ADVENTITIAL ELASTOLYSIS IS A PRIMARY EVENT IN ANEURYSM FORMATION

Purpose: Adventitial elastin degradation is a hallmark of abdominal aortic aneurysm (AAA) formation in human beings, although the quantitative relationship between elastin loss and AAA formation and growth is unknown. This study was undertaken to quantitate the reduction of adventitial elastin for small AAA, to determine whether the loss of this structural component parallels aneurysm growth, and to examine the ultrastructure of the remaining elastin elements. Methods: Longitudinal strips of anterior aneurysm wall were taken from 12 patients having elective repair of small (diameter < 5 cm, n = 4), moderate (diameter < 5 to 7 cm, n = 4), or large (diameter > 7 cm, n = 4) AAA and from six normal control subjects at autopsy. Specimens were prepared with elastin and collagen stains for histologic examination or formic acid for scanning electron microscopic evaluation of elastin architecture. Adventitial elastin content of aneurysmal and control aortas was quantitated with video microscopy and compared by aneurysm diameter. Results: The inner portion of adventitia of normal aortic wall was composed of densely compacted alternating lamellae of elastin and collagen, which were grossly disrupted in all aneurysms. The remaining elastin fibers were disorganized and tortuous. There was an 81.6% +/- 2. 1% reduction in elastin lamellae and an 85.7% +/- 4.2% reduction in fibers per lamellae compared with the number in control aortas (p < 0.001). Size of the aneurysm made no difference in adventitial elastin content. Conclusion: These data strongly suggest that elastolysis is a primary event in AAA formation that occurs before overt loss of adventitial structural integrity and the development of small aneurysms.