## Histopathology of Atherosclerotic AAA **Key Point:** Atherosclerotic abdominal aortic aneurysms are characterized by **fragmentation and loss of elastic fibers** in the media, accompanied by **chronic inflammatory infiltration** (lymphocytes, macrophages, and occasional giant cells). ### Pathological Features | Feature | Atherosclerotic AAA | Cystic Medial Necrosis | |---------|-------------------|----------------------| | **Primary site** | Media and adventitia | Media | | **Elastic fiber loss** | Fragmentation, loss | Smooth loss | | **Inflammation** | Prominent chronic | Minimal or absent | | **Associated condition** | Atherosclerosis | Marfan, Ehlers-Danlos | | **Age of onset** | >50 years | Younger, syndromic | ### Mechanism 1. Atherosclerotic plaques erode into the media 2. Inflammatory mediators (MMP-2, MMP-9) degrade elastic and collagen fibers 3. Loss of structural integrity → progressive dilation 4. Chronic inflammation perpetuates matrix breakdown **High-Yield:** The **chronic inflammatory infiltrate** distinguishes atherosclerotic AAA from cystic medial necrosis, which is a degenerative (non-inflammatory) process. **Clinical Pearl:** Atherosclerotic AAA typically occurs in the infrarenal aorta (95% of cases), below the renal arteries, where hemodynamic stress and atherosclerotic burden are greatest. [cite:Robbins 10e Ch 10]
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