Tuberculosis (TB) is a communicable disease that is a major cause of ill health and the leading infectious disease killer globally [1, 2]. China is one of the high TB burden countries; new TB patients in China accounted for about 8.4% of the world’s cases in 2019, ranking third worldwide [1]. The TB incidence rate has slowly declined since the beginning of the 21st century because of human immunodeficiency virus (HIV) infection, anti-TB drug resistance, and the use of immunosuppressive drugs [2]. Other risk factors for TB are diabetes mellitus, silicosis, smoking, air pollution, malnutrition and protein imbalance [3]. TB typically affects the lungs (pulmonary TB) but can also affect other sites. Tuberculous aortic aneurysm (TBAA) is exceedingly rare [4]. The first case of TBAA was reported by Kamen in 1895 [5, 6]. TBAA is associated with high mortality due to its high risk of sudden rupture. No TBAA patients were known to have survived until the availability of combined technologies of modern imaging, anti-TB therapy, and surgical treatment [5]. Early diagnosis and prompt treatment are important for the improvement of survival [7]. However, early diagnosis is difficult due to the non-specific symptoms and negative blood cultures. Some cases are diagnosed at an advanced stage or after developing complications, such as rupture or aortic fistula.

Current imaging modalities can detect infected aneurysms in clinically suspicious patients including computed tomography (CT), magnetic resonance imaging (MRI), and 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET)/CT [8]. CT angiography (CTA) with arterial and venous phases imaging enables evaluation of the entire aorta. CTA is non-invasive and efficient, with broad coverage and isotropic voxel capabilities; thus, it is increasingly used for the diagnostic assessment of aortic disease including TBAA.

This study retrospectively reviewed 17 cases of patients with TBAA, and collected their clinical data, laboratory tests, imaging findings, and treatment for analysis. The CTA imaging features of all TBAA patients were summarized and analyzed.

Mycotic aortic aneurysm of TB is a rare complication of TB but with high mortality. When mycotic aneurysms are present in the context of TB, and particularly, disseminated TB, TBAA should be suspected [3]. TB of any type was diagnosed on presentation in all of our cases. A few patients had an underlying condition that is known to increase the risk of TB such as HIV infection, diabetes, or oral immunosuppressants [9]. Most of the reported cases of TBAA are symptomatic, but the symptoms are nonspecific and depend on the size, position, and rapid growth of the aneurysm. Patients may describe thoracic, abdominal, or dorsal pain, which may be accompanied by fever. Fever occurs in 35% of patients [3]. In this study group, the proportion of fever was relatively high, with seven patients having fever (41%). They may also present with palpable or a radiographically visible periarterial mass, especially if expanding or pulsatile. Hemorrhage or hypovolemic shock may occur if the aneurysm ruptures or perforates. If a fistula is formed between the aneurysm and the nearby organs, such as the trachea or intestines, massive hemoptysis [10] or gastrointestinal bleeding [11] may occur. The poor prognosis of these patients emphasizes the importance of early diagnosis.

The vast majority of tuberculous aneurysms are pseudoaneurysms (87%), although true (9%) or dissecting (4%) aneurysms have been described [12]. All tuberculous aneurysms of our patients were pseudoaneurysms. About 75% of TBAAs present as a contiguous lesion on the surrounding tissue, such as tuberculous lymphadenitis, pericarditis, empyema, spondylitis, or paravertebral abscess [3]. Caseous necrosis invading the entire arterial wall results in perforation, some with massive hemorrhage or perivascular hematoma formation. Fibrosis gradually forms in the periphery of hematoma, and the hematoma is encapsulated and communicated with the lumen. Thus, the pseudoaneurysm is formed [13]. Extravascular TB was found in all patients of this group. TB adjacent to aneurysm includes miliary pulmonary TB, tuberculous spondylitis, pleural TB, renal TB, IPA or psoas abscess. Some patients have multiple TB sites. Other mechanisms of tuberculous aneurysm formation may include the following: mycobacterium TB reaching the vessel wall through vasa vasorum, spread of bacteria through lymphatic vessels around the artery, and direct implantation of bacteria on the internal surface of the vessel wall after vasculature trauma. Normal arterial intima is very resistant to infection. Atherosclerosis can alter the arterial lining and lower the resistance to infection [14]. At present, the incidence of TB is increasing in the elderly population who have the highest incidence of atherosclerosis; thus, it could be anticipated that seeding of the aorta would be a common finding.

