Mesenteric Ischaemia
MRCS Part A & B — Comprehensive Study Notes
Acute Mesenteric Ischaemia: Overview and Aetiologies
Acute mesenteric ischaemia (AMI) is a vascular emergency with mortality of 60–80% if not recognised and treated promptly. The condition results from sudden loss of intestinal blood supply, causing bowel ischaemia that progresses rapidly from mucosal ischaemia (reversible) to transmural necrosis and perforation (irreversible, fatal if not intervened). Four distinct aetiologies account for nearly all cases:
Aetiology Classification
| Aetiology | Incidence | Mechanism | Clinical Onset | Severity |
|---|---|---|---|---|
| SMA Embolism | ~50% | Cardiac embolus lodges in SMA (usually at middle colic artery origin) | Acute, sudden | Often complete ischaemia acutely; may stabilise with collaterals |
| SMA Thrombosis | ~25% | Acute thrombosis on chronic atherosclerotic SMA stenosis | Gradual onset over hours/days; prodrome of postprandial angina | More diffuse, chronic collaterals present initially |
| Mesenteric Venous Thrombosis (MVT) | ~10% | Thrombosis of superior mesenteric vein | Insidious, often delayed diagnosis | Bowel wall oedema; variable ischaemia progression |
| Non-occlusive Mesenteric Ischaemia (NOMI) | ~20% | Low-flow state causing splanchnic vasoconstriction and mucosal ischaemia; no mechanical occlusion | Progressive, related to underlying low-flow state | Patchy, superficial ischaemia initially; high mortality |
The Clinical Paradox: “Pain Out of Proportion”
The hallmark of acute mesenteric ischaemia is severe, constant abdominal pain that is DISPROPORTIONATE to the physical examination findings. Early in the course (before peritonitis), the abdomen may be soft and non-tender, or only mildly tender, while the patient complains of excruciating pain. This discrepancy—severe pain despite a relatively benign-appearing abdomen—should raise high suspicion for mesenteric ischaemia. The pain reflects visceral ischaemia, not peritoneal irritation (which develops later as necrosis and perforation occur). This paradox is why the diagnosis is often delayed; emergency physicians may underestimate the severity based on examination findings.