Ischemic colitis
Ischemic colitis is inflammation and injury of the colon caused by inadequate blood flow. It typically affects older adults and classically presents with crampy abdominal pain followed by bloody diarrhea.
Ischemic colitis is injury to the large intestine that occurs when its blood supply falls short of its metabolic needs. It is the most common form of intestinal ischemia and predominantly affects older adults with vascular disease. Unlike acute mesenteric ischemia of the small bowel — which is often caused by an embolus and is rapidly catastrophic — ischemic colitis is usually a low-flow (non-occlusive) injury that follows an episode of hypotension, dehydration, heart failure, or vasoconstriction.
The colon's anatomy explains where it strikes. The watershed areas — the splenic flexure, at the junction of the superior and inferior mesenteric artery territories, and the rectosigmoid junction, at the border of the inferior mesenteric and internal iliac supplies — sit farthest from their feeding vessels and are the first to suffer when perfusion drops. Ischemia damages the mucosa first, since the innermost layer is the most metabolically active and the last to receive oxygen.
Clinically, the classic picture is sudden crampy abdominal pain — often on the left side — followed within about a day by an urge to defecate and bloody diarrhea or hematochezia. Most cases involve only the mucosa and resolve with supportive care, but severe (gangrenous) ischemia can progress to necrosis, perforation, and peritonitis. Imaging may show thumbprinting from submucosal edema, and colonoscopy shows a segmental pattern of injury sparing the rectum.
On the USMLE Step 1, ischemic colitis is a favorite gastrointestinal pathology vignette: expect an elderly patient with atherosclerosis or recent hypotension, pain at the splenic flexure, and bloody diarrhea, with questions hinging on the watershed anatomy and the distinction from acute mesenteric ischemia.
Key takeaways
- Ischemic colitis is colon injury from inadequate blood flow, most common in older adults with vascular disease.
- Watershed areas — the splenic flexure and rectosigmoid junction — are most vulnerable because they lie between arterial territories.
- The classic presentation is crampy (often left-sided) abdominal pain followed by bloody diarrhea.
- Most cases are mucosal and self-limited, but severe ischemia can cause necrosis and perforation.
- USMLE Step 1 vignettes hinge on watershed anatomy and distinguishing ischemic colitis from acute mesenteric ischemia.