Gastrointestinal Emergencies Quiz

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Which mechanism is the most common cause of acute mesenteric ischaemia?

A Acute arterial occlusion of the superior mesenteric artery due to embolism ✓
B Non-occlusive mesenteric hypoperfusion due to low cardiac output
C Ischaemia secondary to strangulated abdominal hernia
D Mesenteric venous thrombosis associated with a hypercoagulable state
E Chronic atherosclerotic narrowing of multiple mesenteric vessels

The answer is A: Acute arterial occlusion of the superior mesenteric artery due to embolism

Explanation

Why A is correct

See Acute Mesenteric Ischaemia. Option A is correct. Acute mesenteric ischaemia is a sudden loss of blood flow to the bowel that can lead to bowel infarction (death of the bowel wall). The superior mesenteric artery (SMA) supplies the distal duodenum, the jejunum and ileum, the ascending colon and part of the transverse colon, so blocking it threatens a large stretch of bowel. The most common mechanism, accounting for about half of cases, is an embolus to the SMA. An embolus is a clot that forms elsewhere, most often in the heart, and travels in the blood until it lodges in a vessel too narrow for it to pass. Cardiac causes include atrial fibrillation, a clot on the wall of the heart after a heart attack and prosthetic heart valves. Classically the patient has diffuse, constant abdominal pain that is out of proportion to the findings on examination. The four types occur in roughly these proportions:

Embolus to the superior mesenteric artery: about 50 percent
Thrombosis (a clot forming in a narrowed, atherosclerotic artery): about 25 percent
Non-occlusive ischaemia from shock or low cardiac output: about 20 percent
Venous thrombosis: less than 10 percent

Vessel: the superior mesenteric artery supplies the distal duodenum, jejunum, ileum, ascending colon and part of the transverse colon.
Commonest cause: an embolus to the superior mesenteric artery, in about half of cases.
Source: the clot usually forms in the heart, for example in atrial fibrillation.
Classic sign: diffuse, constant abdominal pain out of proportion to the examination findings.

Why the other options are incorrect

B. Non-occlusive mesenteric hypoperfusion due to low cardiac output: Non-occlusive mesenteric ischaemia occurs when low blood flow, as in shock or heart failure, starves the bowel even though no vessel is blocked. It is an important cause, but it accounts for fewer cases than embolism. It would be the answer for ischaemia in a critically ill patient with low cardiac output.
C. Ischaemia secondary to strangulated abdominal hernia: A strangulated hernia can trap a loop of bowel and cut off its blood supply, but this is a mechanical obstruction of a segment rather than one of the mesenteric ischaemia mechanisms. The mesenteric vessels themselves are not the problem. It would be the answer for a sudden, tender hernia with bowel obstruction.
D. Mesenteric venous thrombosis associated with a hypercoagulable state: Mesenteric venous thrombosis is a clot in the veins that drain the bowel, so blood cannot leave the bowel wall. It is linked to blood clotting disorders, cancer and autoimmune disease, and accounts for a minority of cases. It would be the answer in a young patient with a clotting disorder and slowly progressive pain.
E. Chronic atherosclerotic narrowing of multiple mesenteric vessels: Chronic narrowing of several mesenteric arteries by atherosclerosis is the cause of chronic mesenteric ischaemia, which develops over time. The acute syndrome comes from sudden loss of blood flow. Chronic narrowing can predispose to acute thrombosis, but that is a separate and smaller group of cases.

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1
Which artery is most commonly eroded by a duodenal ulcer causing an upper gastrointestinal bleed?
2
A plain abdominal x‑ray is performed for suspected bowel obstruction. Which radiographic feature most reliably indicates that the dilated loops are from the small intestine rather than the large intestine?

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