Mesenteric ischaemia, also commonly referred to as bowel or intestinal ischaemia, refers to vascular compromise of the bowel and its mesentery that in the acute setting has a very high mortality if not treated expediently. Mesenteric ischaemia is far more commonly acute than chronic in aetiology. This article is focused on acute mesenteric ischaemia.
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Clinical presentation
The presentation can vary with the underlying cause. Severe abdominal pain that is disproportionate to examination findings and that responds poorly to analgesia is a classic mode of presentation for acute mesenteric ischaemia 15.
Pathology
Bowel ischaemia severity ranges from mild (generally transient superficial changes of intestinal mucosa) to more dangerous and potentially life-threatening transmural bowel wall necrosis 1. If ischaemia is severe enough and is not relieved quickly, then a predictable sequence of events will usually be observed:
necrosis of the bowel wall
bacteria proliferation in the bowel wall, releasing gas in the wall itself (pneumatosis intestinalis)
gas goes through mesenteric vessels into the portal vein (pneumatosis portalis)
sepsis and/or intestinal perforation
death
Aetiology
Mesenteric ischaemia can be classified into broad groups according to aetiology 13:
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acute mesenteric ischaemia (95% cases)
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arterial occlusive mesenteric ischaemia (60-85% cases)
non-occlusive mesenteric ischaemia (NOMI) (15-30% cases)
veno-occlusive mesenteric ischaemia / venous acute mesenteric ischaemia (VAMI) (5-15% cases)
mixed: e.g. strangulating bowel obstruction
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Location
Sometimes it may be useful to think of mesenteric ischaemia in the setting of location:
large bowel ischaemia (ischaemic colitis)
Radiographic features
Although historically catheter angiography was the gold standard for imaging of suspected intestinal ischaemia, CT has replaced it, with its ability to volumetrically assess the whole abdomen in multiple vascular phases, e.g. arterial, portal venous, delayed. It also has the added advantage of being able to diagnose alternative causes of acute abdominal pain. As such CT is now the investigation of choice for patients with suspected intestinal ischaemia 16.
CT
Technique
In stable patients, CT of the abdomen and pelvis should be performed with intravenous contrast and a neutral luminal contrast (e.g. water) so that bowel wall enhancement and thickness can be adequately assessed. However, in unstable patients, oral contrast may not be feasible because patient may not be able to tolerate it or the lack of bowel peristalsis to move the contrast into the intestines 7.
Ideally, positive oral contrast is best avoided to better delineate wall enhancement. Administration of positive rectal contrast may help define colonic wall thickening but is more useful when it is the distal colon which is affected (inferior mesenteric artery) - see ischaemic colitis.
Multiple contrast phases are typically obtained:
non-contrast: to identify calcifications, clots, in the blood vessels and bleeding in the bowel lumen 7; however, some studies demonstrate that this phase is not necessary for the diagnosis of acute mesenteric ischaemia 10
arterial phase: e.g. triggered when abdominal aorta reaches >100 HU
portal venous phase: e.g. 30 seconds after arterial phase finishes
Findings
Imaging features can vary depending on the time course and aetiology and are therefore discussed separately in the articles above. A number of features are however common to most advanced acute cases and result from the bowel wall necrosis and perforation:
pneumatosis intestinalis: gas in intestinal wall
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pneumatosis portalis: gas in the portal vein or in mesenteric vein
can be differentiated by pneumobilia because gas usually reaches the periphery of the liver while pneumobilia is usually about 2 cm short of external liver border, and is more clustered at the hilum
pneumoperitoneum: perforation of the bowel
submucosal haemorrhage: sensitivity for diagnosis is low (10%) with all true-positive cases having other CT findings present at diagnosis 10
variable amounts of free fluid
It is important to note that bowel wall thickness is not increased in all causes, and can in fact be thinned in complete arterial occlusion or bowel obstruction 7.
Dual-energy CT
The addition of iodine maps and 40 keV monoenergetic images to standard single energy CT images was found to increase reader confidence and accuracy in diagnosing acute bowel ischaemia. Ischaemic segments have been found to have lower densities and iodine concentrations compared to non-ischaemic segments 12.
Treatment and prognosis
Although treatment will vary according to the severity and cause of the ischaemia, in general treatment is surgical. The bowel needs to be assessed for viability and if necrotic needs to be resected. In some instances, endovascular thrombolysis/thrombectomy may be beneficial 8. Mortality rate is high at ~60% (range 30-90%) 13.
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