Acute mesenteric ischemia is pain out of proportion to the exam. The test is multiphasic CTA, and delay is how small bowel dies. Colon ischemia is cramp, urge, and blood, usually left-sided, usually self-limited. Isolated right-colon ischemia sits on the seam: treat it as a possible SMA warning, not as ordinary colitis. This URL is the differentiator. The clinical colon-ischemia page is the other permalink.

Experienced teaching points

Clinical Pearls

Pain out of proportion: CTA, anticoagulation, call surgery. Do not start with a colonoscope.
Cramp then blood, left colon, stable abdomen: CT, then a limited colonoscopy if there is no peritonitis. That is colon ischemia.
Isolated right-colon ischemia is the overlap. Brandt and Feuerstadt treated it as a possible herald of SMA occlusion. CTA before you discharge.
Four AMI mechanisms: embolus, thrombosis on an ostial plaque, nonocclusive vasospasm, mesenteric venous thrombosis. They do not share one operation.
Lactate and peritoneal signs are late. A soft abdomen does not clear the SMA.

Clinical Bottom Line

Feature	Acute mesenteric ischemia	Colon ischemia
Gut	Small bowel (midgut, SMA). Right colon can tag along.	Colon, usually left-sided watersheds.
Bedside	Pain out of proportion. Bleeding is late.	Cramp, urge, blood within 24 hours.
First test	Multiphasic CTA. Do not wait for lactate to rise.	CT with IV contrast. Colonoscopy if no peritonitis, within 48 hours.
Angiography	Diagnostic and often therapeutic.	Not first-line for typical left-sided disease. Flow has usually returned.
Clock	Hours. Transmural small-bowel infarction is the failure mode.	Most recover with support. Hours still matter for IRCI, gangrene, and shock.
Who operates	Almost everyone with peritonitis, and many with a treatable SMA lesion even before that.	Minority: gangrene, peritonitis, failure of support, selected IRCI and pancolitis.

How do the bedside stories differ?

This permalink is the AMI versus colon-ischemia differentiator. The rectosigmoid watershed anatomy lives at Sudeck's point. They are not the same page.

Colon ischemia announces itself. Longstreth's 401 admissions: pain 87%, rectal bleeding 84%, diarrhea 56%. Montoro: the full triad in sequence in nearly half. The abdomen hurts. It is not usually rigid. Blood is part of the opening scene, not the finale.

AMI hides. The patient looks better than the pain score. The belly is soft. They may be writhing. Early labs can be boring. Lactate rises after bowel has already started to die. Peritoneal signs are the late chapter. If you wait for them, you are documenting transmural infarction, not preventing it.

ACG 2015 is blunt about the mix-up: if the presentation might be a herald of AMI (IRCI, severe pain without bleeding, atrial fibrillation), you do not treat it as ordinary colitis.

What test first?

For suspected AMI, multiphasic CTA is the first test. WSES 2022 and the ACG colon-ischemia guideline agree on the CTA trigger even though they were written for different diseases. Oral contrast is not the point. Arterial and portal venous phases are. ACG: CTA for any suspected IRCI and any case where AMI cannot be excluded (strong). If that study is negative and the story still sounds like SMA disease, conventional angiography is still in play (conditional).

For suspected colon ischemia without AMI features, CT with IV contrast is enough to start (ACG strong). Colonoscopy within 48 hours confirms mucosa if there is no peritonitis. Details, including when to stop the scope, live on the colon ischemia page.

Do not colonoscope a suspected AMI. Insufflation does not open an SMA. It can delay CTA and steal mucosal flow from whatever colon is still alive.

Why is isolated right-colon ischemia the overlap zone?

The SMA feeds the small bowel and the right colon. A thrombus or embolus that is not yet a full midgut disaster can declare itself as isolated right-colon ischemia. Sotiriadis and Brandt already showed IRCI has roughly double the mortality and several-fold the surgery rate of other maps. Feuerstadt then described the subset in whom IRCI is accompanied or followed by AMI. That sequence is the reason ACG wants CTA in every IRCI, on arrival or before discharge.

Pain without bleeding should read as IRCI or AMI, not as "colitis without hematochezia, send home." The clinical colon-ischemia page is where IRCI management sits. This page is why you do not skip the CTA.

What are the four AMI mechanisms?

They do not share one operation. Name the mechanism on the CTA before you pick a device or a knife.

Arterial embolus. Often atrial fibrillation. Lodges in the SMA distal to the middle colic, so the proximal jejunum may look spared. Embolectomy or endovascular clot toilet, plus anticoagulation, plus bowel assessment.
Arterial thrombosis. Ostial plaque, intestinal angina in the months before, weight loss. The occlusion sits at the origin. Endovascular stent or open bypass, depending on what the belly will tolerate.
Nonocclusive mesenteric ischemia (NOMI). Shock and vasopressors. The SMA is patent and thread-like. Fix the pump and the pressors. Papaverine or vasodilator infusion is the interventional remainder, not a routine stent.
Mesenteric venous thrombosis. Hypercoagulable, cirrhotic, or septic portal tree. Anticoagulation is the drug. Surgery is for dead bowel, not for the clot itself.

WSES 2022 still wants source control for dead bowel, a second look when the first look was ugly, and revascularization before resection when the bowel is borderline and the vessel is fixable. Quote that at the consult, not a remembered mortality percentage from a 1970s textbook. Numbers move with time to CTA.

Colon ischemia has a different default: most patients never meet a surgeon. Watershed anatomy that explains the left-colon map is Griffith and Sudeck. Do not paste those names onto an SMA embolus.

Selected references

Last reviewed September 20, 2026. Written for clinicians who have to decide, in the first hour, whether this abdomen is colon ischemia they can watch or mesenteric ischemia they cannot.

Topics

Ischemic Colitis

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