Inflammatory aortic aneurysm

1 - Definition and characteristics

Inflammatory Aortic Aneurysm (IAA) is a distinct clinical entity from atherosclerotic aortic aneurysm that was first described by Walker et al. in 1972. (1). They described a form of aneurysm distinct from atherosclerotic aortic aneurysm in that it is characterised by “an unusually thick wall surrounded by extensive fibrous adhesions involving adjoining tissues and structures making the operative procedure much more difficult".

Inflammatory aortic aneurysm (IAA) is defined by the triad 1) a thickened aneurysm wall, 2) extensive perianeurysmal and retroperitoneal fibrosis, and 3) dense adhesions of adjacent abdominal organs (2).

Retroperitoneal fibrosis is a fibrotic process with progressive encasement and compression of the retroperitoneal region from the renal pedicles to the pelvic structures. Fibrous encasement of the ureters eventually leads to ureteral obstruction, hydronephrosis and varying degrees of renal failure.
These aneurysms are usually seen in the infrarenal abdominal aorta, and their presence in the thoracic aorta is extremely uncommon.
IAA appears in patients with large-vessel vasculitis or without systemic vascular inflammation (isolated idiopathic aneurysms). This article discusses idiopathic Abdominal Aneurysms.

2 - Risk factors and pathogenesis

Male sex, smoking and a genetic predisposition, are the main risk factors for atherosclerotic aneurysms. They are even stronger risk factors for the inflammatory variant. The percentage of patients with inflammatory abdominal aortic aneurysm who smoke is high, ranging from 77 % to 100 % of patients (3). A familial tendency to the development of aneurysms exists in 6.1 % to 15.1 % of IAA (4).
IAA could be also caused by large-vessel vasculitis, particularly by giant cell arteritis and Takayasu arteritis. In published case series of Takayasu arteritis, IAA has been found in up to 45 %  of patients (5).

Controversy exists regarding the pathogenesis of IAAs. Some researchers believe that atherosclerotic aneurysm formation is the preceding phenomena and that the inflammatory response is dependent on the aneurysm itself.

Rose and Dent proposed a novel hypothesis on the cause of the IAA (6). Through histological examination, they showed that an inflammatory process is present in the aneurysm wall in all specimens to varying degrees, (inflammatory and noninflammatory). They noted that no sharp distinction existed between the usual atherosclerotic aneurysm and inflammatory aneurysm and they considered that inflammatory aneurysms are the extreme end of an inflammatory process responsible for both inflammatory and non-inflammatory AAA. It was concluded that IAA is not a distinct clinical and pathologic entity but an “inflammatory variant” of the well-known atherosclerotic AAA.

Similarly, Sterpetti described “a gradual transformation in terms of inflammatory response, from ordinary atherosclerotic to inflammatory response”.

Recent theory suggests that the primary event is an inflammatory response to an unknown antigen presented to the aortic wall, characterised by aortic-wall infiltrating macrophages, T lymphocytes, and B lymphocytes that activate proteolytic activity through the production of cytokines. This proteolytic activity leads to increased turnover in the matrix proteins, elastin, and collagen. Subsequent loss of aortic wall integrity occur and an aneurysm forms (7). This inflammatory process is accentuated in certain persons with environmental risks (e.g., smoking) or a genetic predisposition and results in IAA that appears at a relatively younger age than in noninflammatory aneurysm.

Similar inflammatory pathogenesis of AAA is present in patients with giant cell arteritis and Takayasu arteritis with infiltrating T lymphocytes and macrophages of arterial wall.

Another possible causal factor of inflammatory aneurysm is an infection. The human herpes virus and cytomegalovirus may play a role as an antigen in the pathogenesis of aortic diseases.

3 - Clinical presentation

Inflammatory abdominal aortic aneurysms represent 5% to 10% of all abdominal aortic aneurysms and have a distinct tendency to occur in men. The male to female ratio varies from 30:1 to 6:1 depending on the series. Patients with inflammatory abdominal aortic aneurysms are 5-10 years younger that those with atherosclerotic one (2).

