Introduction
The natural history of undetected abdominal aortic aneurysms (AAA) includes ischemic complications that may affect peripheral limb or organ circulation. Distal embolization or thrombosis is the usual pathological process related to either migration of cholesterolic particles, or extension of wall thrombosis from the aneurysm sac. In that circumstance, ischemic colitis remains an uncommon primary mode of revelation for aneurysm occurring when patency of the inferior mesenteric artery (IMA) is suddenly affected.1 We report a successful endovascular treatment of an AAA initially revealed by reversible colonic ischemia.

Case Report

A 79-year-old man was admitted to our institution for sudden abdominal pain associated with distension and constipation. His recent medical history included uneventful coronary bypass surgery performed 3 weeks earlier for symptomatic coronary disease affecting left anterior descending and circumflex arteries. On physical examination, abdominal palpation was painful on the left lateral side, with distension. Plain abdominal X-ray showed direct signs of ileus. An inflammatory syndrome was noted on the biologic blood test. Computed tomography (CT) of the aorta revealed an infra-renal AAA of 53 mm maximum, with extensive sac thrombosis, but without a sign of rupture or fissuration (Figure 1). The celiac trunk, superior mesenteric artery and hypogastric arteries were fully patent, but IMA was evidently occluded. Early colonoscopy using a flexible endoscope revealed a segmental ischemic colitis concerning the left descending colon and upper sigmoid extending 30 cm, with aspects of grade I non-confluent areas of superficial ischemia, with edema being limited to mucosa (Figure 2).

The histological examination from intra-procedural biopsy did not demonstrate any presence of full-thickness ischemia or necrosis. A sudden occlusion of the inferior mesenteric artery (IMA) not visible on CT angiogram was considered the primary explanation for the symptoms. After a period of observation under medical treatment with intravenous rehydratation and antibiotics, clinical improvement was noticed. After receiving patient and family informed consent, the decision was made to treat this symptomatic aneurysm with endovascular techniques. Graft selection was made after precise sizing. Procedure was conducted under potentialized epidural anesthesia, allowing implantation of Talent™ bifurcated aortic stent-graft (Medtronic Vascular, Santa Rosa, California) via a bilateral lower external iliac approach. Completion aortography showed successful exclusion of the aneurysm (Figure 3) confirmed with a satisfactory immediate follow-up CT angiogram (Figure 4). The patient had uneventful recovery with no abdominal symptom and was discharged one week after the operation under antiplatelet therapy. The midterm control at 6 months demonstrated a normal cardiac and abdominal clinical examination with no sign of endoleak, good positioning of the endograft, and satisfactory patency on CT angiogram (Figure 5).

Discussion
Ischemic colitis revealing an abdominal aortic aneurysm is an uncommon event rarely reported in the literature.¹ Its clinical expression may vary from a transient episode to more severe symptoms. Early diagnosis with colonoscopy using a flexible endoscope is essential to characterize the degree and extent of the lesion on the colonic tract.² In 1966, Marston described three levels of colonic ischemia, including grade I with superficial edema and cyanic spots, grade II with non-concentric ulcers and grade III with necrotic aspect, either superficial or transmural.³ Arterial vasculature of the colonic tract is, in fact, directly related to the inferior mesenteric artery (IMA) and its collateral system coming from superior mesenteric artery (SMA) and celiac trunk (CT).⁴ The hypogastric artery network is linked to this complex anastomotic system.
In that way, patency of IMA is an important feature that must be taken into account during AAA screening and before planning a procedure. Visibility of the artery can be achieved either with conventional angiography or CT angiogram.⁵
Acute colonic ischemia represents a severe potential complication of AAA surgery using open classic procedure.⁶ Many authors report this type of complication, its cause and preventive actions to be taken.^6–8 However, preoperative patency of IMA during open aortic surgery for AAA does not seem sufficient to prevent colonic ischemia. Operative and clamping time, blood loss and preoperative or postoperative hypotensive status increase dramatically the probability of this complication, whereas surgical reimplantation of IMA has not been proven to be sufficient to avoid bowel ischemic complications.⁹ The occurrence of such complications reinforces the use of endovascular repair as a faster and safer procedure, compared to open surgery in selected cases.^10,11 However, a patent IMA may induce a type II late endoleak after endograft for AAA exclusion.¹² Using comparative pre- and postoperative CT angiograms, Fan et al demonstrated that preoperative patency of IMA was significantly associated with early endoleak rate in a retrospective review of 158 patients who underwent stent-graft AAA repair.¹³ Preoperative intraluminal coil embolization with selective microcoils8 or glue and postoperative IMA clip ligation using retroperitoneal laparoscopic approach are the usual procedures currently available to prevent or treat this kind of retrograde endoleak.^14–16

Conclusion
In conclusion, a careful analysis of the colonic circulation is essential before planning endovascular exclusion of AAA. This case report emphasizes the important role played by IMA, of which status, either patent or occluded, may influence operative strategy. During the natural history of AAA, the onset of IMA occlusion can be acute with sudden and severe abdominal symptoms, requiring open surgery in an emergency in order to treat the aneurysm and the associated intestinal damages. A more progressive occlusion inducing transient signs, usually expressing a mild reversible ischemic colitis, does not contraindicate subsequent endografting for aneurysm exclusion, avoiding by evidence any IMA-based type II endoleak after endograft implantation. However, an asymptomatic silent IMA occlusion with functional intestinal collaterals is the best clinical situation before endovascular treatment of AAA.