Teaching Case

A 61-year-old male with a known abdominal aneurism went to the emergency department due to generalised oedema, dyspnoea and abdominal pain that began five days before. At clinical examination, the patient had a distended abdomen with pain and tenderness on the left flank. The inferior members were swollen and cold, and the oxygen saturation was low.

The patient first underwent an ultrasound examination where a voluminous abdominal aortic aneurysm with extensive parietal thrombosis was seen (asterisk in Figure 1). Discrete peri-aneurysmatic fluid was also identified, raising suspicion of a ruptured aneurysm.

On computed tomography (CT) a partial thrombosed abdominal aortic aneurysm with 8 cm of maximum width was proved. A direct communication between the aortic aneurysm and the inferior vein cava (IVC) was seen, raising the suspicion of an aortocaval fistula (arrows in Figures 2a and 2b). Also, in the early arterial post-contrast acquisition, synchronous opacification of the aorta, and the inferior cava vein was evident, proving the aortocaval fistula diagnosis.

An aortocaval fistula is a rare but serious vascular anomaly characterised by an abnormal connection between the abdominal aorta and the IVC. Spontaneous rupture of an abdominal aortic aneurysm into the adjacent inferior vena cava occurs in <1% of all aneurysms [1]. The vast majority of aortocaval fistulas arise in the setting of a ruptured abdominal aneurysm (~3%) [1]. Various other risk factors are known, including atherosclerosis, mycotic aneurysm, syphilis, polyarteritis nodosa and connective tissue disorders like Marfan syndrome and Ehlers–Danlos syndrome [1]. Despite its rarity, it presents significant challenges in diagnosis and management due to its potentially life-threatening consequences [2].

The pathophysiology of an aortocaval fistula involves the creation of a direct communication between the high-pressure system of the aorta and the low-pressure system of the IVC. This results in a shunting of blood from the aorta directly into the IVC, leading to haemodynamic instability and subsequent cardiovascular compromise [1]. Patients may present with symptoms such as abdominal bruit, pulsatile abdominal mass, congestive heart failure, or even shock [2].

Doppler ultrasound, computed tomography angiography (CTA), magnetic resonance angiography (MRA), and conventional angiography are the primary imaging techniques employed in diagnosing aortocaval fistulas [3]. Doppler ultrasound may reveal turbulent flow within the IVC, suggestive of the abnormal shunting of blood from the aorta. However, it may be limited by operator dependency and the inability to visualise the entire extent of the fistula [3].

CTA and MRA have the advantage of providing detailed anatomical information and visualising the abnormal connection between the aorta and IVC. However, due to faster acquisition times and being more widely available, CTA is more commonly used, particularly in emergency settings [4]. On imaging, these modalities typically show a direct communication between the dilated aorta and the IVC, often with a characteristic “jet” of contrast material entering the IVC during the arterial phase, as seen in the present case [2]. Additionally, CTA and MRA can identify associated findings such as aortic aneurysms, dissections, or adjacent tumour involvement, which may predispose to fistula formation [2].

Despite conventional angiography remaining the gold standard for confirming the diagnosis of aortocaval fistula, offering real-time visualisation of the abnormal vascular communication, it has been replaced in many centres by CTA due to its non-invasive nature and ability to provide a detailed 3D view of the involved anatomy [4]. Angiography typically demonstrates contrast material rapidly filling the IVC during the arterial phase, bypassing the normal venous circulation [4]. Additionally, angiography allows for simultaneous intervention, such as embolisation or stent-graft placement, to occlude the fistulous tract [4].

Lastly, transoesophageal echocardiography can be particularly useful in assessing cardiac function and estimating the degree of left-to-right shunt [2].

While imaging findings are crucial in establishing the diagnosis of aortocaval fistula, they also play a significant role in assessing the extent of vascular involvement, guiding treatment decisions, and in post-treatment follow-up. Timely recognition of this condition through imaging modalities is essential for initiating appropriate management strategies and optimising patient outcomes.

Written informed patient’s family consent for publication has been obtained.