Aneurysms and Arteriovenous Fistulas

ANEURYSMS AND ARTERIOVENOUS FISTULAS

Aneurysms are congenital or acquired weakness of the vessel wall media, resulting ina localized dilatation or outpouching. Complications include thrombus formation, thromboembolism, and compression of nearby structures. Rupture or dissection may cause sudden death.

Atherosclerotic aneurysms are due to weakening of the media secondary to ather-oma formation, and typically occur in the abdominal aorta below the renal arteries. They are associated with hypertension. Half of aortic aneurysms >6 cm in diameter will rupture within 10 years. Those >5 cm are treated surgically.

Syphilitic aneurysms involve the ascending aorta in tertiary syphilis (late stage).Syphilitic (luetic) aortitis causes an obliterative endarteritis of the vasa vasorum, leading to ischemia and smooth-muscle atrophy of the aortic media. Syphilitic aneu-rysms may dilate the aortic valve ring, causing aortic insufficiency.

Aortic dissecting aneurysm occurs when blood from the vessel lumen enters an inti-mal tear and dissects through the layers of the media. The etiology usually involves degeneration (cystic medial degeneration) of the tunica media. Aortic dissecting aneurysm presents with severe tearing pain. The dissecting aneurysm may compress and obstruct the aortic branches (e.g., renal or coronary arteries). Hypertension and Marfan syndrome are predisposing factors.

Berry aneurysm is a congenital aneurysm of the circle of Willis.

Microaneurysms are small aneurysms commonly seen in hypertension and diabetes.

Mycotic aneurysms are aneurysms usually due to bacterial or fungal infections.

Arteriovenous (AV) fistulas are a direct communication between a vein and anartery without an intervening capillary bed. They may be congenital or acquired (e.g., trauma). Potential complications include shunting of blood which may lead to high-output heart failure and risk of rupture and hemorrhage.