Silent Rupture: Atypical Presentation of Aorto-Caval Fistula Secondary to Giant Abdominal Aortic Aneurysm

Abstract

Abdominal aortic aneurysm (AAA) is a potentially life-threatening condition characterized by abnormal dilation of the abdominal aorta, typically due to chronic arterial wall degeneration. Aorto-caval fistula (ACF) is a rare but serious complication of AAA, occurring in less than 1% of cases overall, with incidence increasing in the setting of rupture. ACF involves the formation of an abnormal communication between the abdominal aorta and the inferior vena cava (IVC), resulting in blood shunting from the arterial to the venous system. This can cause reduced organ perfusion, high-output cardiac failure, and multiorgan dysfunction. We present the case of a 65-year-old hypertensive male with a known AAA who presented with 2 days of abdominal pain but no systemic or gastrointestinal symptoms. Physical examination revealed periumbilical tenderness, while laboratory results were normal. Contrast-enhanced computed tomography of the abdomen revealed a large (10 cm) infrarenal AAA with evidence of rupture, mural thrombus, and a fistulous connection to the IVC. The patient underwent successful endovascular repair with complete resolution of the aneurysm and fistula, as shown in follow-up imaging. This case highlights the importance of high clinical suspicion and the role of early imaging in diagnosing ACFs, even in the absence of classical signs or lab abnormalities. The aim is to raise awareness of such atypical presentations and emphasize our case’s uniqueness in its silent, stable, yet severe presentation.

Introduction

Abdominal aortic aneurysm (AAA) is a serious condition marked by the abnormal enlargement of the abdominal aorta, typically caused by long-term weakening of the arterial wall. Common risk factors include older age, male gender, smoking, high blood pressure, and atherosclerosis. Aorto-caval fistula (ACF) is a rare complication of AAA, occurring in under 1% of cases overall. Unlike standard aortic ruptures, ACFs may not initially present with major blood loss. An ACF arises when an abnormal connection forms between the abdominal aorta and the inferior vena cava (IVC), allowing blood to flow directly from the high-pressure arterial system into the low-pressure venous system. This can compromise blood flow and oxygen delivery to essential organs, potentially resulting in shock and multiorgan failure.^1-3 We present the case of a 65-year-old hypertensive man with a known AAA who presented with 2 days of abdominal pain. Imaging revealed a 10 cm infrarenal AAA with rupture, mural thrombus, and an aortocaval fistula to the IVC. Despite stable vitals and normal labs, computed tomography (CT) confirmed arterial-phase enhancement of the IVC. He underwent successful endovascular repair. Postoperative imaging showed resolution of the aneurysm and fistula with restoration of normal aortic anatomy.

Case Presentation

A 65 years old male patient, who is known to have hypertension and a history of AAA, presented to the emergency department for abdominal pain of 2 days duration. He reports no other associated gastrointestinal or systemic symptoms. On physical exam, his vital signs were stable and he had periumbilical tenderness upon palpation. An injected CT scan of the abdomen and pelvis showed an aneurysm of the infrarenal abdominal aorta measuring 10 cm, extending to the level of bifurcation with evidence of rupture of the aorta in addition to a mural thrombus and a fistula to the IVC (Figure 1). Laboratory results showed normal values for lactate, Lactate dehydrogenase (LDH), and hemoglobin. Contrast-enhanced coronal reformatted CT scan of the abdomen showed similar findings (Figure 2). The enhancement of the IVC is similar to the enhancement of the aorta in the arterial phase, confirming the presence of the aortocaval fistula (Figure 3). The patient underwent urgent surgical repair of the aneurysm and fistula through endovascular approach. CT scan of the abdomen with axial cut post-op, showed normal caliber abdominal aorta with absence of the fistula (Figure 4a and b). Contrast-enhanced coronal CT scan shows similar finding (Figure 5).

Figure 1.

Injected CT scan of the abdomen and pelvis showed an aneurysm of the infrarenal abdominal aorta measuring 10 cm, extending to the level of bifurcation with evidence of rupture of the aorta in addition to a mural thrombus and a fistula to the IVC. CT, computed tomography; IVC, inferior vena cava.

Figure 2.

Contrast-enhanced coronal reformatted CT scan of the abdomen showed similar findings. CT, computed tomography.

Figure 3.

CT scan of the abdomen shows that enhancement of the IVC is similar to the enhancement of the aorta in the arterial phase, confirming the presence of the aortocaval fistula. CT, computed tomography; IVC, inferior vena cava.

Figure 4.

CT scan of the abdomen with axial cut post-op, showed normal caliber abdominal aorta with absence of the fistula: (a) at the level of L2 vertebra and (b)at the level of L1 vertebra. CT, computed tomography.

Figure 5.

Contrast-enhanced coronal CT scan shows similar finding. CT, computed tomography.