The triad of 1) abdominal or back pain, 2) weight loss, 3) an elevated erythrocyte sedimentation rate and CRP in patients with abdominal aortic aneurysms is highly suggestive of inflammatory aneurysm. Symptoms referable to inflammatory abdominal aortic aneurysm are reported to exist in 65 % to 90 % of patients. This rate of presenting symptoms is significantly higher than in patients with noninflammatory abdominal aortic aneurysm, of which only 8 % to 18 % of patients have symptoms. Weight loss and anorexia occur in at least 20 % to 41 % of patients with inflammatory AAA and can be significant. One quarter of patients with inflammatory abdominal aortic aneurysm had a pulsatile abdominal mass upon examination (8).

Arterial hypertension and arterial occlusive disease are frequent comorbidities associated with both inflammatory and noninflammatory abdominal aortic aneurysm. Similarly, coronary artery disease is a common comorbidity in patients with both inflammatory and noninflammatory abdominal aortica aneurysm.

Entrapment of the ureters in the retroperitoneal fibrotic process is common and occurs in varying degrees. Stella et al. reported involvement of the ureters in the periaortic mass in 53 % of patients with inflammatory aneurysms. Consequently, chronic renal dysfunction is present in 18 % to 21 % of such patients (2).

4 - Diagnostic testing

Preoperative diagnosis of inflammatory AAAs is the exception and occurs in only 13% to 33% of patients. When the diagnosis of aneurysm is suspected, imaging tools for the aorta are indicated.

Although angiography was once the gold standard for diagnosing aortic disease, technological advances in cross-sectional imaging techniques have generally led to the replacement of conventional aortography with CTA or MRA. In addition to their noninvasive nature, CTA and MRA have the advantage of imaging the components of the aortic wall and periaortic structures rather than the lumen only, as is the case of conventional angiography.

Abdominal CT scan is the most reliable diagnostic tool to detect aneurysmal wall thickening and perianeurysmal soft tissue changes suggestive of an inflammatory AAA. A Mayo Clinical study found good sensitivity for detecting an inflammatory AAA by CT scan (in 90 %) (8).

MRA, generally with gadolinium contrast enhancement, is emerging as a noninvasive imaging modality of choice. MR imaging may be used to image the entire aorta without radiation, exposure of iodinated contrast, and it provides excellent resolution of the aortic wall.

High – resolution ultrasound can also detect the inflammatory AAA wall in many patients. The distinct finding is a sonolucent halo. However, the inflammatory halo may not be present in all ultrasound studies. The accuracy of ultrasound in identifying the inflammatory component to an AAA is reported to be less than that of abdominal CT scan.

Recently, the use of 18-fluorodeoxyglucose (F-FDG) positron emission tomography (PET), either alone of in combination with contrast-enhanced CTA or MRA, has emerged as a potential tool for the initial diagnosis and assessment of inflammatory activity of the aorta. The role of F-FDG PET in the diagnosis and follow-up of patients with IAA ultimately needs to be established with larger clinical studies (9).  

5 - Treatment of IAA

The optimal management of patients with IAA is uncertain.

Although inflammatory AAA appears less likely to rupture than atherosclerotic AAA, surgical intervention appears prudent once the diameter of the aneurysm exceeds 5.5 cm. Open surgical repair of inflammatory aortic aneurysms is intrinsically accompanied by a higher complication and mortality rate compared with open surgical repair of noninflammatory aortic aneurysms because of the technical complications caused by inflammation and fibrosis (10). The surgical challenge lies in the difficulty of the dissection and control of the proximal and distal aorta, which leads to a three-times-higher perioperative morbidity and mortality rate (7.9 % vs 2.4 %) (8).

For nonoperative management of IAA, glucocorticoids are used as well. However, reports of management with corticosteroids include only case reports of small series of patients. No controlled clinical trials have evaluated the long-term efficacy of steroids for inflammatory AAAs (10). Most authors agree that corticosteroids do not alter the long-term development of inflammatory aneurysms and that operative repair of the aneurysm is the treatment of choice.

The content of this article reflects the personal opinion of the author/s and is not necessarily the official position of the European Society of Cardiology