CHICAGO (FRONTLINE MEDICAL NEWS) – Postoperative ischemic colitis after abdominal aortic aneurysm (AAA) repair is a feared, potentially devastating complication with a mortality approaching 50%, but early diagnosis can mitigate that risk, Roy M. Fujitani, MD, said at a symposium on vascular surgery sponsored by Northwestern University in Chicago.

The key is to be familiar with the risk factors for this complication. Close surveillance of patients with multiple risk factors is particularly important for the first 4 days after AAA repair since more than three-quarters of cases are diagnosed in this time frame, according to Dr. Fujitani , professor and vice chair of surgery at University of California, Irvine.

The most common etiology of ischemic colitis following AAA repair is hypoperfusion of the mesenteric vasculature leading to nonocclusive ischemia. Caught early – in the initial hyperactive phase of colonic ischemia – the complication is typically transient and can be managed medically without further sequelae. Improvement is generally noted within a day or 2, with complete resolution within 1-2 weeks.

The earliest indicator that a patient is in the hyperactive phase of ischemic colitis following completion of an AAA repair can be defecation while still on the operating table.

“When you’ve just completed an operation and the patient has a bowel movement right on the operating table, that always makes me very, very concerned because of the likelihood of an associated ischemic colitis,” the surgeon noted.

A conscious patient in the first phase of ischemic colitis will describe an urgent desire to defecate, along with crampy pain and loose bowel movements with or without blood in the stool.

In the second, paralytic phase of ischemic colitis, the pain diminishes in intensity but becomes more continuous and diffuse, usually in the lateral borders of the abdomen. The abdomen becomes distended and much more tender, and there are no bowel sounds.

In patients whose ischemic colitis has been misdiagnosed or undiagnosed, the shock phase comes next. This is marked by massive fluid, protein, and electrolyte loss through the gangrenous mucosa. The result is severe dehydration, metabolic acidosis, and hypovolemic shock.

Nonocclusive colonic ischemia most often affects the watershed areas of the colon, such as the Sudeck point at the rectosigmoid junction.

The two other etiologies of ischemic colitis occurring as a complication of AAA repair are acute arterial occlusion, typically caused by iatrogenic embolization from a proximal source, often during endovascular aneurysm repair (EVAR), or rarely, venous thrombosis.

Making the diagnosis

When a patient is suspected of having ischemic colitis, one of the easiest ways of advancing toward a diagnosis is to obtain an abdominal plain x-ray, which classically shows thumb printing indicative of submucosal edema. CT with IV contrast typically shows bowel wall thickening, pericolonic fat stranding, and – most significantly – there may be free air within the colonic wall, an indicator of more advanced ischemia that occurs shortly before transmural gangrenous changes.

Colonoscopy is, however, the mainstay of diagnosis. It should be performed in any patient where postoperative ischemic colitis is suspected.

Ischemic colitis risk factors and outcomes

Dr. Fujitani was senior author of the largest ever study of risk factors for and outcomes of postoperative ischemic colitis in patients undergoing contemporary methods of open and endovascular AAA repair. This retrospective analysis of the American College of Surgeons National Surgical Quality Improvement Program (NSQIP) database included 3,486 patients who underwent AAA repair in U.S. hospitals during 2011-2012. Twelve percent had an open repair, while the other 88% underwent EVAR.

The incidence of postoperative ischemic colitis was 2.2%. The median time of diagnosis was on postoperative day 2. The rate was nearly threefold higher in the open repair group: 5.2% versus 1.8%. However, the open-repair group had a higher rate of emergency admission, ruptured aneurysm before surgery, and other high-risk features. Upon multivariate analysis, the adjusted risk of postoperative ischemic colitis was no longer significantly different in the open-repair and EVAR groups.

The mean hospital length of stay in patients with postoperative ischemic colitis was 20 days, compared with 5 days in those without the complication. The unadjusted in-hospital mortality rate in patients with ischemic colitis was 39% versus 4% in those without ischemic colitis.

Of the 75 patients who developed postoperative ischemic colitis, 37 were managed medically, 38 surgically.

“What was quite surprising was that there was a 56.8% in-hospital mortality in the surgically treated patients. The point being that if you end up having ischemic colitis, there’s a 50% chance you’ll end up requiring an operation, and if you do undergo an operation you have more than a 50% chance of succumbing from the process,” Dr. Fujitani observed.

Dr. Fujitani and his coinvestigators scrutinized a plethora of potential risk factors for postoperative ischemic colitis. Six emerged as significant upon multivariate analysis: ruptured aneurysm before surgery, with an associated adjusted 4.1-fold increased risk; need for intra- or postoperative transfusion, with a 6-fold increased risk; renal failure requiring dialysis, with a 3.9-fold risk; proximal extension of the aneurysm, with a 2.2-fold elevation in risk; diabetes, with a 1.9-fold risk; and female sex, with an adjusted 1.75-fold increased risk ( J Vasc Surg. 2016 Apr;63[4]:866-72 ).

Of note, these risk factors are largely unmodifiable, which underscores the importance of vigorous surveillance for possible signs of ischemic colitis during the first 4 days after AAA repair, especially in patients with multiple risk factors, Dr. Fujitani said.

Also, careful intraoperative assessment of the collateral mesenteric vascular anatomy is important in assessing a patient’s risk for postoperative ischemic colitis. This assessment should include the superior and inferior mesenteric arteries, as well as the celiac and internal iliac arteries. It’s worth bearing in mind that, even though collateral flow may appear adequate, it can be affected by hypovolemia, hypotension, or low cardiac output, the surgeon continued.

In the NSQIP data analysis, no patients who underwent reimplantation of the inferior mesenteric artery during open repair developed postoperative ischemic colitis. While this is an encouraging finding, the numbers were too small to draw definitive conclusions as to whether reimplantation of the artery is protective. It’s an important issue for further study, though, since so few of the recognized risk factors for the complication are modifiable, Dr. Fujitani noted.

He reported having no financial conflicts regarding his presentation.