A 74-year-old man with abdominal pain

Stent graft infection

Infection has been reported in 1% to 6% of patients receiving a stent graft for aortic aneurysm.⁶ They occur most commonly in the first year after placement; one study showed that 42% of patients diagnosed with graft infection presented within 3 months of endovascular repair.⁷

The leading cause of graft infection is contamination during the original procedure, but secondary infection from hematologic seeding and contamination from adjacent bowel are also possible.⁸ In our patient, who underwent graft placement followed by endovascular repairs of endoleaks, bacterial seeding of his aortic aneurysm from the procedures should be considered.⁹

The most common organisms are staphylococcal species, with Staphylococcus aureus more common in early infection and coagulase-negative staphylococci more common in late infection.¹⁰ Methicillin-resistant S aureus has been reported in as many as 25% of cases of graft infection. Diphtheroids and gram-negative enteric organisms should also be considered.¹¹

CT is the most effective imaging test for graft infection. Perigraft soft tissue, fluid, and gas are the major CT findings.¹²

Given that our patient presented with abdominal pain, leukocytosis, and the CT finding of perigraft gas, graft infection should be high on our list differential diagnoses.

Retroperitoneal fibrosis

Retroperitoneal fibrosis is most often idiopathic, although many believe it is due to an exaggerated local inflammatory reaction to aortic atherosclerosis or is a manifestation of a systemic autoimmune disease.¹³ Secondary retroperitoneal fibrosis may be due to drugs, infection, or malignancy.

Pathologic findings include sclerotic plaques, typically around the abdominal vessels and ureters. Clinical presentations are often nonspecific, with early symptoms that include back or abdominal pain, malaise, anorexia, edema, and hematuria.^14,15 Progressive ureteral obstruction can occur in later stages. CT with contrast is the imaging test of choice to visualize the extent of disease, with the fibrosis exhibiting attenuation similar to that of muscle.¹⁶

Initial treatment of idiopathic retroperitoneal fibrosis is with a glucocorticoid or other immunosuppressive agent, whereas treatment of secondary retroperitoneal fibrosis is aimed at the underlying cause.¹⁷ Late stages complicated by ureteral obstruction typically require surgery.¹⁸

Our patient did have some nonspecific complaints that could be due to retroperitoneal fibrosis. He also had an intra-abdominal malignancy, which could lead to secondary retroperitoneal fibrosis. However, his CT findings of periaortic gas are not consistent with this diagnosis.¹⁹

Aortoenteric fistula

Aortoenteric fistulas can be either primary or secondary.

Primary aortoenteric fistulas occur de novo in patients who have never undergone any surgery or procedure in the aorta. This type of fistula usually results from pressure erosion of an atherosclerotic abdominal aortic aneurysm into the gastrointestinal tract. They are rare, with an annual incidence of 0.04% to 0.07% in the general population.^20,21

Secondary aortoenteric fistulas are complications of aortic reconstructive therapy. After open repair, a perianastomotic or pseudoaneurysmal fistula can develop into the gastrointestinal tract.⁴ Endovascular repair leaves the aortic wall intact with no exposed suture lines, but an aortoenteric fistula can still develop²² and in fact occur in 0.4% to 3.1% of recipients of stent grafts for abdominal aortic aneurysm repair.²³ In such cases, it is commonly thought that graft infection can lead to formation of an aortoenteric fistula, but a penetrating gastrointestinal ulcer, tumor invasion, radiation therapy, and trauma have also been implicated.^19,24–26 An aortoenteric fistula can present several months to several years after either open or endovascular abdominal aortic aneurysm repair.^4,23

One of the main CT signs of an aortoenteric fistula is periaortic ectopic gas at least 3 to 4 weeks after surgery or endovascular repair.¹⁹ Gas around the stent graft is most commonly caused by infection, but an aortoenteric fistula must also be considered in our patient, as roughly one-third of graft infections present as aortoenteric fistula.²⁷ Our patient denied having any gastrointestinal bleeding, but his hemoglobin concentration at presentation was 8.9 g/dL.

Highlight point. Perigraft gas after abdominal aortic aneurysm repair can be seen in graft infection and aortoenteric fistula.

SIGNS AND SYMPTOMS OF AORTOENTERIC FISTULA

2. What is the most common clinical sign or symptom of an aortoenteric fistula?

• Gastrointestinal bleeding
• Sepsis
• Abdominal pain
• Back pain

Gastrointestinal bleeding occurs in 80% of patients who have an aortoenteric fistula, sepsis in 40%, abdominal pain in 30%, and back pain in 15%.¹⁹ The classic triad of symptoms is gastrointestinal bleeding, abdominal pain, and a pulsatile abdominal mass. However, symptoms can vary widely, and the classic triad is present in fewer than 25% of cases.²⁸ Sepsis may be the predominant clinical manifestation, particularly in the early stages of fistula formation. Unexplained fever is an underrecognized early manifestation.²⁴

Highlight point. The classic triad of symptoms of an aortoenteric fistula (gastrointestinal bleeding, abdominal pain, and a pulsatile abdominal mass) is seen in fewer than 25% of cases.

Case continued: The patient develops frank bleeding

The vascular surgery service was consulted because of concern for an aortic graft infection, since surgical removal of the infected material is recommended.¹⁰ The patient was deemed to be a poor surgical candidate, given his stage IV colon cancer, so he was treated conservatively with broad-spectrum antibiotics.

Over the next 2 days, he had two episodes of dark, bloody bowel movements, but he remained hemodynamically stable. He subsequently developed frank bleeding per rectum with symptoms of lightheadedness, and his hemoglobin concentration fell to 6.9 g/dL. He was given a total of 3 units of packed red blood cells, which raised his hemoglobin level, but only to 8.3 g/dL. The gastroenterology service was consulted to evaluate for the source of the bleeding.

Comment. In a situation like this, an aortoenteric fistula is high on our list of differential diagnoses as the cause of bleeding, but other causes of frank bleeding per rectum such as diverticulosis, arteriovenous malformation, hemorrhoids, or a rapid upper-gastrointestinal bleed cannot be ruled out.

Upper-gastrointestinal endoscopy is the most commonly used diagnostic test for aortoenteric fistulas. It can also find other possible sources of gastrointestinal bleeding. CT with contrast is another option. It can depict the fistula itself or reveal signs of infection, such as gas or liquid surrounding the graft. In an emergency, when there is not enough time for diagnostic testing and an aortoenteric fistula is strongly suspected on clinical grounds, emergency surgical exploration is warranted.^4,24

In our patient, the gastrointestinal service elected to first perform endoscopy to look for an aortoenteric fistula.