Key Facts

Acute mesenteric ischaemia is a life-threatening emergency requiring a high index of suspicion. The classic triad of severe pain, minimal examination findings, and AF should prompt immediate CT angiography. Delay in diagnosis is the primary reason for the persistently high mortality.

Epidemiology

Acute mesenteric ischaemia: incidence ~1-2 per 100,000 per year; increasing with aging population. Accounts for ~1% of acute abdominal admissions. Mortality >60% (up to 80% if diagnosis delayed). Chronic mesenteric ischaemia is rarer but increasingly recognised.

Aetiology

• Arterial embolism (~50%): usually from left atrial thrombus (AF) or ventricular thrombus (post-MI); lodges at SMA origin or branch points
• Arterial thrombosis (~25%): in situ thrombosis of atherosclerotic SMA; usually proximal; may have preceding chronic symptoms
• Non-occlusive mesenteric ischaemia (NOMI) (~20%): splanchnic vasoconstriction in low-flow states (cardiogenic shock, sepsis, vasopressors, post-cardiac surgery)
• Mesenteric venous thrombosis (~5%): hypercoagulable states, portal hypertension, intra-abdominal sepsis

Pathophysiology

The SMA supplies the entire small bowel and right colon. Occlusion leads to mucosal ischaemia within minutes → transmural infarction within 6-12 hours → perforation and peritonitis. Initially, pain is visceral (poorly localised, severe) but examination is unremarkable ("pain out of proportion to findings"). As infarction progresses, peritoneal signs develop (late and ominous). Reperfusion injury following revascularisation can further damage already ischaemic bowel.

Acute Mesenteric Ischaemia

• Severe periumbilical/diffuse abdominal pain — sudden onset, constant
• Pain out of proportion to clinical findings (abdomen soft early on)
• Nausea, vomiting, diarrhoea (may be bloody)
• History of AF, recent MI, or vascular disease
• Rapid deterioration → peritonitis, shock, multi-organ failure

Chronic Mesenteric Ischaemia

• Postprandial pain ("intestinal angina") — 15-60 minutes after eating
• Food fear and weight loss (patients avoid eating due to pain)
• Abdominal bruit (not always present)
• Cardiovascular risk factors

Red Flags

• Severe pain disproportionate to examination in patient with AF
• Metabolic acidosis + raised lactate + abdominal pain (late — bowel infarction)
• Peritonitis (transmural necrosis — poor prognosis)
• New AF/recent MI + abdominal pain

Bedside

• A-E assessment: signs of shock, peritonitis
• ECG: AF, recent MI (embolic source)
• ABG/VBG: metabolic acidosis, raised lactate (late signs — bowel infarction)

Bloods

• Lactate: raised (late indicator — >2 mmol/L concerning, >4 mmol/L ominous)
• ABG: metabolic acidosis
• FBC: leucocytosis (>12 × 10⁹/L in most)
• CRP: raised
• Amylase: may be mildly raised (not specific)
• LDH: raised (tissue ischaemia)
• D-dimer: raised (non-specific but supports vascular event)
• Group and save/crossmatch: for surgery

Imaging

• CT angiography (CTA): investigation of choice — arterial and venous phases; sensitivity >90% for occlusive disease
  • Shows: thrombus/embolus in SMA/SMV, bowel wall thickening/thinning, pneumatosis intestinalis (gas in bowel wall — late), portal venous gas (very late), mesenteric stranding, free fluid
• Plain AXR: often normal early; late signs include thumbprinting, pneumatosis, portal venous gas

Special Tests

• Mesenteric duplex USS: first-line for chronic mesenteric ischaemia screening (fasting SMA peak systolic velocity >275 cm/s suggests >70% stenosis)
• Diagnostic laparoscopy/laparotomy: if clinical suspicion high and imaging inconclusive

Non-pharmacological

• Resuscitation: aggressive IV fluids, correct electrolytes and acidosis
• NBM, NG tube: if ileus/obstruction
• Broad-spectrum antibiotics: perioperative cover (piperacillin-tazobactam 4.5 g TDS or meropenem 1 g TDS)

Pharmacological

• IV unfractionated heparin: immediate anticoagulation (for embolic/thrombotic disease and venous thrombosis)
• NOMI: treat underlying cause (optimise cardiac output, wean vasopressors if possible); consider intra-arterial vasodilators (papaverine) via mesenteric catheter
• Long-term anticoagulation: warfarin/DOAC for AF-related embolism or mesenteric venous thrombosis
• Antiplatelet therapy: for atherosclerotic disease

Surgical/Interventional

• Acute arterial embolism: SMA embolectomy (open or endovascular catheter-directed thrombolysis/aspiration)
• Acute arterial thrombosis: mesenteric bypass or endovascular stenting
• Bowel resection: of non-viable bowel; often requires second-look laparotomy at 24-48 hours to reassess bowel viability
• Damage control surgery: in critically unwell patients
• Mesenteric venous thrombosis: anticoagulation (IV heparin → warfarin/DOAC × 6+ months); surgery only if peritonitis/infarction
• Chronic mesenteric ischaemia: SMA stenting (endovascular — first-line) or mesenteric bypass (open surgery)

Referral Criteria

• Emergency vascular surgery + GI surgery referral for all acute mesenteric ischaemia
• ITU/HDU for post-operative care
• Vascular surgery for chronic mesenteric ischaemia

Acute mesenteric ischaemia mortality is >60% overall and up to 80% if diagnosis is delayed beyond 12-24 hours. SMA embolism has the best prognosis (~50% mortality) because the proximal SMA is often spared. NOMI has the worst prognosis (~70-80% mortality) as it occurs in critically ill patients. Mesenteric venous thrombosis has better outcomes (~20-30% mortality) as onset is more gradual. Short bowel syndrome is a significant complication of extensive resection. Chronic mesenteric ischaemia: revascularisation is effective with >85% symptom relief.

Types of Mesenteric Ischaemia

CT Angiography Findings in Mesenteric Ischaemia