Mesenteric ischaemia (acute mesenteric arterial occlusion; mesenteric venous occlusion)
DEFINITION AND TYPES
Mesenteric ischaemia occurs when intestinal blood flow is insufficient for metabolic demand. Acute forms include arterial embolism, arterial thrombosis, mesenteric venous thrombosis and non-occlusive mesenteric ischaemia (NOMI). Chronic mesenteric ischaemia is usually due to atherosclerotic stenosis of major mesenteric arteries.
AETIOLOGY AND RISK FACTORS
Arterial emboli commonly arise from the left atrium in atrial fibrillation or from cardiac/ aortic sources and often lodge in the superior mesenteric artery. Arterial thrombosis occurs on pre-existing atherosclerotic disease, often at the vessel origin. Venous thrombosis is associated with hypercoagulability, malignancy, cirrhosis/portal hypertension, pancreatitis, inflammation and inherited thrombophilia. NOMI occurs with low-flow states, shock, heart failure, sepsis and vasoconstrictor exposure.
PATHOPHYSIOLOGY
Reduced perfusion causes mucosal injury first, followed by transmural infarction if flow is not restored. Mucosal barrier failure allows bacterial translocation, endotoxin release, peritonitis and septic shock. Venous thrombosis causes congestion, oedema and haemorrhagic infarction. Early clinical findings may be deceptively mild despite severe ischaemia.
CLINICAL FEATURES
Acute arterial occlusion classically causes sudden severe abdominal pain out of proportion to early physical findings, often with vomiting and diarrhoea; later there may be bloody stool, guarding, rigidity, shock and sepsis. Mesenteric venous thrombosis may evolve more gradually with diffuse pain and nausea. NOMI occurs in critically ill patients and may present as unexplained abdominal distension, feeding intolerance, GI bleeding or worsening shock. Chronic disease causes postprandial pain, food fear and weight loss.
DIAGNOSIS
Maintain high suspicion in older patients with atrial fibrillation, atherosclerosis or low-flow states. Obtain FBC, renal function, electrolytes, blood gas and lactate, but normal lactate does not exclude early disease. Urgent CT angiography of abdomen/pelvis is the investigation of choice and should not be delayed for oral contrast. It can show arterial/venous occlusion, bowel-wall changes, pneumatosis, portal venous gas and reduced enhancement. Catheter angiography may be diagnostic and therapeutic in selected cases.