Inside The Exam Room™ 11/14/2006
ABDOMINAL AORTIC ANEURYSMS Part 2
By Mark Ombrellaro, MD
Abdominal aortic aneurysms (AAA) are a major public health concern. The overall incidence of aneurysm disease is rising independent of the fact that there are a growing number of elderly individuals at risk for the disease. Aneurysms are found in approximately 9% of men over age 65 in the US and have been found to run in families. A family history of aneurymal disease is noted in about 20% of individuals with AAA and there is thought to be a genetic mechanism that predisposes one to develop the condition. Abdominal aortic aneurysms have also been associated with long-term cigarette use, emphysema, and hypertension. Over the last 2 decades, there has been much research into identifying the actual underlying cause of aneurysm formation. Research has linked aneurysm formation with inflammation of the arterial wall tissue, an increased content of proteins capable of breaking down the structural components of the artery itself, and weakening of the arterial wall tissue. The elasticity of normal arterial tissue is related to the amount of the protein elastin contained within the arterial wall while its tensile strength is attributable to its collagen content. As with all living tissue, these components undergo a continuous cycle of break down and renewal. Rather than being thought of as an all or nothing process, I prefer to think of aneurysm formation as a disease state where the breakdown processes tend to predominate over those that ” build up” the arterial wall. The result is destruction, or an overall net loss, of the elastic component of the arterial wall which characterizes aneurysmal disease.
First degree relatives of a patient with an AAA are at risk for having AAA themselves. In one study by Webster (J Vasc Surg 13: 9-14,1991) 25% of male siblings of patients with AAA were also found to have aneurysms. The relative increased risk for aneurysmal disease in other family members was 4% for parents and 23% for sisters. The natural history of aneurysmal disease is that over time, most will increase in size until they rupture. Since expansion and rupture rates are extremely unpredictable, it is aneurysm size that physicians use as the most significant factor in assessing risk from rupture. The average growth rate of an aneurysm is between 2-4 mm per year. Annual rupture risk increases exponentially with arterial size and is <3% for aneurysms below 4 cm and approximately 8-10% for 5 cm aneurysms. Operative repair of abdominal aortic aneurysms is indicated when the risk of rupture exceeds the risk of repair. In general, the mortality rate for elective aneurysm repair is less than 5%. Risk is obviously increased in patients with other significant health problems such as advanced age, lung disease, renal (kidney) insufficiency, and active heart disease. In patients with ruptured aneurysms, over 90% of patients die before reaching health care and for those who make it to the ER alive, about 50% will die after a successful operative repair. This high mortality rate is due to the impact of nearly bleeding to death prior to a very complex operative procedure. These patients have not had the benefit of being able to optimize their heart, lung, and kidney function before their operation and often succumb to failure of these other organ systems. In order to best balance risk and benefit between rupture and operative repair, most surgeons recommend repair for AAA 5cm or greater in diameter. Since untreated aneurysms never shrink or regress, younger patients who are in good health and considered low operative risk may be offered elective aneurysm repair for aneurysm size between 4-5 cm on an individualized basis. This is predicated on the assumption that the operating surgeon can successfully repair the aneurysm with an expected mortality rate of 2-3% in these circumstances. Occasionally circumstances will arise where undertaking repair of an AAA is not appropriate. It is thought that patients with life expectancies of less than 2 years from other unrelated causes, advanced malignancies, or very poor quality of life (mentally debilitated or incapacitated) may not be suitable candidates for AAA repair. Again, these decisions should be individualized and consider the overall risks and benefits of undergoing treatment versus allowing the natural history of the disease process to evolve on its own.