Dartmouth researchers find new imaging method may lower risks for abdominal aortic aneurysms

Results of a study by researchers at Dartmouth-Hitchcock Medical Center (DHMC) and Dartmouth Thayer School of Engineering could have implications for choosing which patients with abdominal aortic aneurysms should have surgery and which ones should simply have follow-up with noninvasive studies.

In an article published in the April issue of the Journal of Vascular Surgery, Dr. Mark Fillinger and colleagues describe a new noninvasive method for evaluating abdominal aortic aneurysms (AAA). They found that the new method – examining aneurysm wall stress – predicts AAA rupture risk better than aneurysm diameter, which has been used to predict rupture risk for over 40 years.

The multidisciplinary study was sponsored by the National Institutes of Health and the National Heart Lung and Blood Institute. In the study, conducted at Dartmouth-Hitchcock Medical Center, over 100 patients who had computed tomography scans (CT scans) during the course of routine care had AAA “wall stress analysis”. The CT scan is processed through a series of computer programs, including an engineering process called finite element analysis. Finite element analysis breaks the structure into thousands of tiny elements so a computer can calculate the wall stress using the three-dimensional shape of the AAA (from the CT scan), the patient’s blood pressure, and the tissue properties of typical AAAs. The result is a computer-generated “stress map” that displays the aneurysm wall stress (the force trying to pull the aneurysm apart and cause rupture).

The patients in the study were generally under observation for their aneurysm for one of three reasons: 1) because the aneurysm was felt to be too small to repair, 2) because the risks of repair were felt to be too high compared to the risk of aneurysm rupture, or 3) because the patient decided not to have aneurysm repair. The outcomes of observation were then compared based on the standard method of determining rupture risk (maximum AAA diameter) versus the maximum stress within the aneurysm wall. A prior study by this group had determined that wall stress was high at the time of rupture, but this was the first large study of AAA wall stress in patients under extended periods of observation.

Researchers found that the new technique (aneurysm wall stress analysis) predicted the risk of rupture better than maximum AAA diameter, with a 25 fold increase in rupture risk for patients with high AAA wall stress, and only 9 fold higher rupture risk for patients with large diameter (over 5.5 cm). The location of rupture (when known) was also consistent with the location of maximum stress predicted by the computerized stress map. Interestingly, some patients with small aneurysms had high wall stress (high risk of rupture) and some patients with large aneurysms had low wall stress (low risk of rupture).

“This study has several important aspects. Some patients with small aneurysms (based on diameter) have an unexpectedly high risk of rupture and should have surgery earlier than is typically recommended” says Dr Fillinger. “Other patients with larger aneurysms, but high risks for surgery, may delay surgery and have observation with noninvasive studies if we can reliably predict that they have a low rupture risk. Another impact of this study may be that blood pressure control in aneurysm patients will be examined with more scrutiny, since blood pressure plays a key role in AAA wall stress. We have made great strides with minimally invasive surgery for aortic aneurysms, but not everyone is a candidate for these techniques. AAA wall stress analysis may be able to prevent rupture in some patients and prevent unnecessary surgery in others.”

The risk of rupture relative to AAA diameter and female gender (known risks for aneurysm rupture) were consistent with recent large clinical trials in the US and UK. In those studies, patients with AAAs under close observation with ultrasound every 6 months still had a small risk of rupture, with higher risks for women. “The consistency of our current study with the US and UK small aneurysm trials is encouraging. In future work, we hope to determine why females are at greater risk for rupture than males. We would like to improve the aneurysm tissue model, and hope to start a larger multicenter study within the next year.”

It is estimated that 2 million people in the US have abdominal aortic aneurysms, and AAA rupture is the thirteenth leading cause of death in men. Many people are unaware they have aortic aneurysms because they cause no symptoms prior to rupture. The increasing incidence of AAA nationwide has lead to more attention regarding screening for AAA, including a recent feature in the Wall Street Journal. Numerous famous people have died of AAA rupture, including Albert Einstein, Lucille Ball, and George C Scott. Senator Bob Dole and Rodney Dangerfield are among celebrities who have recently had AAA repair to prevent rupture.

Dartmouth-Hitchcock Medical Center (DHMC) is an integrated academic medical center located on a 225-acre campus in the heart of New Hampshire’s Upper Connecticut River Valley in Lebanon. DHMC comprises Mary Hitchcock Memorial Hospital, the Dartmouth-Hitchcock Clinic, Dartmouth Medical School and the Veterans Affairs Medical Center in White River Junction, VT.

For more information contact Tamsin Stubbs at (603) 653-1997.

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