CECT can provide valuable information about the morphology of AA, aortic wall enhancement, and the relationship between the aneurysm and adjacent tissue because of the higher quality spatial resolution. TBAA typically appears on CT as a focal, contrast-enhancing, saccular lumen, with an indistinct, irregular aortic wall [8]. Tuberculous aneurysms may occur anywhere along the arterial system [15] and usually occur as a solitary lesion [16]. The thoracic aorta is the most common location [17], because it is adjacent to the lungs and mediastinum where TB most commonly occurs. In this group, the incidence of TB pseudoaneurysm is the same in the thoracic or abdominal aorta [3]. Less frequently, femoral [18], iliac [19] and subclavian [12] arteries can also be affected. It has been reported that most of the aneurysms are saccular (98%) [2]. The shapes of TBAA in this study were all saccular, and nine of them were lobulated—the wall of the aneurysm was irregular. A lobulated aneurysm indicates more instability and higher risk of rupture. All aneurysms of the six patients who died were lobulated based on the saccular shape. The diameter of aneurysm ranged from 3 to 12 cm. The size of the aneurysm is neither a risk factor of rupture nor the necessity for influencing treatment [20]. Because one or more layers of mycotic aneurysm wall are missing, they all have the risk of rupture, no matter the size. However, the rapidly progressive growth of aneurysms ($>$5 mm in 2 weeks) is suggestive of an infectious etiology [8]. CT is the most sensitive imaging modality for the detection of calcification and gas bubbles. Calcification is reportedly very uncommon in TBAA [3]. Twelve patients in this group had calcification. The higher incidence of calcification in this study may be due to the older age of these 12 patients, with an average age of 68 years. Here, calcification was consistent with atherosclerosis. No patients showed gas bubbles, which may appear in and around aneurysm and could be indicative of high diagnostic reliability of bacterial infection [8]. Although gas bubble is an important sign of arterial infection, it is too rare to make the differential diagnosis. In this study, all cases of aneurysm ruptured at different locations of arterial wall, which was a process of forming pseudoaneurysms. In fact, three-tier structure of the aneurysm wall was incomplete, especially the sparse elastic fiber fracture of middle smooth muscle had broken. CT showed that the wall of some aneurysms was thin and there was no obvious soft tissue around it. Those lobulated, tension-free aneurysms are more likely to rupture, some of which look like a mess of mud and may have ruptured. CT can well display soft tissue and adjacent organ damage around tubercular aneurysm.

Eccentric periaortic surrounding soft tissue can show as a rim or septum enhancement by the administration of contrast material (venous phase) on CECT. Significant exudation around aneurysm was in three patients in this study. Exudation and edema around aneurysm suggest that the aneurysm was unstable and may have ruptured with extravasation. CT cannot differentiate between the exudation and edema from hematoma, whereas MR can provide more information due to its high tissue resolution. Lymph nodes adjacent to TBAA might also appear swollen and enhanced. These enlarged lymph nodes showed ring enhancement and necrosis in the center. TB can cause progressive enlargement of the surrounding lymph nodes, and the rupture of lymph nodes can spread to the adjacent aorta to form an aneurysm. Tuberculous aneurysm can also cause lymph node hyperplasia. A causal relationship between aneurysm and enlarged lymph nodes was not identified, especially in the late stage of the disease. IPA or psoas abscess is a common complication in the abdominal TBAA, presenting as a direct invasive infection with purulent materials occurring within the iliopsoas or psoas muscles. The typical features on CT are enlarged and swollen muscles with single or multiple relatively low-density areas and contrast enhanced rim of the abscess wall. In tuberculous spondylitis patients, TBAA can involve secondary spread from spine lesions. Primary and secondary pyogenic spondylitis manifests as erosion of the vertebral body and/or intervertebral disc on CT. Soft tissue swelling or abscess may be detected around the vertebral body. Pseudoaneurysm may develop adjacent to the eroded vertebral body, which greatly increases the risk of rupture during surgery. It was reported that an abdominal AA was iatrogenically ruptured during surgery for lumbar tuberculous spondylitis with psoas abscess [21]. In one case in this study, the patient’s AA was found during thoracic spine tuberculosis surgery. It is very important to evaluate the presence of aneurysm before surgery in patients with tuberculous spondylitis. Tuberculosis in other parts can usually be found in TBAA patients by CT scan, such as pulmonary TB, renal TB, and TB of the reproductive system.

Treatment of TBAA includes anti-TB chemotherapy treatment, open surgery (in situ reconstruction or extra-anatomic bypass), and endovascular treatment (embolization, aortic stent grafting) [22]. The mortality rate of tuberculous aneurysm is 35% in this group, which is still high. Early diagnosis and timely treatment are critical in reducing the mortality of TBAA. CT plays an important role in the diagnosis of TBAA, especially for some patients without conditions (economic reasons, no MRI/PET equipment), or with contraindications to MRI [3, 23]. TBAA shows saccular shapes on CT scans, which are imaged as unstressed. Another distinguishing feature is that the surrounding tissues and adjacent organs of TBAA are usually infected with TB, and most of them are accompanied by other sites of TB. Regular CT follow-up is also important for diagnosis. The limitation of this study was the absence of TB etiology in most cases with no follow-up in those who survived treatment. Future studies will address this limitation.

In conclusion, this study showed that TBAA typically appears on CT as a single saccular pseudoaneurysm, and the incidence is the same in the thoracic or abdominal aorta. Surrounding tissues and adjacent organs of pseudoaneurysm are infected with TB, and other sites of TB may be found.

The datasets used during the current study are not publicly available due to strict requirements set out by the Human Ethics Research Committee regarding the storage and use of the data by authorised investigators.

Conceptualization—YL; methodology—XX and WX; validation—ZS; formal analysis—NZ; writing and original draft preparation—XX; writing–review and editing—LC, and ZS. All authors have read and agreed to the published version of the manuscript.

This study was conducted in accordance with the declaration of Helsinki. The study was approved by the Ethic committee of the 3rd Affiliated Hospital of Shenzhen University. Written informed consent to publish the clinical details and images of the patient was obtained.

We would like to thank Qinxiang Mao from Liuzhou Hospital and Huaiping Yuan from Fuyang Hospital for providing data for this study.

This study was supported by Natural Science Foundation of Liaoning Province of China (No. 2019-MS-200).

The authors declare no conflict of interest. Zhonghua Sun is serving as Editorial Board Member and Guest Editor of this journal. We declare that Zhonghua Sun had no involvement in the peer review of this article and has no access to information regarding its peer review. Full responsibility for the editorial process for this article was delegated to Dinesh Kalra and Grigorios Korosoglou.