DISCUSSION
Aortoenteric fistulas (AEF) constitute a rare clinical entity, concerning a pathological connection between the aorta and the gastrointestinal tract. Although their incidence barely reaches 4% (between 1.6 and 4%), they are associated with high mortality rates (24% to 45.8%).2, 3
They are divided into two major categories, the Secondary AEF (SAEF), which most commonly occur after previous aortic surgery, either open of endovascular, and the Primary AEF (PAEF), that take place spontaneously without any previous intervention in the aorta and are even rarer (incidence <0.07%)4.
High-risk patients of developing SAEFs include those who undergo emergent surgery for a ruptured aneurysm, have post-operative complications such as reoperation or bowel injury, and those with endoleaks or stent migration.5
Most commonly, an AEF presents with an initial or multiple “herald bleedings”, meaning episodes of transient, seemingly self-limiting and minimal lower GI bleeding. A catastrophic bleed follows, resulting in significant hemodynamic instability, requiring urgent surgical repair. Other clinical signs and symptoms might be sepsis (44%), abdominal pain (30%), back pain (15%), groin mass (12%), and abdominal pulsatile mass (6%).6
Clinical symptoms are very important in diagnosis of an aortoenteric fistula, because of the lack of high sensitivity and specificity of a single imaging modality. In previous reports authors suggested the diagnostic triad of pain, sepsis, and gastrointestinal bleeding, although the septic element is not always present in a SAEF.8
In our case, the patient presented with septic shock. This was explained by the infected pseudoaneurysm, which was the cause of the fistula. The previous medical history should arise the suspicion of such a diagnosis, creating the question of the best diagnostic imaging test.
Upper endoscopy is useful to exclude other sources of bleeding, such as peptic ulcer disease, but a normal endoscopy result never rules out AEF. When in suspicion of a SAEF, CT scan is considered the work-horse exam, although literature reports variable sensitivity and specificity.8 The most important imaging finding in an AEF is ectopic gas in the aortic lumen or in direct contiguity to the aortic lumen. In the most obvious cases, a direct tract of gas can be traced directly from the involved bowel loop towards the aorta.7 However, while highly suggestive; the presence of gas is not completely specific. For example, perigraft soft-tissue edema, fluid, and ectopic gas may be normal CT findings immediately after surgery.9 However, after 2-3 months, identification of any ectopic gas should be considered a sign of perigraft infection with the possible presence of AEF until proven otherwise.
Other CT findings that are suggestive of both perigraft infection and aortoenteric fistula include pseudoaneurysm, effacement of the periaortic fat, tethering of a bowel loop immediately adjacent to the aorta, disruption of the aortic wall or a graft or significant graft migration.7
The most specific sign in AEF, direct extravasation of contrast from the aortic lumen into a intestinal loop, is especially rare to identify on CT. Similarly, the leakage of enteric contrast directly into the periaortic space is a highly specific sign, but extremely rare. Notably, in a series by Hagspiel the extravasation of contrast from the aorta into the bowel lumen was present in only 11% of cases.14
Digital Subtraction Angiography (DSA) is frequently requested for further evaluation of gastrointestinal bleeding. However, in most patients with bleeding in the gastrointestinal tract, DSA offers few, pointing of to be a poor diagnostic tool.14 On the other hand CTA is proven to be a helpful tool for identifying disruption of the aortic wall, outline the a pseudoaneurysm, or demonstrating the presence of an AEF by describing direct extravasation from the aorta into the gastrointestinal lumen.10-11
The nonspecificity of these features is responsible for a considerable overlap with a variety of other disorders, with perigraft infection being the most important, as it can appear identical to a fistula. Other entities which intrigue the diagnostic procedure include aortitis, mycotic aneurysms, and perianeurysmal fibrosis, all of which can demonstrate periaortic inflammation, fluid, or soft tissue.7, 12, 13
Intramural gas and aortic wall thickening in a patient with positive blood cultures should raise suspicion of the presence of infectious aortitis. Similarly, when a patient is presented with a saccular aneurysm of the aorta accompanied with clinical evidence of sepsis, the surgical team should act prompt to treat the infected aortic aneurysm before it ruptures.7
As noted in the beginning, a secondary aortoenteric fistula occurs most commonly after an aortic aneurysm reconstruction. Whether this complication occurs more frequently after an open or an endovascular repair remains open to interpretation.16, 17
Considering that arterial suture line appears to be the main predisposing factor in open repair, endovascular treatment of AAA was initially thought to confer little to no risk of AEF. However, persistent endoleak, with the continued presence of blood flow into the aneurysm sac and aneurysm sac enlargement, represents a significant complication of endovascular aneurysm repair.18
Further, degeneration of the aneurysm neck causes graft migration with proximal endoleak and presence of pulsatile blood flow into aneurysm sac which eventually will lead to continued aneurysm expansion and rupture or subsequent bowel erosion and creation of AEF.19
Identification of impending or contained rupture is critical because these patients are at risk for frank rupture but can generally benefit from a more thorough preoperative assessment, followed by urgent surgery.20
Regardless the initial reconstructive surgery, when a SAEF is present, traditionally is managed by graft explantation, wide debridement of the infected tissues, infrarenal aortic stump ligation, and extra-anatomic revascularization with axillo-bifemoral bypass.21
This former gold-standard procedure has been doubted, as it hides the risk of aortic stump blow-out syndrome, specifically in long term follow up. As a result, in situ bypass grafting using homografts, allografts, prosthetic or vein grafts was developed. 23-25
In a series published by Kakkos et al, open surgery had higher in-hospital mortality (246/725, 33.9%) than endovascular methods (7/89, 7.1%, p<0.001). The reduced postoperative mortality after endovascular surgery indicated that expedient haemostasis achieved with former methods without any further insults is probably all that is needed in this patient population with haemmorhagic shock. The statistical difference, however, mostly disappeared during the first 18-24 months after the procedure.22
Most common causes of death in short-term post-operative period were irreversible shock, cardiac arrest, bleeding – either from stump blowout or homograft rupture- sepsis, MODS and GI complications including leak from the GI repair. During long-term follow up bleeding (recurrent AEF, stump blowout, disruption of the proximal anastomosis or homograft rupture), sepsis MOF or coronary syndromes were noted as the commonest causes of death.