Imaging features of AF can be divided into direct and indirect signs.Direct signs [Fig 1]:- Rarely visualised.- Visualisation of the fistula tract.- Extravasation of intravenous contrast medium into the fistulising organ or oralcontrast medium coursing into the periaortic/prosthetic space.- Aortic graft migration into the adjacent fistulizing structure.Indirect signs [Fig 1]:- Often the only available evidence.- Abnormalities of the aortic wall: aortitis, aneurysm or pseudoaneurysmformation, aortic mural thrombus, a malpositioned graft, or discontinuousatherosclerotic plaque.- Interface irregularities between the aorta and the fistulizing structure, loss of fatplanes and periaortic foci of gas.- Intra-aortic air, any surrounding fluid collection, or free hemorrhage.- Abnormally thickened or tethered walls of the fistulizing organ.
1. Aorto-atrial fistula (AAF).AAF is an intracardiac communication between the aorta and atrium, being morecommon with the left atrium due to anatomical relationships. The main cause isendocarditis which is more common in prosthetic valves. AAF may also be secondaryto complications of aortic root or sinuses of Valsalva aneurysms, trauma, andinterventions on the aortic root, aortic valve or atria. Few are congenital and oftenassociated with other malformations.AAF clinically presents with a left-to-right shunt that overloads the right heartchambers. TEE is the gold standard for fistulous tract detection allowing flowquantification, especially in unstable patients requiring bedside or intraoperativeassessment.
2. Aorto-pulmonary fistula (APF).APF is a rare diagnosis. Causes include pathology of the ascending aorta (rupturedaneurysms or dissections), procedures on the pulmonary valve and pulmonary artery,trauma and sepsis. There are also congenital cases that are often associated withother malformations.APF leads to the development of left-to-right shunt, acute pulmonary edema, and rightheart failure. TEE may show flow between the aorta and pulmonary artery.
3. Aorto-bronchial fistula (ABF).The left tracheobronchial tree is more commonly involved due to anatomicalrelationships. Main etiologies include interventions on the thoracic aorta, trauma orendobronchial interventions. Other causes are oesophagectomies, cardiac surgery,radiotherapy or vasculitis. Surgery-related fistulas can occur both acutely and yearsafter surgery.The main symptom is haemoptysis, especially when massive and/or repetitive. Othersymptoms include chest or back pain and dyspnoea.Radiologically, indirect signs such as ground glass opacities due to haemorrhage aremore common, while it is very unlikely to see direct signs such as the passage ofcontrast from the aorta into the airways.
4. Aorto-esophageal fistula (AESF).AESF are mainly associated with rupture or repair of aneurysms, aortic dissections,neoplasms or foreign body ingestion. Classically, the Chiari triad has been described(midthoracic pain or dysphagia, a sentinel episode of minor hematemesis, thenmassive hematemesis), although other bleeding patterns or symptoms such as fever orback pain may be observed.Radiologically it may manifest with direct signs such as contrast extravasation into theoesophagus or stent migration, however, indirect signs such as air within the aorta,mediastinal collections or thickening of the oesophageal wall are more common.
5. Aorto-enteric fistula (AENF).AENF is the most common type of AF. It is most frequently secondary to surgical orendovascular interventions on the abdominal aorta, with infection of surgical orprosthetic material also playing a very important role in the process. Primary fistulasare rarer, arteriosclerosis being their main cause, and any chronic inflammation can bea trigger (peptic ulcers, radiotherapy, neoplasms, infections, etc).Clinical manifestations include various forms of gastrointestinal bleeding, sepsis,abdominal or back pain and shock. A limited, intermittent ‘herald bleeding’ precedingmassive haemorrhage may be observed. The classic triad of gastrointestinal bleeding,abdominal pain and a pulsatile mass is not so common.AENF mainly occur in the third duodenal portion [Fig 1], due to the proximity betweenthe aorta and the retroperitoneal duodenum, followed by the fourth portion, jejunum,ileum, and large bowel [Fig 2].The sensitivity and specificity of CT are higher for secondary fistulas. Similar to otherAF, direct signs are less common than indirect signs such as air in the aortic lumen orfocal bowel wall thickening [Fig 1].
6. Aorto-caval fistula (ACF).
The main causes are a ruptured aortic aneurysm, aortitis, vena cava filters, trauma,spinal surgery or inferior vena cava dissections.Shock is not as common as other AF as the blood remains intravascular. Symptomsmay include abdominal or lower back pain, pulsatile abdominal mass and high-outputcardiac failure.ACF usually occur in the infrarenal aorta. The main direct sign is early enhancement ofthe vena cava, with the same density as the aorta, in the arterial phase [Fig 3]. Indirectsigns include retroperitoneal haematoma and dilatation of the pelvic veins due tovolume overload [Fig 4].
7. Differential diagnosis.Some normal aortic variants can mimic aneurysms or pseudoaneurysms, causingconfusion when an AF is suspected. They appear as a focal bulge which forms anobtuse angle with the aorta, suggesting benignity.Some examples:- Ductus diverticulum: located at the anterior undersurface of the isthmic region.- Aortic spindle: located at the descending aorta.- Aortic nipple: prominence of the third intercostal artery origin located at thelateral aortic contour just distal to the isthmus.- Kommerell diverticulum [Fig 5]: dilatation in either an aberrant right or leftsubclavian artery arising from a left or right-sided aortic arch.Postoperative changes overlap with AF imaging features. Perigraft air can persist up to7 weeks after surgery but it usually disappears after the first week. Perigraft soft tissue,fluid, or hematoma should be considered pathologic after 3 months. Surgical materialand haemostatic packing agents can have foci of air within them, simulating perigraftair. However, these materials tend to be uniform, do not enhance, do not changeposition across studies, and are higher in attenuation (-104 to -458 HU) than pure air.Other pathologic processes can have imaging findings similar to AF:- Retroperitoneal fibrosis: variably enhancing soft-tissue plaques encasing theaorta that can sometimes be asymmetric, focal, and infiltrating, mimicking aninflammatory process.- Lymphadenopathy.- Inflammatory pathology [Fig 6]: entities such as Erdheim-Chester disease canmanifest with a thick inflammatory periaortic rind. Lack of other imaging featuressuggests AF.- Mycotic aneurysm [Fig 7]: an important risk factor for the formation of AF. Someof the imaging features can overlap (aortic wall irregularity, fat stranding, airfoci, …). AF suspicion must be high when some imaging features such ascontiguity with an adjacent organ and direct signs are found.
8. Treatment.AF have a poor prognosis. Open surgery is the treatment of choice although it has ahigher complication rate. Endovascular intervention is associated with less morbidity
and mortality in the short term, however, due to its high rate of re-intervention, it isrecommended as a bridge therapy in the acute phase for later open surgery.