This study shows our experience of reporting clinical and CT imaging features in patients with confirmed tubercular aortic aneurysm. Main CT findings include: TBAA is mostly single saccular and lobulated pseudoaneurysm and often occurs in the thoracic aorta and abdominal aorta. Extravascular tuberculosis usually can be found such as miliary pulmonary tuberculosis, lymph node tuberculosis, tuberculous spondylitis, iliopsoas abscess and psoas abscess.
Mycotic aortic aneurysm of tuberculous is a rare complication of TB but with very high mortality. The incidence may increase in the future because of the greater number of immunodeficient patients and the emergence of drug resistant tuberculosis [1]. When mycotic aneurysms are present in the context of TB and, particularly, disseminated TB, TBAA should be suspected [6]. Active tuberculosis of any type was diagnosed on presentation in all of our cases. Sputum tuberculosis test was positive in patients with pulmonary tuberculosis. The concentration of CRP and ESR were high in all patients of this group, although cultures of the blood were negative. A few patients had an underlying condition that was known to increase the risk of TB such as HIV infection, rheumatic diseases, or oral immunosuppressants [11]. In addition, diabetes is a risk factor for tuberculosis [11]. In this group one patient had HIV infection and 5 cases had diabetes history. 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. Some patients may be accompanied by fever. It has been reported that fever occurs in 35% of patients [6]. In this group the proportion of fever is relatively high, with 7 patients having fever (41.2%). They may also present as palpable or radiographically visible periarterial mass, especially if expanding or pulsatile. Haemorrhage 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 (Fig. 4) or intestines, massive haemoptysis [12] or gastrointestinal bleeding [13] may be caused. The poor prognosis of these patients emphasizes the importance of early diagnosis.
It is reported that the vast majority of tuberculous aneurysms are pseudoaneurysms (87%), although true (9%) or dissecting (4%) aneurysms have been described [14]. All tuberculous aneurysms of our patients were pseudoaneurysms (Fig. 1–4). About 75% of TBAA present as a contiguous lesion on the surrounding tissue, such as tuberculous lymphadenitis, pericarditis, empyema, spondylitis, or paravertebral abscess [6]. 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 [15]. Of 17 cases in this group, extravascular tuberculosis was found in all patients. Tuberculosis adjacent to aneurysm includes miliary pulmonary tuberculosis, tuberculous spondylitis, pleural tuberculosis, renal tuberculosis, iliopsoas abscess or psoas abscess. Some patients have multiple tuberculosis sites. Twelve of them had enlarged lymph nodes. Other mechanisms of tubercular aneurysm formation may include the following: mycobacterium tuberculosis reaching the vessel wall through vasa vasorum; spread of bacteria through lymphatic vessels around the artery; direct implantation of bacteria on the internal surface of the vessel wall after vasculature trauma. Normal arterial intima is very resistant to infection. In this study, thirteen patients (76.5%) had atherosclerosis, their average age was 62.5 years. The atherosclerosis can alter the arterial lining and lower the resistance to infection [16]. At present, the incidence of tuberculosis is increasing in the elderly population who have most of the atherosclerosis, it could be anticipated that seeding of the aorta would be a common finding.
