Aneurysms

An aneurysm is a localized abnormal dilation of a blood vessel or the wall of the heart.

Classification of aortic aneurysms

Aneurysms can be classified by macroscopic shape and size.

• Saccular aneurysms are essentially spherical (involving only a portion of the vessel wall) and vary in size from 5 to 20 cm in diameter, often partially or completely filled by thrombus.
  
• Fusiform aneurysms involve a long segment and vary in diameter (up to 20 cm) and length; many involve the entire ascending and transverse portions of the aortic arch, whereas others may involve large segments of the abdominal aorta or even the iliacs.
  

The shape of an aneurysm is not specific for any disease or clinical manifestations.

Causes of aortic aneurysms

• atherosclerosis - causes arterial wall thinning through medial destruction secondary to plaque that originates in the intima.
• cystic degeneration of the arterial media
  
• trauma (traumatic aneurysms or arteriovenous aneurysms)
• congenital defects such as those causing berry aneurysms (in the brain)
  
• infection resulting in mycotic aneurysms,
• systemic diseases e.g. vasculitides

Abdominal aortic aneurysms

• Usually positioned below the renal arteries and above the bifurcation of the aorta.
• Saccular or fusiform, sometimes up to 15 cm in greatest diameter and of variable length (up to 25 cm).
  
• The aneurysm and the nearby aorta often contain atheromatous ulcers covered by granular mural thrombi, prime sites for the formation of atheroemboli that may lodge in the vessels of the kidneys or lower extremities.
• AAAs rarely develop before age 50 and are more common in men.
  
• There is a genetic susceptibility to AAA beyond the genetic predisposition to atherosclerosis or HT.
  

Aneurysm growth

Most aneurysms expand at a rate of 0.2 to 0.3 cm/year, but 20% expand more rapidly. The most important clinical factor affecting aneurysm growth is blood pressure.

Clinical consequences of AAAs

• Rupture into the peritoneal cavity or retroperitoneal tissues with massive, potentially fatal, hemorrhage.
• The risk of rupture is directly related to the size of the aneurysm.
• Risk varies from zero for a small AAA (less than approximately 4 cm in diameter), to 1% per year for aneurysms measuring 4.0 to 4.9 cm indiameter, 11% per year for aneurysms between 5.0 and 5.9 cm in diameter, and 25% per year for those larger than 6.0 cm.

• Obstruction of a vessel, particularly of the iliac, renal, mesenteric, or vertebral branches that supply the spinal cord leading to ischemic tissue injury
• Embolism from atheroma or mural thrombus
• Impingement on an adjacent structure, such as compression of a ureter or erosion of vertebrae
• Presentation as an abdominal mass (often palpably pulsating) that simulates a tumo

Management Large aneurysms are managed aggressively; operative mortality for unruptured aneurysms is approximately 5%, whereas emergency surgery after rupture carries a mortality rate of more than 50%.

The treatment of abdominal and thoracic aortic aneurysms is evolving toward endoluminal approaches using stent grafts (expandable wire frames covered by a cloth sleeve) rather than surgery for some patients.