Acute Mesenteric Ischaemia: The Abdominal Emergency You Can't Afford to Miss

Ever been on a busy A&E shift, called to see a patient writhing in agony from abdominal pain, only to find a completely soft, non-tender abdomen on examination? It’s a classic and unsettling diagnostic puzzle. This mismatch—severe pain with a surprisingly benign exam—is the hallmark of a condition every junior doctor needs to have high on their radar: acute mesenteric ischaemia.

Let's walk through a typical presentation that highlights why this diagnosis is so tricky—and so critical to make quickly.

A Classic Case in A&E

A 70-year-old woman is brought into A&E with severe abdominal pain that started suddenly 90 minutes ago. She can't get comfortable, and the pain is diffuse and overwhelming.

The F2 doctor examines her and notes: "Abdomen is soft and non-tender, with no guarding or rebound tenderness. Bowel sounds are present. No specific signs of cholecystitis or renal colic. A bedside ultrasound scan shows a thin-walled gallbladder with no stones, no hydronephrosis, a normal-calibre aorta, and no free fluid."

Despite receiving analgesia, her pain barely subsides. Routine blood tests are requested. They come back showing a mild leukocytosis, but her inflammatory markers, U&Es, and kidney function are all normal.

And yet, she remains in severe distress. The sheer intensity of her pain, completely at odds with the normal physical exam and initial tests, screams that something is wrong. This is the classic pain out of proportion to exam scenario that should immediately make you think of a vascular catastrophe.

The next step? An urgent CT scan of her abdomen with IV contrast, specifically a multiphasic CT covering the arterial and venous phases. The suspicion is acute mesenteric ischaemia, and the scan confirms it: a complete superior mesenteric artery occlusion (SMA occlusion).

What is Acute Mesenteric Ischaemia (AMI)?

Acute mesenteric ischaemia (AMI), also known as ischaemic bowel disease or, in its final stages, mesenteric infarction, is a life-threatening condition caused by a sudden reduction in blood flow to the intestines.

It's a notoriously difficult diagnosis because its early symptoms are non-specific. The patient reports intense, poorly localised abdominal pain that often doesn't respond to standard analgesics. The prognosis is poor, and the chance of survival depends almost entirely on how quickly the diagnosis is made and treatment is started.

AMI can be caused by several mechanisms:

• Arterial Embolism: An embolus, most commonly from the heart (e.g., in atrial fibrillation), travels and lodges in a mesenteric artery. This is the most frequent cause, often leading to an SMA embolism.

• Arterial Thrombosis: A blood clot forms directly within a mesenteric artery, usually at a site of pre-existing atherosclerotic disease. This is a common cause of mesenteric artery thrombosis.

• Mesenteric Venous Thrombosis: A clot forms in the veins draining the intestines, leading to congestion, oedema, and eventually ischaemia.

• Non-Occlusive Mesenteric Ischaemia (NOMI): This occurs without a specific blockage. Instead, it's caused by a state of severe, generalised low blood flow (splanchnic vasoconstriction), often seen in critically unwell patients in shock, on high-dose vasopressors, or following cocaine use.

This article will focus primarily on the occlusive causes, as these are the ones you're most likely to encounter first on an acute medical or surgical take.

Clinical Presentation: Reading the Subtle Signs

The cardinal symptom is severe abdominal pain. It's often described as diffuse or periumbilical and doesn't fit a neat pattern. Vomiting is also common.

The most important clinical clue in the early hours is the physical exam—or rather, the lack of findings.

Initially, the abdomen is soft, and palpation may not worsen the pain. You won't find guarding or signs of peritonitis. In the first few hours—the golden window for effective treatment—the physical exam is often deceptively normal. This discrepancy is the core feature of acute mesenteric ischaemia.

As the ischaemia progresses, the intestinal wall begins to break down. This leads to inflammation, necrosis, and eventually perforation. Only then do the classic signs of peritonitis appear: guarding, rebound tenderness, absent bowel sounds, and signs of shock. By the time these signs develop, the patient is often critically unwell and the chance of salvaging the bowel is much lower.

When Should You Suspect Acute Mesenteric Ischaemia?

Think of acute mesenteric ischaemia as the ultimate pain out of proportion diagnosis. The patient's distress level is a 10/10, but your exam findings are a 1/10.

Our advice: Think in terms of risk factors! If a patient presents with severe, diffuse abdominal pain and has any of the following risk factors, you must actively investigate for AMI.

Key risk factors to ask about:

• Atrial Fibrillation (AF): Especially if poorly controlled or if the patient is not compliant with anticoagulation therapy. Always get an ECG.