The contrast-enhanced CT can provide valuable information about the morphology of aortic aneurysm, 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, lobulated, with an indistinct, irregular aortic wall [10]. Tuberculous aneurysms may occur anywhere along the arterial system [17] and usually occur as a solitary lesion [18]. In this study tubercular aneurysms were mostly solitary and only one patient with multiple aneurysms. It is reported that the thoracic aorta is the most common location [19], because it is adjacent to the lungs and mediastinum where TB most commonly occurs. In this group tuberculous pseudoaneurysms were located in the thoracic aorta, abdominal aorta, junction of thoracic and abdominal aorta and iliac aorta in 8, 7, 1 and 1 cases, respectively. Thus, TB pseudoaneurysm often occurs in the thoracic and abdominal aorta [6]. Less frequently, femoral [20], iliac [21] and subclavian [14] arteries can also be affected. Most of the aneurysms were saccular (98%) [6]. The shapes of TBAA in this study were saccular, lobular, both saccular and lobular in 8, 5 and 4 cases respectively. One of aneurysms was large saccular which cystic wall was separated (Fig. 1). A lobulated aneurysm indicates more instability and higher risk of rupture. The diameter of aneurysm ranged from 39 to 120 mm. The size of the aneurysm is neither a risk factor of rupture, nor of the necessity for influencing treatment [22]. However, rapidly progressive growth of aneurysms (> 5 mm in 2 weeks) is suggestive of an infectious etiology [10]. CT is very sensitive to detect calcification and gas bubbles. Calcification is very uncommon in TBAA [6]. In this group of patients, only one patient had calcification in the arterial wall suggesting atherosclerosis (Fig. 1B). No patients showed gas bubbles which may appear in and around MAAs which could be indicative of high diagnostic reliability of bacterial infection [10]. 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 rim or septum enhancement by the administration of contrast material (venous phase) on contrast-enhanced CT. Significant exudation around aneurysm was in 3 patients in this study. Exudation and edema around aneurysm suggested acute infection, and also suggests that the aneurysm was unstable and may have ruptured with extravasation. The exudation and edema around the aneurysm cannot be identified by CT examination, which needs to be identified in combination with clinical symptoms. Lymph nodes adjacent to TBAA might also appear swollen and enhanced. Twelve patients of this group showed enlarged lymph nodes connected to or around the aneurysm. These enlarged lymph nodes showed ring enhancement and necrosis in the center (Fig. 1C). Tuberculosis 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. It couldn't be identified that the causal relationship between aneurysm and enlarged lymph nodes especially in late stage of disease. Iliopsoas abscess (IPA) or psoas abscess are common complications 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 area and contrast-enhanced rim of the abscess wall (Fig. 3B). In tuberculous spondylitis patients, TBAA can involve secondary spread from spine lesions. Sometimes it is difficult to identify the causal relationship between spinal infection and infectious aneurysms. Primary and secondary pyogenic spondylitis manifests erosion of the vertebral body and/or intervertebral disc on CT (Fig. 1A). Soft tissue swelling or abscess may be detected around the vertebral body (Fig. 1A). Pseudoaneurysm may develop adjacent to the eroded vertebral body,which greatly increases the risk of rupture during surgery. It was reported that an abdominal aortic aneurysm was iatrogenically ruptured during surgery for lumbar tuberculous spondylitis with psoas abscess [23]. In one case of this study, the patient’s aortic aneurysm was found during thoracic spine tuberculosis surgery. It is very important to evaluate the presence of aneurysm before operation in patients with tuberculous spondylitis. Tuberculosis in other parts usually can be found in patients of TBAA by CT scan, such as pulmonary tuberculosis (Figs. 1,2,4), renal tuberculosis, tuberculosis of reproductive system, etc.
Treatment of TBAA include antituberculosis chemotherapy treatment, open surgery (in situ reconstruction or extra-anatomic bypass), and endovascular treatment (embolization, stent grafting) [24]. Mortality rate of tubercular 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. Compared with other infectious aneurysms, tuberculous aneurysms show more cysts and lobulated shapes on CT scan, which are imaged as poured and unstressed. Another point of differential diagnosis is that the surrounding tissues and adjacent organs of tubercular aneurysms are usually infected with tuberculosis and most of them are accompanied by other sites of tuberculosis. Regular CT follow-up is also important for diagnosis.
There are some limitations in this study which should be acknowledged. This study analyzed the CT features of TBAA in order to assist its clinical diagnosis based on a single centre experience, however, the number of cases was relatively small. We will continue to collect cases of TBAA in our work and analyze their imaging characteristics in order to obtain more CT imaging characteristics of TBAA and help diagnosis. Further, we did not follow-up these patients who survived in terms of treatment outcomes. This will be addressed in further studies when more cases are included.
In conclusion. This study shows that TBAA typically appears on CT as a single saccular and lobulated pseudoaneurysm, usually occurring in the thoracic aorta or abdominal aorta. Surrounding tissues and adjacent organs of pseudoaneurysm are infected with tuberculosis and other sites of tuberculosis may be found.