Discussion

AAA is a potentially life-threatening condition characterized by the abnormal dilatation of the abdominal aorta. Risk factors include advanced age, male sex, smoking, hypertension, and atherosclerosis. ¹ An ACF is an uncommon complication of AAAs, occurring in less than 1% of cases, though the incidence rises to 2% to 6.7% in ruptured AAAs. Unlike typical aortic ruptures, ACFs often present without significant blood loss upon admission. ² An ACF forms when a connection develops between a dilated abdominal aorta and the IVC, causing blood to shunt from the high-pressure arterial system to the low-pressure venous system. This can reduce arterial perfusion and oxygen delivery to vital organs, potentially leading to shock and organ failure. ³ Computed tomography angiography is the primary diagnostic modality. The primary symptoms result from rapid arteriovenous shunting leading to secondary venous hypertension, and typically include an abdominal bruit, abdominal pain, pulsatile mass, reduced urine output, rapid heartbeat, low blood pressure, and signs of right-sided heart failure. ² Risk factors for ACF include large aneurysm size, rapid aneurysm expansion, and the presence of a mural thrombus. Additional reported risks involve female gender, a history of previous AAA repair, and the use of anticoagulant or antiplatelet medications. ³ Other associated factors include atherosclerosis, connective tissue disorders (like Marfan and Ehlers–Danlos syndromes), vasculitis, infections, prior spinal surgery, malignancy, radiation exposure, and penetrating trauma or spinal injuries. ⁴ Rapid shunting can result in high output heart failure. Therefore, the timely placement of intra-aortic balloon occlusion (IABO) plays a crucial role in controlling severe venous bleeding and stabilizing hemodynamics. IABO should be deployed promptly once significant bleeding or instability is detected, aiming to lower venous pressure and stabilize the patient ahead of definitive surgical intervention. ⁵

Traditionally, open surgical repair has been the standard treatment, but it carries a high mortality risk. Although endovascular repair has been attempted, its long-term outcomes remain unclear due to the rarity of ACFs and limited long-term follow-up data. ² Endovascular repair has become more common in recent years, with various techniques reported. Compared to open surgery, it offers lower early mortality but higher recurrence rates. Treatment options include placing devices within the aorta or IVC to seal the fistula, and in complex cases, adapting existing devices. ⁶

The choice of the type of surgical repair to an ACF secondary to AAA depends on the physician experience, preference, and chance of survival. A systematic review analyzed 196 cases of ACF secondary to AAA, comparing outcomes of open surgical (78%) and endovascular repair. Technical success was high for both methods (99%-100%). Thirty-day survival favored endovascular repair (97.6%) over open surgery (87.5%), as did mid-term survival (95.2% vs 86%). However, endovascular repair had a higher reintervention rate (35.7% vs 2.5%) and frequent type II endoleaks. Despite limited data and male-dominant cases, both approaches showed excellent technical outcomes and acceptable survival, though long-term durability remains a concern for endovascular treatment. ⁷ A retrospective study conducted by Davidovic et al analyzed outcomes in 39 male patients (mean age 67.4 years) who underwent open surgical repair for ACF caused by spontaneous rupture of AAA between 2012 and 2023 in Belgrade, Serbia. The 30-day mortality rate was 35.9%, including 7.7% intraoperative deaths. Of the 25 postoperative survivors, followed for a mean of 3.7 years, the long-term mortality was 3.0 per 100 person-years. Despite the high risk and complexity, open repair remains a viable option, though contemporary literature suggests endovascular repair may offer better outcomes when feasible. ⁸ Furthermore, it is important to diagnose ACF and mural thrombus prior to surgical repair of the aneurysm to prevent further risk of embolization of the thrombus during manipulation of the aneurysm. ⁹ Endoleaks after endovascular aneurysm repair (EVAR) is common to occur. Accarino et al present the case of an aortocaval fistula with type 1 and 2 endoleaks that occurred after 6 years of EVAR and were treated by treated by placing a polytetrafluoroethylene endograft in the IVC. ² McDonald et al used a triple endovascular approach through both the aorta and caval systems to successfully embolize the ACF and treat a type II endoleak following prior EVAR. ⁶

Some casers of infrarenal AAA complicated by ACF have been reported. Wu et al reported a 75-year-old male with an 8.7 cm infrarenal AAA and ACF underwent successful endovascular repair after bilateral internal iliac artery embolization. A persistent type Ia endoleak was resolved with an aortic cuff. ⁷ Delbare et al present a case of a large infrarenal aortic aneurysm with extensive para-aortic hematoma. In this case, the patient presented for sharp lower back pain, nausea, vomiting and syncope with a pulsatile mass in the abdomen along with tachycardia and hypotension with an elevated lactate level. ¹⁰ Sohail et al report a case of ACF with an unruptured AAA with no precipitating factors; however, the patient had perturbed laboratory results such as high lactate level and LDH. ⁴

In our case, the patient had solely abdominal pain and was hemodynamically stable with no elevated lactate levels, which made the suspicion of rupture aneurysm with ACF difficult to suspect initially. He did not have all the typical symptoms reported in the literature. In addition, the patient’s follow-up revealed no endoleaks present postendovascular repair, showing the success of the fistula and aneurysm’s repair, unlike other cases.

Conclusion

This case stands out due to the atypical clinical presentation and favorable outcome. Unlike most reported ACF cases associated with ruptured AAAs, our patient remained hemodynamically stable with normal lactate and hemoglobin levels, lacking the typical signs of rupture or arteriovenous shunting. Early imaging played a crucial role in detecting the fistula and guiding successful endovascular repair. The absence of endoleaks on follow-up further highlights the efficacy of the intervention. This report underscores the importance of high clinical suspicion and timely imaging in identifying ACFs, even in patients presenting without classic symptoms or laboratory abnormalities.