Abdominal Aortic Aneurysm
Definition
- Abdominal aortic aneurysm (AAA) is the localized dilation of the abdominal aorta beyond 1.5 times its normal diameter, typically ≥3 cm.
- It is the most common type of large vessel aneurysm, affecting ~2% of the population, with 95% occurring below the renal arteries.
- Risk of rupture increases with aneurysm size (>5.5 cm in men, >5.0 cm in women).
- Many remain asymptomatic until rupture occurs, which is a surgical emergency.
Classification
True vs. False Aneurysm
- True aneurysm: Involves all three layers of the arterial wall (intima, media, adventitia).
- False aneurysm (pseudoaneurysm): Blood collection between layers, typically due to trauma or surgery.
Morphological Types
- Fusiform (most common): Uniform dilatation of the artery.
- Saccular: Localized outpouching.
Etiology
A. Degenerative Causes (Most Common)
- Atherosclerosis (primary cause in 95% of cases).
- Pathogenesis: Chronic inflammation → elastin/collagen degradation → arterial wall weakening.
- Risk factors:
- Smoking (strongest modifiable risk factor).
- Hypertension (increases wall stress).
- Hyperlipidemia.
- Male sex (4-6x more common than in women).
- Age >65 years.
- Family history (genetic predisposition).
B. Infectious Causes ("Mycotic Aneurysms")
- Bacterial infections
- Organisms: Salmonella, Staphylococcus aureus, Syphilis, Tuberculosis.
- Mechanism: Hematogenous spread or direct invasion → inflammatory destruction of the aortic wall.
C. Connective Tissue Disorders
- Marfan's syndrome (FBN1 gene mutation).
- Ehlers-Danlos Syndrome (Type IV, vascular type).
- Loeys-Dietz Syndrome (TGF-β pathway mutation).
D. Inflammatory Causes
- Takayasu’s Arteritis, Giant Cell Arteritis → Chronic vessel wall inflammation leading to aneurysm formation.
E. Traumatic Causes
- Blunt or penetrating trauma → Can cause pseudoaneurysm rather than true AAA.