Aortic Aneurysm: A High-Yield USMLE Review

An aortic aneurysm is a pathologic dilation of an intact aortic wall, most commonly involving the infrarenal abdominal aorta (AAA). Clinical decision-making revolves around weighing the natural-history risk of rupture against the procedural mortality of repair. Rupture is a catastrophic surgical emergency with extremely high mortality, making surveillance and elective repair at defined size thresholds critical exam concepts.

Presentation

• Frequently asymptomatic and discovered incidentally on imaging obtained for another reason (e.g., CT for nephrolithiasis) or as a pulsatile abdominal mass on exam
• Ruptured AAA presents with the classic triad of hypotension, sudden severe abdominal and back (or flank) pain, and a pulsatile abdominal mass
• Rupture is accompanied by hemodynamic instability (e.g., BP 78/50) — a surgical emergency
• Common in older patients, particularly men with a history of smoking, hypertension, and atherosclerosis
• Thoracic aortic aneurysms may be associated with connective tissue disease (e.g., Marfan syndrome) or, historically, tertiary syphilis (ascending aorta)

Diagnosis

• Ultrasound — first-line for screening, surveillance, and measuring aneurysm diameter over time (e.g., serial measurements showing enlargement from 4.9 cm to 5.2 cm)
• CT angiography — detects and sizes the aneurysm and defines anatomy for repair planning; however, in a clinically obvious ruptured AAA with an unstable patient, do NOT delay for CT — proceed directly to the operating room
• Clinical diagnosis of rupture is made on the triad of hypotension, abdominal/back pain, and pulsatile mass in an unstable patient

Management

• One-time screening ultrasound is recommended for men aged 65–75 who have ever smoked
• Surveillance for aneurysms below the size threshold; elective repair is recommended for AAA ≥5.5 cm or for rapid expansion, weighing rupture risk against procedural mortality
• Risk-factor modification — smoking cessation and blood pressure control — to slow aneurysm growth
• Elective repair is either open surgical repair or endovascular aneurysm repair (EVAR), chosen based on anatomy and surgeon preference/availability
• Ruptured AAA requires emergent surgical repair — large-bore IV access, type and cross for massive transfusion, permissive hypotension (avoid excessive fluids that increase bleeding), and emergent vascular surgery consultation

High-yield

• Ruptured AAA triad = hypotension + abdominal/back pain + pulsatile abdominal mass — go straight to the OR if unstable, do not wait for CT
• Elective repair threshold is AAA ≥5.5 cm; below that, surveillance ultrasound
• Screen men 65–75 who have ever smoked with a one-time abdominal ultrasound
• Even with surgery, ruptured AAA mortality is very high — the rationale for screening and elective repair
• Fluoroquinolones are associated with an increased risk of aortic aneurysm and rupture
• The intervention decision is about rupture risk crossing the procedural mortality threshold — not the mere presence of an aneurysm

Pitfalls

• Do NOT delay for CT imaging in a hemodynamically unstable patient with a clinically obvious ruptured AAA — proceed directly to emergent surgery
• Avoid aggressive fluid resuscitation in ruptured AAA; excessive fluids raise blood pressure and can increase bleeding (permissive hypotension is preferred)
• Treating the size threshold as an automatic rule ignores the individualized weighing of rupture risk against operative risk and comorbidities
• Do not confuse an aneurysm (dilation of an intact aortic wall) with an aortic dissection (an intimal tear creating a false lumen) — they are distinct entities with different workup and management