A 52-year-old man came to the emergency department complaining of increasing epigastric pain in the last 48 hours. An emergent Ultrasound was performed by a non-radiologist physician arising the suspicion for a perforated ulcer. No significant alterations were found on the complete blood count and an abdominopelvic contrast-enhanced CT was ordered for a better characterisation.
A contrast-enhanced abdominopelvic CT in portal phase was performed depicting a huge aneurysm (66 x 72 x 73 mm; AP, T and CC diameters respectively) in the intrarenal abdominal aorta (Fig. 1, arrowhead) with a massive retroperitoneal haematoma associated (Fig. 1, star) indicating rupture. Note also the ‘draped aorta’-sign (Fig. 1, arrow) showing the posterior wall of the aneurysm moulding to the anterior surface of the vertebral body close to the rupture point. The caudal limit of the aneurysm was delimited by the aortoiliac bifurcation (Fig. 2a). At this level, an arterialised inferior vena cava (IVC) with blood-contrast level can be seen representing an acute aortocaval fistula (ACF) (Fig. 3a, arrowhead) as a main CT finding of this entity. The fistulous tract was also demonstrated in further multiplanar reconstructions (Figs. 3b and 3c, arrowheads). A 3D-reconstruction for teaching purposes was also performed exhibiting the fistulous tract (Fig. 4, arrowhead) and the filling of the IVC (Fig. 4, star).
Aortocaval fistula (ACF) is a very rare and devastating complication of ruptured abdominal aortic aneurysms (AAA). It presents in <1% of all AAA but in 3 to 7% in all ruptured AAA and is spontaneous in 80% of cases.  The high mortality of this complication (20 to 66%) is due to the blood flow derivation from a high-resistance arterial system (aorta) to a low-resistance venous circulation (IVC) through a fistulous tract. This abrupt blood flow derivation can lead to a high output cardiac failure, bilateral pedal oedema (secondary to venous congestion and hypertension in the lower half body) and haematuria among other clinical presentations. 
In our patient, an initial Ultrasound evaluation performed by a non-radiologist physician in the emergency department missed the aneurysm. An important teaching point is to always carefully look at retroperitoneum and abdominal aorta in the emergency setting. If the clinical suspicion were properly addressed, a different CT protocol would have been performed with a non-contrast, arterial and delayed phase instead of a single portal phase.
Only 17% of the patients with ACF present the classic triad of abdominal or lower back pain, machinery bruit and pulsating mass. Furthermore, half of the patients do not have any classical manifestation. A high degree of suspicion is crucial to accurately diagnose this complication .
Treatment options include surgical and/or endovascular reparation. When haemodynamic instability is present as in our patient, an emergent surgery should be performed due to the high mortality rate . In this case, the patient was treated surgically confirming the presence of the aneurysm and the ACF which was not bleeding due to the presence of a fresh clot. After graft reparation and exclusion of the ruptured aneurysm, the clot in the ACF was removed by suction. Digital compression of the ACF was performed and after controlling the posterior bleeding, the ACF was sutured.
Written informed patient consent for publication has been obtained.
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