Images in medicine: Spontaneous aortocaval fistula complicating abdominal aortic aneurysm.

Aortocaval fistula represents a rare, life-threatening complication of abdominal aortic aneurysm that needs emergency surgical treatment. The presentation couples that of a rupturing aneurysm with other more characteristic features resulting from the massive arteriovenous shunt. Early recognition and emergency surgical treatment are essential in reducing mortality and morbidity. Prompt investigation with multidetector computed tomography (CT) angiography quickly and accurately establishes a precise preoperative diagnosis, thereby enabling proper planning of operative treatment.

INTRODUCTION

Whereas the vast majority of abdominal aortic aneurysms (AAA) rupture into the retroperitoneum, in up to 3%-5.5% of the patients bleeding in the duodenum, inferior vena cava (IVC), iliac, or left renal vein may occur. Most usually observed in elderly people, spontaneous aortocaval fistula (ACF) represents one of the rarest complications of an atherosclerotic AAA. Alternatively, in sporadic cases, ACF may develop secondary to trauma, spine surgery, or interventional procedures, syphilis, or mycotic aneurysm from infectious endocarditis, Takayasu's arteritis, Marfan syndrome, or Ehlers-Danlos syndrome.[123]

CASE REPORT

A 63-year-old man was brought to our hospital with progressively worsening dyspnea since three days, intermittent thoracic pain, and lower limb weakness. A heavy smoker, he had an otherwise unremarkable past medical history.

At emergency department admission, he was found agitated and tachycardic (115 beats/minute), with increased respiratory frequency and 120/70 blood pressure. Physical examination disclosed distended jugular veins at 45°, extensive cyanosis over his legs and lower abdomen, and cold extremities with faintly appreciable peripheral pulses. Arterial blood gas values included pH 7.34, pO2 83 mmHg, pCO2 27 mmHg, HCO2 14 mmol/L, and 95% oxygen saturation on room air. Electrocardiography (ECG) and echocardiography failed to disclose acute abnormalities.

Due to a palpable abdominal pulsatile mass, the clinical suspicion of previously AAA was confirmed by bedside emergency ultrasound (not shown). Therefore, despite renal dysfunction (3 mg/dL serum creatinine), contrast-enhanced multidetector CT (MDCT) was immediately performed [Figure 1]. Timed arterial-phase MDCT acquisition after high-flow intravenous contrast medium (CM) injection showed early enhancement of the dilated IVC, which appeared synchronous and equivalent to that of the aortic lumen. A large (8 cm maximum transverse diameter) extensively thrombosed infrarenal aortic aneurysm was confirmed, without extravascular blood collections indicating retroperitoneal rupture. Furthermore, at the right posterolateral aspect of the aneurysmal sac, a focal CM-opacified communication consistent with a spontaneous ACF was identified between the distal inferior vena cava and the patent aortic lumen and effectively visualized by means of angiographic maximum intensity projection (MIP) and three-dimensional volume-rendering (3D-VR) reformations. Distally to the ACF, the iliac veins appeared collapsed and poorly opacified due to the massive left-to-right shunt. Both kidneys showed reduced, delayed enhancement.

Figure 1

Contrast-enhanced multidetector computed tomography (CT) angiography. Early enhancement of the inferior vena cava (IVC) and hepatic veins, synchronous and equivalent to that of the aortic lumen (a). Aortocaval fistula (ACF) identified as direct communication between the distal IVC and patent lumen of large, extensively thrombosed infrarenal aortic aneurysm (arrowheads), depicted on axial (b), oblique coronal (c), and three-dimensional volume-rendered (d) images. Note the poorly enhancing left kidney in C and collapsed poorly opacified iliac veins in D

Following intensive care support and blood transfusions, emergency laparotomic vascular surgery was performed, including aneurysm repair, aortocaval fistula suture, and insertion of an aortobifemoral prosthesis. The patient finally recovered after a prolonged postoperative hospitalization.

DISCUSSION

The clinical presentation of ACF couples the signs and symptoms of hemorrhagic shock suggesting AAA rupture with other more characteristic features that result from the massive left-to-right shunt and increased venous return. The classical clinical triad including severe abdominal and low back pain, pulsatile abdominal mass, and machinery-like bruit is observed in up to 50%-80% of the patients. Among the other variable manifestations, edema and cyanosis in the lower extremities or scrotum or both occur in 60%-70% of the patients. Venous hypertension often causes liver congestion, hepatomegaly, and sometimes ascites. Reported in 12%-33% of the patients, hematuria, oliguria, and renal failure result from the combination of venous hypertension and decreased median arterial pressure with kidney hypoperfusion. Furthermore, the decreased peripheral resistance and diastolic hypotension cause a sharp increase in cardiac output, which can rapidly lead to left ventricular high-output decompensated cardiac failure in nearly half of the patients, particularly in those with underlying heart disease. Finally, paradoxical pulmonary embolism may result from emboli originating within the aneurysmal sack trespassing on the IVC.[12]

Due to these varied, complex manifestations of ACF, the spectrum of acute thoracic abdominal conditions that may be hypothesized by the attending emergency physician encompasses many different entities including pulmonary thromboembolism, cardiac failure, AAA rupture, and aortic dissection, along with other nonvascular abdominal emergencies such as acute pancreatitis or renal colic.[124]

Although ultrasound may promptly detect the AAA, CM-enhanced MDCT currently represents the gold standard modality to provide a prompt and precise preoperative diagnosis in critically ill patients. At MDCT, the massive arteriovenous shunting is heralded by the early, synchronous contrast enhancement in the dilated IVC cephalad to the ACF, which appears equivalent to the patent AAA lumen. As this case exemplifies, MDCT with multiplanar angiographic and 3D-VR reformatted images effectively visualizes the presence, site, and caliber of the abnormal arteriovenous communication. ACF almost always forms between the posterolateral aspect of the distal AAA and the infrarenal IVC, where the normal anatomic space is lost. Furthermore, early enhancement of the renal veins and enlarged poorly perfused kidneys are commonly seen.[2] The differential diagnosis of ACF includes alternative causes of early IVC contrast enhancement such as highly vascular tumors in the lower extremities and retrograde flow of opacified blood from the renal vein from suprarenal IVC obstruction or tricuspid regurgitation.[5]

In the vast majority of cases, ACF requires emergency surgical treatment, which may be performed by a transperitoneal approach with aneurysmal sac opening, venous bleeding control by manual pressure, direct repair, and insertion of a prosthetic graft. Successful surgical treatment allows relieving congestive heart failure, edema, and renal function. The mortality from ACF is estimated to be equivalent to that associated with ruptured AAA and depends largely on the entity of blood loss, hemodynamic impairment, and comorbidities. In conclusion, as this case demonstrates, an early preoperative diagnosis with MDCT is crucial to provide an easier and timely surgical approach, and allows to limit the mortality rate to 7.2%-19.2% of patients.[13]