What is an Abdominal Aortic Aneurysm?
An abdominal aortic aneurysm (AAA) is defined as a dilated aorta and is diagnosed when the aortic diameter exceeds 3.0 cm. The most common location for a AAA is between the division where the aorta supplies blood to the kidneys and where it supplies blood to the pelvis and legs. When an abdominal aortic aneurysm occurs, it weakens the walls of the artery and can rupture or leak, causing bleeding into the abdomen. When this occurs, it is a very serious medical emergency requiring immediate medical attention.
Most AAAs are small when identified, either by intentional screening or as an incidental finding from a radiologic exam (e.g., ultrasound, computed tomography, magnetic resonance) done for other reasons. A noticeable pulsation near the navel may not be observed by the patient, but as a known symptom of AAAs, will often be detected by healthcare providers during routine physical examination. Approximately 30 percent of asymptomatic AAAs are discovered in this manner. Undiagnosed AAAs are often asymptomatic until the development of rupture, which is generally acute and often fatal. AAAs are a leading cause mortality and morbidity but often go undiagnosed until late stage or become symptomatic.
People who have a history of smoking are three to five times more likely to develop an abdominal aortic aneurysm, and four to five times more often in men than women. Accordingly, AAA Screening is recommended for all smokers, especially older men, as the risk is directly related to number of years smoking. The U.S. Preventive Services Task Force recommends that men aged 65–75 years who have ever smoked should get an ultrasound screening for abdominal aortic aneurysms, even if they have no symptoms. People with other medical conditions, such as coronary heart disease and peripheral vascular disease, or a known family history of AAAs, are also advised to receive screening. Targeted screening based on history of smoking has been found to detect 89% of prevalent AAAs.
Why screening and surveillance is important
Approximately 10,000 deaths occur each year in the United States due to AAAs, usually because of rupture. In the United States, ruptured AAA is estimated to cause 4 to 5 percent of sudden deaths. Once rupture has occurred, the success rate of surgery is much lower than if surgery is performed electively, prior to rupture. The goal of a screening and surveillance program is to locate, diagnose, track and treat the aneurysm before rupture.
The risk of rupture of small aneurysms (smaller than 4.0 centimeters) is much lower than the risk of rupture of large aneurysms (larger than 5.0 centimeters). Most patients have little warning before a AAA ruptures. Of those patients that experience a AAA rupture, 50% reach the hospital alive and 30-50% of those who reach the hospital don’t make it through surgery.
Incidental AAAs are aneurysms identified when the abdomen is imaged for other reasons, such as colorectal cancer screening. Incidental findings of positive AAAs are not well documented or followed up with. When incidental AAAs are identified, they may not be as effectively surveilled as those detected in a structured screening program. Within a cohort of 191 patients with incidental AAA found by ultrasound, magnetic resonance imaging, or CT, and a median observation time of 4.4 years, 29.3 percent of subjects had no follow-up imaging of their aneurysm,174 and only 26 percent of those who were inpatients had discharge summaries mentioning the finding. Another retrospective study found that there was no documentation in the electronic medical record of primary care physicians being made aware of patients with AAA diagnosis within 3 months of a CT scan for 61.4 percent of 83 incidental AAAs patients identified.175 Using a multivariate regression model, it was determined that subjects with incidental AAAs not receiving radiological monitoring for 1 year were significantly more likely to die compared with subjects receiving recommended radiological monitoring.
How to build a program
Serial AAA surveillance has now been deemed beneficial for over 10 years. Although several screening methods exist, ultrasonography is accepted as the standard screening imaging method for AAA because it has a high sensitivity and specificity. Ultrasonography is noninvasive, can be conducted at a low cost, and avoids radiation exposure. This test is painless and involves the use of a wand, which is applied to the abdomen and uses high-frequency sound waves to create an image of the abdominal aorta. The screening is conducted while the participant lays on their back and the technician uses ultrasound to take images and measurements of the abdominal aorta. It is recommended that screening AAA ultrasounds be performed by a registered diagnostic medical sonographer with vascular expertise who will take measurements of the abdominal aorta to look for any abnormalities that might require further examination. The American Institute of Ultrasound in Medicine (AIUM) offers detailed guidelines on the proper performance and reporting of AAA screening ultrasound exams. For reporting AAA screening ultrasounds, the AIUM recommends that exams be classified as “positive” (infrarenal AAA present), “negative” (infrarenal AAA absent), or indeterminate (partial or inadequate abdominal aortic visualization).
Most people will need AAA screening every three years, but some, depending on risk factors and earlier screenings, may need it annually. The recommended frequency of surveillance ultrasounds depends on the size of the aneurysm; very small AAAs may only require surveillance every few years, while larger ones need to be monitored yearly or even more frequently. A one-time screen is sufficient for a population-based screening program with regard to initial negative scans and development of large AAAs. Screening reduces the incidence of AAA ruptures, and decreases rates of emergency surgical repair for AAA and AAA-attributable mortality.
- van Walraven C, Wong J, Morant K, et al. The influence of incidental abdominal aortic aneurysm monitoring on patient outcomes. J Vasc Surg. 2011;54(5):1290–7. [PubMed]
- Gordon JR, Wahls T, Carlos RC, et al. Failure to recognize newly identified aortic dilations in a health care system with an advanced electronic medical record. Ann Intern Med. 2009;151(1):21–7. [PubMed]
