Aneurisma Aorta Abdominalis (AAA)

01Pathophysiology

The abdominal aorta is the most common site for aortic aneurysm formation, with the overwhelming majority occurring in the infrarenal segment. This location is vulnerable because of several factors: it has fewer vasa vasorum (the small vessels that supply the aortic wall itself), it is subjected to greater hemodynamic stress from the aortic bifurcation, and it lacks the structural support provided by thoracic skeletal and muscular attachments.

The core mechanism involves chronic inflammation and enzymatic degradation of the tunica media. Infiltrating macrophages and lymphocytes release matrix metalloproteinases (MMPs), particularly MMP-2 and MMP-9, which break down elastin and collagen in the aortic wall. Simultaneously, smooth muscle cell apoptosis reduces the wall's structural integrity. Over time, the weakened wall cannot withstand normal arterial pressure and begins to dilate.

Once dilation begins, Laplace's law becomes the driving force behind progressive expansion. The law states:

$$T=\frac{P\times r}{w}T\; =\; \backslash frac\{P\; \backslash times\; r\}\{w\}$$

where $TT$ is wall tension, $PP$ is intraluminal pressure, $rr$ is the vessel radius, and $ww$ is wall thickness. As the aneurysm diameter increases, wall tension rises proportionally, which promotes further dilation and thinning. This creates a vicious cycle that explains why larger aneurysms grow faster and are at significantly higher risk of rupture.

This pathophysiology directly explains the clinical picture. Most patients are asymptomatic because the aneurysm expands slowly without compressing surrounding structures or disrupting blood flow. The pulsatile abdominal mass on exam is simply a dilated, pulsating aorta that is now large enough to be palpable. Pain occurs when the aneurysm stretches the wall rapidly (impending rupture) or erodes into adjacent structures. Rupture causes abrupt hemorrhage, typically into the retroperitoneal space, producing sudden severe flank or back pain with hemodynamic collapse.

Atherosclerosis is strongly associated with AAA and shares overlapping risk factors, but current understanding views it as a contributing process rather than the sole cause. The disease is better classified as a degenerative and inflammatory condition of the aortic wall.

02Classification and Clinical Manifestation

Classification by Diameter

Clinical Manifestation by Presentation Type

03Diagnostic Workup

Abdominal ultrasound is the best initial test for AAA. It is noninvasive, widely available, has no radiation, and has a sensitivity and specificity both approaching 95 to 100% for detecting aneurysms. Ultrasound is used in two key contexts: (1) screening and (2) serial surveillance of known small aneurysms. According to the USPSTF, one-time screening ultrasound is recommended for men aged 65 to 75 who have ever smoked. This is one of the most commonly tested screening recommendations on the exam. For women and non-smokers, screening is not routinely recommended unless they have other significant risk factors.

When AAA is confirmed or when surgical repair is being considered, CT angiography (CTA) is the most accurate test and the gold standard for preoperative planning. CTA provides precise measurement of the aneurysm diameter, delineates its craniocaudal extent, identifies the relationship to the renal arteries and iliac bifurcation, and detects complications such as mural thrombus, retroperitoneal hemorrhage, or contained rupture. On the exam, if you are asked for the "next step" after identifying a large AAA on ultrasound and the patient is a surgical candidate, the answer is CTA.

A critical exam principle: In a patient presenting with the classic triad of rupture (sudden abdominal/back pain, pulsatile mass, hypotension) who is hemodynamically unstable, do not delay for imaging. The next best step is emergent surgical repair. Imaging (CTA) is appropriate only in hemodynamically stable patients where rupture is suspected but the diagnosis is uncertain.

04Management and Treatment

Conservative management is indicated for all asymptomatic AAAs below the surgical threshold. The cornerstones are:

• Smoking cessation: This is the single most important intervention. Smoking is the strongest modifiable risk factor for both development and progression of AAA. Continued smoking accelerates aneurysm growth.

