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Abdominal aortic aneurysm

An aortic aneurysm is a general term for any swelling (dilatation or aneurysm) of the aorta, usually representing an underlying weakness in the wall of the aorta at that location. While the stretched vessel may occasionally cause discomfort, it is the risk of rupture causing severe pain, massive internal hemorrhage and, without prompt treatment, resulting in a quick death. In addition the aneurysm may split (Aortic dissection) which may block vessels that branch off from the aorta or release blood clots (emboli) causing blockage to blood-flow elsewhere. more...

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Pathology

The physical change in the aortic diameter can occur secondary to an intrinsic defect in the protein construction of the aortic wall, trauma, infection, or due to progressive destruction of aortic proteins by enzymes. The last is the most common cause of aneurysmal disease although the origin of this enzymatic destruction is not known.

Signs, symptoms and diagnosis

  • Most intact aortic aneurysms do not produce any symptoms. Untreated, aneurysms tend to become progressively larger, although the rate of enlargement is unpredictable for any individual. Rarely, clotted blood which lines most aortic aneurysms can break off and result in an embolus. They may be found on physical examination. Medical imaging is necessary to confirm the diagnosis.

Abdominal Aortic Aneurysm

Aortic aneurysms are more common in the abdominal aorta, one reason for this is that elastin, the principle load bearing protein present in the wall of the aorta, is reduced in the abdominal aorta as compared to the thoracic aorta (nearer the heart). Most are true aneurysms that involve all three layers (tunica intima, tunica media and tunica adventitia), and are are generally asymptomatic before rupture.

The prevalence of AAAs increases with age, with an average age of 65-70 at the time of diagnosis. AAAs have been attributed to atherosclerosis, though other factors are involved in their formation.

An AAA may remain asymptomatic indefinitely. There is a large risk of rupture once the size has reached 5 cm, though some AAAs may swell to over 15 cm in diameter before rupturing. Before rupture, an AAA may present as a large, pulsatile mass above the umbilicus. A bruit may be heard from the turbulent flow in a severe atherosclerotic aneurysm or if thombosis occurs. Unfortunately, however, rupture is usually the first hint of AAA. Once an aneurysm has ruptured, it presents with a classic pain-hypotension-mass triad. The pain is classically reported in the abdomen, back or flank. It is usually acute, severe and constant, and may radiate through the abdomen to the back.

The diagnosis of an abdominal aortic aneurysm can be confirmed at the bedside by the use of ultrasound. Rupture could be indicated by the presence of free fluid in potential abdominal spaces, such as Morrison's pouch, the splenorenal space, subdiaphragmatic spaces and peri-vesical spaces. A contrast-enchanced abdominal CT scan is needed for confirmation.

Only 10-25% of patients survive rupture due to large pre- and post-operative mortality. Annual mortality from ruptured abdominal aneurysms in the United States alone is about 15 000. Another important complication of AAA is formation of a thrombus in the aneurysm.

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Risk Factors for Rupture of Abdominal Aortic Aneurysm
From American Family Physician, 2/1/00 by Richard Sadovsky

Rupture of an abdominal aortic aneurysm has high morbidity and mortality rates. Many patients die before reaching the operating room. Surgeons often electively repair abdominal aortic aneurysms that measure 4 to 5.5 cm in diameter even though the long-term survival benefit of early elective surgery is uncertain. The decision is made more difficult because of uncertainty about the rupture risk of abdominal aortic aneurysms of different sizes. Abdominal aortic aneurysm diameter is probably a determinant of rupture, but measurements are poorly reproducible. The rupture rate of abdominal aortic aneurysms that measure less than 5 cm in diameter vary widely, from zero to 1 percent per year to as high as 6 percent per year. Modeling studies have suggested that the rupture risk for abdominal aortic aneurysms measuring 6.5 and 7.5 cm in diameter is 9 percent and 12.5 percent per year, respectively. Knowledge of other risk factors would help management decisions.

The U.K. Small Aneurysm Trial and the additional follow-up of patients who were ineligible or who refused randomization provided a large cohort for evaluating factors that influence the rupture risk of abdominal aortic aneurysms. Patients who had abdominal aortic aneurysms that measured 4 to 5.5 cm in diameter were randomized to undergo serial ultrasound surveillance or surgery. Patients who did not meet the enrollment qualifications were also followed. The primary end point of the study was rupture of the abdominal aortic aneurysm.

Among the 1,090 randomized patients, 25 abdominal aortic aneurysm ruptures occurred. In the 1,167 nonrandomized patients, abdominal aortic aneurysm rupture occurred in 78 patients. Of the 103 ruptures, 26 patients died without ever reaching the hospital, 53 patients died in the hospital without undergoing surgery, 13 patients died within 30 days of surgery and 11 patients died more than 30 days following surgery. Factors significantly and independently associated with abdominal aortic aneurysm rupture included initial aneurysm diameter (much higher risk of rupture in patients when the diameter of the abdominal aortic aneurysm measured greater than 6 cm), female gender, higher mean blood pressure reading and current smoker as measured by baseline plasma cotinine level. Although women typically have smaller abdominal aortic aneurysm measurements than men (mean diameter at rupture: 5 versus 6 cm), a surprising finding of this study was a rate of aneurysm rupture three times higher in women than in men. This might suggest that the ratio of infrarenal/ suprarenal diameter is an important determinant of abdominal aortic aneurysm stability or rupture, although suprarenal diameters are not reproducible by ultrasonography. This study sheds new light on the risk factors associated with the rupture of abdominal aortic aneurysms, particularly smaller ones.

The authors conclude that patients with abdominal aortic aneurysm who are at marginal fitness levels, refuse surgery or are awaiting surgery should maintain adequate control of blood pressure and receive counseling and support to stop smoking. Although small abdominal aortic aneurysms do rupture, the risk of rupture of an abdominal aortic aneurysm that measures smaller than 5 cm in diameter is notably low; the risk of rupture of abdominal aortic aneurysms that measure 5 to 5.9 cm is also low but escalates sharply for abdominal aortic aneurysms that measure 6 cm or more in diameter. Different thresholds for abdominal aortic aneurysm repair should apply to women than to men because of the greater tendency for smaller aneurysms to rupture.

Brown LC, Powell, JT. Risk factors for aneurysm rupture in patients kept under ultrasound surveillance. The U.K. Small Aneurysm Trial Participants. Ann Surg September 1999;230:289-97.

COPYRIGHT 2000 American Academy of Family Physicians
COPYRIGHT 2000 Gale Group

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