Aneurysms and Aortic Dissection

1. Aneurysm — definition & types

A localized abnormal dilation (out-pouching) of a vessel or the heart; congenital or acquired.

• True aneurysm: involves all 3 wall layers (intima, media, adventitia) or the attenuated heart wall.
• False / pseudoaneurysm: a breach of the wall → extravascular hematoma that freely communicates with the lumen (“pulsating hematoma”).
• Shapes: saccular (focal out-pouching, often containing thrombus) vs fusiform (circumferential dilation).

2. Causes

Aneurysms arise when connective-tissue structure/function is compromised: inadequate/abnormal CT synthesis, excessive CT degradation, loss of VSMCs / phenotype change → cystic medial degeneration. Contributing: atherosclerosis, hypertension, trauma, congenital defects, infection (syphilis) — all weaken the wall.

3. Abdominal aortic aneurysm (AAA)

• Morphology: saccular or fusiform, typically below the renal arteries and above the aortic bifurcation; usually with extensive atherosclerosis ± mural thrombus.
• Subtypes: inflammatory AAA (dense periaortic fibrosis + infiltrate); mycotic AAA (atherosclerotic lesion infected by circulating microorganisms).
• Pathogenesis: atherosclerosis (main) → ECM degeneration, plaque compresses media → medial degeneration/necrosis → wall thinning → dilation; genetic factors alter collagen balance.
• Clinical: rupture → retroperitoneal hemorrhage; compression of adjacent structures (ureter, vertebrae); secondary thrombosis → ischemia of kidney/lower limb/GI.

4. Syphilitic (luetic) aneurysm

Tertiary syphilis (T. pallidum) → endarteritis of the vasa vasorum of the thoracic aorta (ascending aorta + arch) → ischemic injury of media → inflammation/scarring → dilation. Clinical: aortic ring dilation → valvular insufficiency → LV hypertrophy, mediastinal compression (respiratory problems), rupture → hemorrhage.

5. Berry aneurysm

Saccular aneurysm at branch points of the circle of Willis. Causes: congenital medial defect at branch points, hypertension in middle age, possible collagen disorder. Wall is bright/translucent with hyalinized intima. Rupture → subarachnoid hemorrhage → ↑ ICP; treated by coiling or clipping.

6. Aortic dissection

• Definition: intimal tear → blood enters the media → blood-filled false channel.
• Who: ages 40–50 with hypertension, or younger patients with CT disorders (Marfan).
• Pathogenesis: atherosclerotic intimal injury; hypertension impairing the vasa vasorum; CT disorders (abnormal ECM).
• Morphology: intimal tear usually in ascending aorta within ~10 cm of the valve; propagates antegrade/retrograde, can reach iliac/femoral arteries; may rupture through adventitia (massive hemorrhage / cardiac tamponade) or re-enter the lumen (second tear). Histology: cystic medial necrosis/degeneration, fragmented elastic fibers.

7. Dissection classification

Type	Extent	DeBakey
Type A (proximal — most common & dangerous)	Involves ascending aorta (any origin)	I = ascending + descending; II = ascending only
Type B (distal)	Does not involve ascending aorta	III = begins distal to left subclavian artery

Consequences: severe MI-like pain; rupture into pericardial/pleural/peritoneal cavity → hemopericardium/tamponade, hemothorax, right-sided HF; disruption of the valve apparatus; false channel ± thrombosis. Dx: angiography, CT, MR.

💡 High-yield: True aneurysm = all 3 layers; false = wall breach with contained hematoma. AAA = atherosclerotic, infrarenal, rupture → retroperitoneal bleed. Syphilitic = vasa vasorum endarteritis of the ascending/arch thoracic aorta. Berry = circle of Willis → SAH. Dissection = blood in the media (HTN or Marfan/cystic medial necrosis); Type A (ascending) is the dangerous one and can cause tamponade.