Incidence and histologic characteristics of blebs in patients with abdominal aortic aneurysms

Purpose: Aortic blebs-focal outpouchings within aortic aneurysyms-may contribute to their eventual rupture. In this study we determine the incidence of aortic blebs and describe their microscopic features. Methods: Computed tomographic scans of the abdominal aorta were obtained in 188 patients with aortic diameters measuring ≤3 cm and were independently evaluated by a radiologist. The number and location of blebs were recorded, and each was measured with calipers. Sixteen blebs, with an adjacent uninvolved aneurysmal segment of aorta, and tissue from two patients with ruptured aneurysms were examined by light microscopy and immunohistochemical analysis. Specimens from six blebs and five aneurysms were examined for α 1 (I) procollagen messenger RNA by in situ hybridization. Results: Twenty blebs, ranging in size from 5 to 30 mm (mean, 12 ± 7 mm), were detected in 11% (20 of 188) of computed tomographic scans. Blebs were observed in 10% (11 of 111) of patients with aortic diameters between 3.0 and 4.9 cm, 10% (6 of 61) of patients with aneurysms between 5.0 and 6.9 cm, and 19% (3 of 16) of patients with aortic diameters ≤7 cm. Histologically, the major difference between the aneurysmal aortic wall and blebs was found in the media. In aneurysmal aortas, the media consisted of multiple layers of fragmented elastic lamellae, whereas the number of elastic tissue elements along the circumference of the blebs progressively decreased; only a few isolated fragments of elastic tissue were present at the apices. Histologic evidence of rupture was evident in two specimens. A chronic inflammatory cell infiltrate composed of T and B lymphocytes, plasma cells, and macrophages, common to both the aneurysms and the blebs, was most prominent in the adventitia of aneurysmal tissue, but involved both the media and adventitia of the blebs. In situ hybridization demonstrated the presence of α 1 (I) procollagen messenger RNA in four of the five aneurysm segments that were evaluated, compared with only one of six blebs. Conclusions: Blebs were discovered in aneurysms of all sizes; their frequency appeared to be unrelated to aneurysm size. The presence of inflammatory cell infiltrates and absence of α 1 (I) procollagen messenger RNA in five of six blebs suggest that a local imbalance of matrix degradation and repair plays a role in the cause of these lesions. Attenuation of the aortic wall accompanying the formation of blebs may predispose these sites to rupture.