A 68-year-old man is evaluated on postoperative day 2 following an elective open repair of… — USMLE Step 2 CK

A 68-year-old man is evaluated on postoperative day 2 following an elective open repair of a 6.5-cm infrarenal abdominal aortic aneurysm. The surgical note indicates that the inferior mesenteric artery was ligated during the procedure. Today, the patient reports the sudden onset of left lower quadrant abdominal pain. Two hours later, he passes a large volume of bright red blood mixed with stool. His temperature is $38.1^\circ\text{C}$ ( $100.6^\circ\text{F}$ ), blood pressure is $135/85\text{ mmHg}$ , heart rate is $98/\text{min}$ , and respiratory rate is $16/\text{min}$ . Physical examination reveals a soft abdomen with tenderness to deep palpation in the left lower quadrant. There is no rebound tenderness, guarding, or rigidity. The midline laparotomy incision is clean and intact. Laboratory studies show a leukocyte count of $14,500/\text{mm}^3$ and a serum lactate level of $1.8\text{ mmol/L}$ (normal: $0.5-2.2\text{ mmol/L}$ ). Which of the following is the most appropriate next step in management?

A
Stool assay for Clostridioides difficile toxins
B
Nasogastric tube decompression and serial abdominal examinations
Flexible sigmoidoscopyAnswer
D
Immediate exploratory laparotomy
E
Empiric initiation of broad-spectrum intravenous antibiotics

Answer

Flexible sigmoidoscopy

The correct answer is flexible sigmoidoscopy. This patient's presentation of new-onset left lower quadrant pain and bloody diarrhea following an open abdominal aortic aneurysm (AAA) repair is highly suspicious for ischemic colitis. During open infrarenal AAA repair, the inferior mesenteric artery (IMA) is routinely ligated, rendering the descending and sigmoid colon entirely dependent on collateral blood flow from the superior mesenteric artery via the marginal artery of Drummond. If this collateral flow is inadequate, colonic ischemia results. Because this patient is hemodynamically stable, has a normal serum lactate, and lacks peritoneal signs, the ischemia is likely limited to the mucosa. The most appropriate next step is to directly visualize the colonic mucosa via flexible sigmoidoscopy to confirm the diagnosis and assess severity. Full colonoscopy is generally avoided due to the higher risk of perforation in an ischemic bowel.

Step-by-Step Solution

Recognize the clinical syndrome based on the presentation.

The patient has left lower quadrant pain and bloody diarrhea on postoperative day 2, which is the classic presentation for ischemic colitis.

Identifying the specific symptoms and their timing distinguishes this from other postoperative complications like C. difficile infection or small bowel obstruction.

Correlate the symptoms with the surgical history.

The patient had an open abdominal aortic aneurysm repair with ligation of the inferior mesenteric artery (IMA).

Ligating the IMA makes the descending and sigmoid colon entirely dependent on collateral blood flow from the superior mesenteric artery via the marginal artery of Drummond. Inadequate collateral flow leads to left-sided colonic ischemia.

Assess the severity of the ischemia using clinical and laboratory markers.

The patient is hemodynamically stable, has a normal serum lactate (

1.8\text{ mmol/L}

), and lacks peritoneal signs (no rebound or guarding).

These findings suggest the ischemia is limited to the colonic mucosa rather than full-thickness (transmural) infarction, which would dictate an immediate surgical response.

Determine the most appropriate diagnostic intervention based on the severity.

Flexible sigmoidoscopy is selected as the next step.

Direct endoscopic visualization is required to confirm the diagnosis and stage the mucosal injury. Flexible sigmoidoscopy is preferred over full colonoscopy because it does not require bowel preparation and carries a lower risk of perforating the compromised bowel wall.

Key Concept

Management of ischemic colitis following open abdominal aortic aneurysm repair.

Estimated Time:1m 30s

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