This is the largest study on INAA patients to report detailed findings using CT [1, 10]. The main findings of this study were that the majority of patients had saccular aneurysms often with a multilobulated contour and an infrarenal aortic location. Also, as opposed to prior conception, many of these aneurysms contained atherosclerotic plaques.
Although calcification at the aneurysmal wall is commonly found in degenerative aortic aneurysms, it has been rarely reported in INAA, which seems to be an errant misconception [1, 10]. Several pathological studies of INAA have shown acute transmural inflammation superimposed on the atherosclerotic aorta as the typical pathogenesis of INAA [11, 12], thereby supporting calcified plaque as its common CT feature. Therefore, the presence of calcified plaque could not distinguish INAA apart from a degenerative aortic aneurysm . Moreover, the infrarenal aorta has been the most common site of both INAA and degenerative aortic aneurysm, inferring that INAA is a disease commonly affecting elderly patients with aged aortas and comorbidities causing atherosclerosis [2, 6, 13].
The inflammation process involving the compromised aortic wall would also cause an aggressive appearance of the aneurysm including the saccular shape, multilobulated contour, large initial aneurysm sac, and rapid progression in size as the primary CT features of INAA [1, 10, 14, 15]. Furthermore, evidence of aneurysm instability (i.e., impending rupture, contained rupture, or free rupture) from the CT findings was also found in more than half of the patients with INAA.
The typical periaortic characteristics of INAA in this study were soft-tissue mass, enhancement, fluid, and fat stranding, which were likely secondary to the infection and subsequent inflammation [1, 8, 10, 16,17,18].
Although some CT findings, including periaortic fluid and ectopic gas, were previously described as uncommon [1, 8, 10, 19,20,21], periaortic lymphadenopathy, which supported the diagnosis of INAA, was found in two-thirds of the patients in this study. However, periaortic lymphadenopathy was reported in only a few previous studies [14, 17]. Since lymphadenopathy has been one of the characteristics to indicate surrounding active infection/inflammation, its role in determining the activity of infection and the treatment response of INAA could be another opportunity for future researchers .
The features associated with the surrounding structures were less often noticed. However, they could be found as INAA culminated in secondary complications and thus were noteworthy for urgent management. The psoas muscle could either have been the primary site of abscess or hematoma secondary to a ruptured INAA [1, 23, 24]. These etiologies can be indistinguishable on CT, which leaves percutaneous aspiration as the most appropriate means to simultaneously treat and secure a specimen for culture and polymerase chain reaction analysis and to differentiate the two conditions from each other [25, 26].
Despite the rare incidence, ectopic gas should raise suspicion of a gastrointestinal structure or bronchus involvement apart from being evidence of infection [18, 23, 27]. However, contrast extravasation from INAA into a hollow viscus was rarely seen on CT in this study. Having said that, a definite diagnosis of aortic fistula prior to exsanguination is only possible from CT findings when appropriate clinical information, such as herald bleeding, is present [28, 29]. Also, thoracic INAA is relatively infrequent compared to abdominal INAA [30, 31]. On the contrary, there is a greater proportion of hollow viscus complications from esophagus and bronchus involvement in thoracic INAA than bowel involvement in abdominal INAA in this study. Spondylitis and perinephric involvement were also uncommon concomitants, as reported in previous studies [1, 10, 23, 32, 33]. Only a few previous cases of aortocaval fistula in the acute setting of INAA have been reported [10, 23].
Our study had some limitations. Even though this is the largest study of CT characteristics of INAA, the number of cases is relatively small due to the rare prevalence. The high rates of previous antibiotic treatment before obtaining blood culture were responsible for the low rate of positive blood culture results in this study. However, the rate of negative cultures in this study was in line with the literature [2, 34]. Maximal diameter from the outer border to the outer border of the aneurysm on axial CT image was the standard measurement method in this study. Such simplicity led to a good comparison to determine treatment response in the follow-up examinations. However, to date no standard measurement of INAA sac size has been proposed [16, 35].
In conclusion, infective native aortic aneurysms most commonly occur in the infrarenal aorta and are usually associated with calcified atherosclerotic plaques. The most common aneurysmal features are saccular shape, multilobulated contour, aneurysmal instability, and rapid expansion. The typical periaortic findings are enhancement, fat stranding, and soft-tissue mass. Involvement of the adjacent psoas muscle, gastrointestinal structures, and bronchi are infrequent but do occur as critical complications to INAA.