Intestinal ischaemia

Intestinal ischaemia refers to vascular compromise of the bowel which in the acute setting has a very high mortality if not treated expediently. Diagnosis is often straight forward provided appropriate imaging is obtained and sometimes subtle findings sought out. The disease can be arbitrarily classified into broad groups according to time of onset, or the portion of bowel involved or the underlying cause

• acute or chronic
• occlusive or non-occlusive
• small bowel or large bowel
• superior mesenteric artery  or vein occlusion
• small bowel obstruction

This article is a general discussion. For discussion of specific types of intestinal ischemia, please refer to the following:

• acute superior mesenteric artery occlusion
• acute superior mesenteric vein occlusion
• small bowel ischemia
• large bowel ischemia (ischaemic colitis) 
• small bowel obstruction
• chronic intestinal ischaemia

Pathology

Impairment of normal vascular supply can result from a number of insults including:

• general hypotension / hypoxia especially in the setting of arterial insufficiency due to stenosis
• arterial occlusion
• bowel obstruction
• venous outflow obstruction

In other words anything which result in a deficiency in the normal supply of blood and metabolites to the bowel can result in ischemia. 

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 ischemia is severe enough, and is not relieved quickly, then a predictable sequence of events will usually be observed:

1. necrosis of the bowel wall
2. bacteria proliferation in the bowel wall, releasing gas in the wall itself (pneumatosis intestinalis)
3. gas goes through mesenteric vessels into portal vein (pneumatosis portalis)
4. sepsis and / or intestinal perforation
5. death

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. 

CT

Technique

In general 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 ⁷. 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:

1. non-contrast
2. arterial phase (e.g. triggered when abdominal aorta reaches >100HU)
3. 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
• 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
• 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. 

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.