The severity of aneurysmatic disease and its evolution were described long ago. Since that time, therapeutic strategies have been based on surgical procedures, which have been modified according to scientific and technical developments in the medical fields. The Papyrus of Erbes, written in approximately 2,000 B.C., described peripheral aneurysms and suggested surgical treatment with a glowing iron: “treat with the knife and burn with fire …” [1,2].

The term inflammatory abdominal aortic aneurysm (AAA) was created in 1972 by Walker et al. [5] to describe an AAA with an unusually thick wall surrounded by extensive fibrous adhesions involving adjoining tissues and structures and treated as a different clinical entity from degenerative aneurysms. This characteristic was found in 2.5–15 % of all AAAs with a 20:1 predominance in males and occurred in patients an average of 10 years younger than those with noninflammatory AAAs [3–5].

The inflammatory AAA, once believed to have a different etiology and pathogenesis, now seems to be the final result of the inflammatory process found in degenerative aneurysms. The etiology of the aortic aneurysm, attributed long ago to atherosclerosis, has not yet been fully defined.

Aneurysmatic degeneration is the end result of a multifactorial process that leads to the destruction of the connective tissues of the arterial wall. Evidence indicates that the most important structural elements of the aorta wall, the interstitial collagen and elastin, are associated with the degradation of the extracellular matrix in AAA. Many of these findings are related to matrix metalloproteinases (MMPs). Among these are 72-kDa gelatinase (MMP-2), 92-kDa gelatinase (MMP-9), matrilysn (MMP-7), and macrophage elastase (MMP-12), which are capable of degrading elastic fibers. Others, such as proteinase, plasminogen activator serum elastase, and cathepsin, also contribute to aneurysmatic degeneration [6].

Atherosclerosis is still being investigated as a participatory factor in degenerative aneurysmatic processes of the aorta. Experimental studies including atherogenic diets for extended periods produced areas of lateral inclination of the middle arterial layer, with aneurysm formation in primates (13 % in cynomolgus monkeys and 1 % in rhesus monkeys) [7].

Atherosclerotic plaques are usually more plentiful, pronounced, calcified, and complicated in the abdominal aorta compared with those in the thoracic segment. These changes are related to local differences in blood flow conditions, wall stress mechanisms, and the composition and nutrition of the structures of the arterial layers.

In humans, the abdominal aorta expands with the progression of atherosclerotic plaque. This is associated with the narrowing and loss of structural architecture of the middle layer, being particularly pronounced at its midpoint and at the fork of the iliac arteries [8].

Preliminary studies suggest cathepsin S and K as potent elastases, and the genes of this enzyme are expressed in human atheroma plaque. Atheromas also have high activity in destroying elastin, which is sensitive to the cysteine protease inhibitors. Levels of cysteine C, the most abundant of the cathepsin inhibitors, decreased in atherosclerosis arteries and in the walls of abdominal aortic aneurysms [6].

Evaluating the causal factors, elastin and collagen are the most important components of the arterial wall and are associated with smooth muscle cells and the middle layer of the abdominal aorta [9].

Elastin is the arterial wall component responsible for ­elasticity of the artery, allowing physiological swelling and shrinkage, determined by the heartbeat. Much evidence exists concerning the participation of aortic dilatation in elastin lesions.

The presence of swelling occurs most often in the abdominal aorta, where there are fewer elastic layers. Elastin is not synthesized in the aorta of adult individuals, and the elastic capacity of the average 70 year old (period when AAAs appear most frequently) is three times less than that of a 20 year old. The use of elastase in the arterial walls in animal experiments produces dilatation, but without a relation to the proportions of the elastin lesion and the final diameter of the aorta [10]. Elastin is responsible for 35 % of the weight of the middle layer of normal aortas and is reduced up to 8 % in aneurysmatic arteries. Comparing the quantity of elastin in 81 fragments of the AAA wall surgically removed from corpses 82 with normal aortas, a reduction of approximately 90 % was found and a total reduction of 50 % in amino 84 acid aneurysms [11–13].

An additional finding was the elevated levels of elastase in these walls, suggesting an important participation of elastin in the genesis of aortic dilation in humans [14].

Collagen types I and III are the main forms of this component of the aortic wall, which is characterized by tensile strength and low distensibility in order to allow the increase in arterial diameter. Different from elastin, collagen is produced throughout life (absorbed and reconstituted) and has 20 times the tensile strength of elastin. The collagen is laid in such a way that, supported by elastin, it allows an increase in diameter (heartbeat) and upon reaching a particular stretching point is used as a protective barrier containing the arterial increase. Its concentration increases in the AAA walls. A deficiency in type III collagen (the most frequent in the aortic wall) has been associated with the rupture of cerebral aneurysms and identified as an abnormality in Ehlers-Danlos syndrome type IV (arterial weakness with formation of aneurysms) [13]. At the same time, the collagenolytic activity is increased in individuals with AAA ruptures [15]. Contrarily, metalloproteinase inhibitor (which decreases the activity of protease enzymes, such as elastase, collagenase, and gelatinase) is found in decreased concentrations in the walls of dilated arteries [6,14].

RNA messenger expression of 92-kDa gelatinase ­(MMP-9) on the wall of the aorta has been correlated with the presence and size of the aneurysm, being also ­associated with high levels of MMP-9 in the serum of these individuals. These surveys suggest there is a change in the ability to repair the arterial wall in case of an exaggerated expression of MMP-9, which results in the destruction of the arrangement [6].