• Cardiovascular Disease: Ask about hypertension, known atherosclerosis, heart failure, and previous myocardial infarctions or strokes.

• Hypercoagulable States: A history of DVT/PE, known conditions like polycythaemia vera, or a family history of clotting disorders.

• Age: AMI is most common in patients over 65.

• Recreational Drug Use: Cocaine and amphetamines can cause severe splanchnic vasoconstriction, leading to non-occlusive mesenteric ischaemia (NOMI).

Diagnosis: Time is Bowel

While you'll always run a standard set of bloods for an acute abdomen, their role in diagnosing AMI is limited.

• Laboratory Tests: Patients may have a raised D-dimer, lactate, amylase, or LDH. However, these markers are neither sensitive enough to rule out the diagnosis nor specific enough to confirm it. In the early stages, all bloods can be completely normal.

• The Gold Standard: The definitive investigation is  CT angiography (CTA) of the abdomen. This scan visualises the mesenteric vessels and can identify an occlusion, confirm blood flow, and assess the bowel for signs of ischaemia. It's the one test that will confirm or exclude the diagnosis while also helping to rule out other causes of an acute abdomen.

• What about ultrasound? A point-of-care ultrasound in A&E is great for ruling out other immediate life-threats like a ruptured AAA or identifying gallstones. However, it cannot reliably diagnose acute mesenteric ischaemia.

Thinking Through the Differential Diagnosis

Because the main symptom is non-specific pain, the differential is broad and includes nearly every other acute abdominal emergency.

From our experience, the conditions that can most closely mimic AMI are:

• Aortic dissection

• Acute pancreatitis

• Perforated viscus (e.g., peptic ulcer)

• Internal hernia

A CTA of the abdomen is your best tool for differentiating between these conditions. It's also worth remembering extra-abdominal causes of abdominal pain, like an inferior MI.

A related condition to be aware of is chronic mesenteric ischaemia, often called abdominal angina or intestinal angina. This is caused by stable atherosclerotic disease, much like stable angina of the heart. Patients typically experience severe, crampy abdominal pain shortly after eating (when the gut's oxygen demand increases), which resolves after a few hours. These patients also need a mesenteric angiography and a referral to a vascular surgeon, but the situation is less immediately time-critical.

Management: A Race Against Time

Once the diagnosis is suspected or confirmed, management in A&E involves four key steps:

• Stabilisation: Secure IV access, give fluids, and correct any electrolyte imbalances.

• Analgesia: Provide strong pain relief.

• Anticoagulation: After confirming the diagnosis and discussing it with the surgical team, start an unfractionated heparin infusion. A typical starting regimen is a bolus followed by a continuous infusion, but always follow your local hospital trust guidelines.

• Urgent Referral: This is a surgical emergency. Contact the on-call general and vascular surgery registrars immediately.

Definitive treatment aims to restore blood flow. This might involve endovascular techniques like local thrombolysis, mechanical thrombectomy, or SMA angioplasty. In other cases, or if the bowel is already necrotic, the patient will need to go to theatre for a laparotomy and resection of  non-viable bowel.

Every hour of delay dramatically increases mortality. There's no clear time limit after which treatment is futile, as the extent of damage depends on the location of the occlusion and any collateral blood supply. The goal is always to achieve reperfusion as quickly as possible.

The CT Scanner Dilemma: To Scan or Not to Scan?

The diagnosis of AMI, and indeed many acute abdominal emergencies, hinges on getting a CT scan. So, should we just scan every patient with severe abdominal pain?

While  CT is a powerful tool, performing it on every patient would quickly overwhelm any radiology department and lead to significant delays. It also involves radiation exposure and contrast risks that aren't justified in patients at low risk of serious pathology.

This is where your clinical judgement is essential. By carefully assessing the clinical presentation and risk factors, you can narrow the differential and select the patients who truly need urgent, advanced imaging. When you request the scan, telling the radiologist that you are specifically looking for a superior mesenteric artery occlusion can speed up the reporting process and improve accuracy.

Key Takeaways for Your Next On-Call Shift

• Suspect AMI in any patient with severe abdominal pain but minimal or no findings on physical examination. This pain out of proportion is your biggest clue.

• Check the risk factors. A patient with AF and sudden, severe abdominal pain should be considered to have a mesenteric artery embolism until proven otherwise.

• Don't be falsely reassured by normal blood tests. They are unreliable in the early stages. The definitive test is  CT angiography of the abdomen.

• Time is bowel. Delays in diagnosis are fatal. Act fast.

• Once diagnosed, get the patient stabilised, start heparin (as per local protocol), and make an urgent referral to the vascular and general surgical teams.