Up to 5% of sudden deaths in the United States are caused by AAA rupture 1
• Emergent surgical intervention is mandatory but carries high operative mortality 4

Critical Pitfalls to Avoid

Ensure consistent measurement technique across surveillance studies, using measurements perpendicular to the longitudinal axis of the aorta. 2

• Document the same measurement method consistently (inner-to-inner, outer-to-outer, or leading-to-leading edge) 2
• Women have four-fold higher rupture risk than men at equivalent AAA sizes, which is why surgical thresholds are lower (50 mm vs 55 mm) 1, 2
• Do not delay scheduled surveillance imaging, as AAAs can expand unpredictably 5
• Physical examination alone has poor sensitivity for detecting AAA (especially in obese patients) and cannot reliably detect changes in size 1, 5
• Consider screening first-degree relatives, especially siblings, due to genetic component 5