The enzyme 72-kDa gelatinase (MMP-2) appears at high levels in AAA walls, in its active form, next to the extracellular matrix, suggesting a protease action. High-level membrane type I MMP (MMP-14), which constitutes the primary activator Pro-MMP-2, was also found. Doxycycline inhibits MMP-2, and this is an important factor in aneurysm formation [38].

A model of aortic wall lesion induction in rats, with administration of elastase, demonstrated the presence of transmural infiltrated mononuclear phagocytes, increased production of MMP, and progressive destruction of the ­protein matrix from the wall. The deletion of the degenerative process in vivo using nonselective antiinflammatory inhibitors of MMP (doxycycline) was also investigated. MMP-9 deficiency induced resistance to aneurysm formation, but with a bone marrow transplant to correct this deficiency, mice recovered the potential to develop aneurysms. The administration of doxycycline resulted in a five-fold decrease in the expression of MMP-9 in the aortic wall of patients undergoing elective treatment of AAA. In vitro, it suppressed expression of MMP-9 stimulated by forbol in mononuclear THP1 phagocytes [6].

In intermediate situations, increasing the process of reconstitution of collagen (types I and III) can maintain stable levels, but with the continuous increase of degradation, this can provide an unfavorable balance with rapid expansion and rupture of the aneurysm. Collagenase-3 (MMP-13) is most frequently found on the AAA wall [6].

In vitro studies suggest that reactive oxygen forms mediate activation of MMP-9 and MMP-8. Recent studies have shown that activated phagocytes produce oxygen, which then forms hydrogen peroxide and myeloperoxidase in the process of phagocytosis; the latter enables MMP-9 and MMP-8, creating the oxidative theory of aneurysm formation [6].

An inflammatory infiltrate composed of T cells, monocytes/macrophages, B-lymphocytes, and plasma cells associated with the presence of HLA-DR  +  and antigenic markers on the wall of AAAs suggests an inflammatory component. This also indicates that purified immunoglobulin G in a segment of the AAA wall in contact with the normal aorta wall will produce an immunoreaction [6,16].

Surveys on the presence of Chlamydia pneumoniae in aneurysmatic wall biopsies of the abdominal aorta proved to be positive in 77 % of the cases, which highlights the relationship between infection and AAA [17]. The use of plasma markers, such as IgA and IgG using ELISA or immunofluorescence, showed the presence of Chlamydia pneumoniae in 36 % of aneurysms. A review showed their sensitivity and specificity for the need for surgery (expansion) for AAAs to be 80 % and 66 %, respectively [17,18].

Among the mechanisms that cause weakening of the aortic wall, some are particularly crucial: genetics; smoking; factors that increase the size of the artery, such as hypertension, inflammatory, processes, and atherosclerosis.

Family history is one of the background factors related to the prevalence of aneurysm and has special characteristics, such as being most likely to be present in women or in individuals under the age of 65 years [19]. Although conditions such as mutations in type III collagen, changes in MZ alpha-1-antitrypsin, and metalloproteinase inhibitors indicate a genetic basis for AAAs, a genetic origin for the formation of the aneurysm has not yet been established. Surveys suggest that 12–19 % of people with AAA features are related to one or more first-degree relatives with this disease [20,21]. The relative risk for relatives of people with AAA is 18 times higher than for individuals without a family relationship [6]. When searching for deficiency of type III collagen in 56 people with AAA, 16 (28.6 %) had a family history and 6 (10.7 %) a deficiency in type III collagen [21].

Smoking is strongly associated with weakening of the arterial wall. The products of combustion of tobacco inactivate alpha 1-antitrypsin, oxidizing methionine, increasing wall degradation, and contributing to the genesis of AAAs and an increase in their size, which entail an increased risk of rupture [22,23]. The rupture rate in a study of small aneurysms in the UK (UK Small Aneurysm Trial) reached 1.9 % per year in patients with high serum levels of nicotine compared to 0.5 % in those with only trace amounts (AAAs of 4–5.5 cm diameter) [24].

It is still uncertain whether hypertension plays a role in the formation of AAAs or exacerbates an already existing situation. The fact is that hypertension is related to the presence of AAAs and contributes to the increase in arterial diameter [22] in the elderly (individuals aged 60 years or more, according to the World Health Organization). As described earlier, age above 60 years is related to the presence of AAAs not only in association with atherosclerotic disease, but also with changes in the elastin and collagen recruitment engine [25].

Atherosclerosis is invariably associated with AAAs in spite of different risk factors. Hyperlipidemia and diabetes are not related to AAAs, with great frequency observed in atherosclerotic disease [26].

Local factors such as composition, biomechanical strength, and nutrition are different in the abdominal and thoracic aorta [6]. There is a decrease in the vasa vasorum in the human abdominal aorta when compared with other arterial segments considering the thickness of the wall. Thus, the complement of this blood perfusion deficit is provided by direct absorption through the aortic lumen. It is believed that the presence of atherosclerotic plaques in the aorta or in the nurturing artery (vasa vasorum) helps to create layers of wall ischemia that can be related to the genesis of dilation.

The difference is basically the intense inflammatory process involving fibroses and dense adhesions to the adjacent abdominal viscera.