• Blood pressure control: Target blood pressure below 130/80 mmHg. ACE inhibitors or ARBs are commonly used. There is no strong evidence that beta-blockers slow AAA growth despite earlier enthusiasm for propranolol.

• Statin therapy: Statins reduce cardiovascular morbidity and mortality in AAA patients, who almost universally have coexisting atherosclerotic disease. Some observational data suggest statins may also slow aneurysm growth.

• Surveillance ultrasound: Intervals are determined by aneurysm size as outlined in the classification table above.

Surgical repair is indicated when the aneurysm reaches the size threshold ($\ge \backslash geq$ 5.5 cm in men, $\ge \backslash geq$ 5.0 cm in women), expands rapidly (> 0.5 cm over 6 months or > 1.0 cm per year), or becomes symptomatic.

Endovascular aneurysm repair (EVAR) is the preferred method for elective repair when the patient's anatomy is suitable. EVAR involves percutaneous placement of a stent graft through the femoral arteries under fluoroscopic guidance. Compared to open repair, EVAR has lower perioperative mortality (approximately 1 to 2% vs. 4 to 5%), shorter hospital stays, and faster recovery. However, EVAR requires lifelong surveillance with serial CT imaging to monitor for endoleaks, which are persistent blood flow into the aneurysm sac around or through the graft. Endoleaks are the most important long-term complication of EVAR.

Open surgical repair involves laparotomy with direct clamping of the aorta and replacement of the aneurysmal segment with a synthetic graft (typically Dacron). Open repair is indicated when the anatomy is not suitable for EVAR (e.g., short or angulated infrarenal neck, iliac artery disease) or when the patient is young and can tolerate a more durable repair without the need for lifelong graft surveillance.

Ruptured AAA is a surgical emergency with an overall mortality rate of 80 to 90% (including prehospital deaths). For patients who reach the hospital alive, the management priorities are:

1. Activate the surgical team immediately.

2. Establish large-bore IV access and initiate resuscitation. Permissive hypotension (target systolic BP around 70 to 80 mmHg) is recommended to avoid disrupting any retroperitoneal tamponade.

3. Do not delay for imaging in the unstable patient. Proceed directly to the operating room.

4. Type and crossmatch blood; initiate massive transfusion protocol as needed.

A notable pharmacological pearl for the exam: fluoroquinolones (ciprofloxacin, levofloxacin) have been associated with an increased risk of aortic aneurysm and dissection through collagen degradation. This is increasingly tested as a drug safety question.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students get AAA questions wrong involves failing to recognize the urgency hierarchy. When a vignette describes an elderly male smoker with sudden severe abdominal or back pain and hypotension, students sometimes choose "order a CT scan" because CTA is the most accurate test. This is a trap. In an unstable patient with the classic triad of rupture, the correct answer is emergent surgical repair, not imaging. Imaging is only appropriate when the patient is hemodynamically stable and the diagnosis is uncertain.

A second common trap involves the screening recommendation. Students often over-apply screening to all elderly patients. The USPSTF recommends one-time ultrasound screening only for men aged 65 to 75 who have ever smoked. This is not extended to all men, all women, or all patients with hypertension.

Another frequently tested concept is the size threshold for elective repair. Students may confuse the threshold or fail to recognize that rapid growth (> 0.5 cm in 6 months) is an independent indication for surgery regardless of the absolute diameter. A 4.5 cm aneurysm that was 4.0 cm six months ago requires surgical consultation, not continued surveillance.

Finally, be aware of post-EVAR surveillance. After endovascular repair, patients require lifelong follow-up CT imaging to detect endoleaks. If a vignette describes a patient with a history of EVAR who now has an enlarging aneurysm sac, the answer relates to identifying and managing an endoleak, not assuming the repair has failed entirely.

The core competency being tested is the ability to risk-stratify patients based on aneurysm size and symptoms, choose the correct diagnostic modality based on hemodynamic stability, and distinguish AAA from other causes of acute abdominal or back pain in a vascular-risk patient.