Comparative Study of Hemodynamics and Pathological Consequences of Saccular Versus Fusiform Experimental Aortic Aneurysms

Introduction - Saccular aneurysms are thought to have a worst evolution than fusiform aneurysms in humans, probably for hemodynamic reasons (1,2). Some authors advocate earlier aggressive therapy towards saccular aneurysms to prevent death from rupture (3). However, data is lacking regarding comparison of saccular versus fusiform aneurysms. The aim of this study was to compare the differential hemodynamics and pathological consequences of fusiform versus saccular aneurysms in rats. Methods - Forty Lewis rats, at 9 weeks of age, were prospectively randomized in two groups of twenty individuals each: “saccular” (Group A) and “fusiform” groups (Group B). Decellularized thoracic aortas from guinea pigs were xenografted to the abdominal aorta in either an end-to-end (fusiform) or side-to-end (saccular) anastomosis manner (4). Main objective was to evaluate hemodynamic outcomes through duplex-scan and magnetic resonance imaging (MRI), one week and three weeks after surgery. Rats were sacrificed 4 weeks after surgery in order to analyze aneurysms histology, dosage of matrix metallopeptidase-9 (MMP-9)/matrix metallopeptidase-2 (MMP-2), plasmin/anti-plasmin complex in the plasma and in conditioned media. Results - All rats displayed a patent aortic aneurysm at one and three weeks. At three weeks, duplex-scan showed a mean growth of 34.7±9.1 % in group A, compared to the first week. Similar results were found in group B with a mean growth of 33.2±9.8 % (p=not significant). A complete flow recirculation was found in saccular aneurysms unlike fusiform aneurysms where recirculation was incomplete (persistent longitudinal flow). These results were confirmed by MRI examination. Corresponding histology showed an organized intraluminal thrombus (ILT) as a background cup in group A whereas it was partially developed on one side in group B (Figure 1). The ILT from saccular aneurysms was enriched in iron coming from lysis of Red Blood Cells as compared to fusiform aneurysms. Similarly, calcifications were more important in group A as compared to group B, corresponding to more tissue retention of free DNA, potentially released by dying neutrophils. Conclusion - Saccular and fusiform aneurysms show a similar growth over time but differential specific hemodynamics impact the form, the aspect and the biology of ILT in each type of aneurysm. In particular, ILT remains more active in saccular aneurysms whereas fusiform aneurysms display a healing process through fibrosis. The active ILT in saccular aneurysms maintain an inflammatory environment by recruiting neutrophils and might be a risk factor to their worst evolution in time compared to fusiform aneurysms. References1.Frösen J, Tulamo R, Paetau A, Laaksamo E, Korja M, Laakso A et al. Saccular intracranial aneurysm: pathology and mechanisms. Acta Neuropathol 2012;123:773-86.2.Michel JB, Martin-Ventura JL, Egido J, Sakalihasan N, Treska V, Lindholt J et al. Novel aspects of the pathogenesis of aneurysms of the abdominal aorta in humans. Cardiovasc Res 2011;90:18-27.3.Nathan DP, Xu C, Pouch AM, Chandran KB, Desjardins B, Gorman JH et al. Increased wall stress of saccular versus fusiform aneurysms of the descending thoracic aorta. Ann Vasc Surg 2011;25:1129-37.4.Allaire E, Guettier C, Bruneval P, Plissonnier D, and Michel JB. Cell-free arterial grafts: morphologic characteristics of aortic isografts, allografts, and xenografts in rats. J Vasc Surg 1994;19:446-56.