Researchers propose new method for predicting AAA growth, rupture

Using multimodality imaging to assess Fluorine-18-sodium fluoride (18F-NaF) activity could help clinicians better predict whether abdominal aortic aneurysms (AAAs) are likely to grow more rapidly and eventually rupture, researchers reported in the Journal of the American College of Cardiology.

The small, proof-of-concept study is the first to make this connection, according to lead author Rachael O. Forsythe, MD, with the British Heart Foundation Centre for Cardiovascular Science, and colleagues.

“In this prospective series of clinical studies, we have demonstrated for the first time that 18F-NaF uptake is specifically increased in AAA and relates to areas of advanced aneurysmal disease,” they wrote. “Moreover, 18F-NaF uptake is a major predictor of aneurysm expansion and clinical outcome that is additive to standard clinical risk factors, including aneurysm diameter. This is the first study to demonstrate that an imaging biomarker of disease activity can add to the risk prediction of AAA and to suggest that this approach might refine clinical decisions regarding the need for surgery and improve patient outcomes.”

AAA surveillance currently relies on serial measurements of aneurysm diameter, which are used to predict further expansion and guide clinician decision-making regarding if and when to perform aneurysm repair. But AAA growth is nonlinear and AAA rupture occurs at unpredictable thresholds, Forsythe et al. noted, driving the need for more specific methods to predict AAA growth and clinical outcomes.

The researchers correctly hypothesized that 18F-NaF—a marker of vascular calcification associated with atherosclerotic plaque—could add predictive value when assessed via PET and CT.

In a study of 72 patients with a mean baseline aneurysm diameter of 48.8 millimeters, the median AAA expansion rate was 2.20 mm/year. When separated into three tiers based on 18F-NaF activity, aneurysms in the highest tertile expanded 2.5 times more rapidly than those in the lowest tertile and were nearly three times more likely to rupture or require repair. Twenty-two patients met this endpoint, with 19 undergoing elective AAA repair and the other three dying after experiencing AAA rupture without repair.

“Although we know that larger aneurysms tend to expand more rapidly and are more prone to rupture, disease evolution is not straightforward,” Forsythe and colleagues wrote. “Better AAA disease prediction using 18F-NaF uptake could be particularly useful for patients in whom the decision to intervene is challenging, such as those with high-risk aneurysms smaller than 55 mm, those with borderline aneurysm sizes, and those with larger aneurysms where the balance of risk and benefit is uncertain.”

In a separate study of 20 AAA patients (aortic diameter greater than 40 mm) matched to 20 control individuals based on age, sex and smoking status, the researchers found 18F-NaF uptake was increased more in dilated aortic regions versus nonaneurysmal regions within the same aorta and to the aortas within the control subjects. Uptake was nearly twice as high in the aneurysmal region than in the descending thoracic aorta of AAA patients.

The authors acknowledged their study contained a small number of rupture events, which makes it difficult to analyze which covariates make patients particularly susceptible. Also, the study population was largely men, which is indicative of the disease population, but the findings may not be generalizable to women.

In a related editorial, Parmanand Singh, MD, and Jagat Narula, MD, PhD, said further studies can help molecular imaging become a routine part of AAA care.

“The identification of aortic features linked to aortic vulnerability is crucial, both in guiding selection of patients for pre-emptive surgical repair and for optimizing timing of intervention to prevent complications,” the editorialists wrote. “Noninvasive molecular imaging holds promise to identify markers of aortic instability earlier in the course of disease progression, and could offer a major advance in the diagnosis, surveillance and management of AAA.”