Ischemic colitis 

• Ischaemic bowel can be classified anatomically into
  • ischaemia of small bowel (mesenteric ischaemia)
    • Mortality for patients undergoing revascularization ranges from 44% to 90%.
    • Mesenteric ischaemia may be acute or chronic
    • Chronic mesenteric ischemia (also called intestinal angina) refers to episodic or constant intestinal hypoperfusion, which usually develops in patients with mesenteric atherosclerotic disease.
  • large bowel (ischaemic colitis)
    • is usually not related to blood vessel occlusion but to a compressive lesion in the large bowel (e.g., colon carcinoma, stricture, diverticulosis, fecal impaction)
    • is commonly transient, but may be chronic or recurrent. 
    • Although most cases will resolve on their own, 15% will develop gangrene and up to 20% will require surgical intervention.
• may be caused by
  • arterial thromboembolism
  • non-occlusive ischaemia (due to hypoperfusion, vasospasm)
  • venous thrombosis
• may be acute or chronic
• omental infarctions are rare

Risk Factors

Acute mesenteric ischaemia	Colonic ischaemia
Most patients over 50 years	90% of patients over 60 years
Acute precipitating cause is usual (e.g.,myocardial infarction, congestive heart failure, cardiac arrhythmias, hypotensive episodes)	Acute precipitating cause is rare
Predisposing lesion is uncommon (excluding atherosclerosis)	Associated predisposing lesion (e.g., colon carcinoma, stricture, diverticulosis, fecal impaction)
Patients usually appear seriously ill	Patients do not appear ill
Pain more severe; abdominal findings minimal early in course but become pronounced later	Mild abdominal pain with tenderness and guarding usual
Rectal bleeding and diarrhea uncommon	Moderate rectal bleeding or bloody diarrhea

Acute Mesenteric Ischaemia

• Pathophysiology
  • Arterial embolism (50%)
    • Risk factors to consider for arterial thromboembolism:
      • Age>60
      • AF
      • Recent AMI
      • Valvular heart disease
      • Aortic atlerosclerosis or aneurysm
      • Aortoiliac instrumentation or surgery.
  • Arterial thrombosis (15-25%)
  • Non-occlusive (due to hypoperfusion, vasospasm) (20-30%)
  • Venous thrombosis (5%)
    • occurs in younger patients and risk factors include:
      • hypercoagulable states
      • dehydration
      • portal hypertension
      • abdominal infections
      • blunt abdominal trauma
      • pancreatitis
      • splenectomy
      • malignancy in the portal region.
• Symptoms
  • Severity of abdominal pain out of proportion to physical findings.
  • May be associated with shock from dehydration and excessive fluid loss – mental confusion, tachycardia, tachypnoea and circulatory collapse.
  • In bowel infarction – peritoneal signs, haemodynamic instability, signs of sepsis with multiorgan failure.
• Management
  • Consideration of other life threatening diagnoses: acute pancreatitis, ruptured AAA, perforated viscus, septic shock from intraabdominal source.
  • History and examination concurrent with resuscitation.
  • Bedside tests: 12 lead ECG, VBG, FAST to rule out ruptured AAA if this is a differential. 
  • An elevated or rising lactate is suggestive of ischaemic bowel. 
  • However it is non-specific and a normal lactate does not exclude the diagnosis (sensitivity 77-100%, specificity 42%).
  • Blood tests should include FBC, EUC, LFT, lipase, Coagulation profile and Group and Hold.
  • Imaging
    • X-rays :
      • Not diagnostic of ischaemic bowel but may help in exclusion of differential diagnoses.
      • AXR if done, may show thumbprinting or thickening of bowel loops (40% of ischaemic gut), air in the portal vein is a late finding.
    • CT
      • focal or segmental bowel wall thickening
      • intestinal pneumatosis with portal vein gas
      • bowel dilation
      • mesenteric stranding
      • portomesenteric thrombosis
      • solid organ infarction,
      •  angiography may show thromboembolic occlusion which increases the specificity for the diagnosis (64% sensitivity, 92% specificity).
      • The absence of occlusions is not sufficiently sensitive to rule out ischaemic bowel.
  • Early notification of surgeons and ICU (and interventional radiology if appropriate).
  • Early laparotomy or arteriography.
  • Broad spectrum IV antibiotics (ampicillin and gentamicin or 3rd generation cephalosporin and metronidazole).

Ischaemic colitis

• Pathophysiology
  • Non-occlusive (95%)
  • Arterial thromboembolism
  • Mesenteric vein thrombosis.
• Symptoms
  • Mild-moderate abdominal pain, often left sided, diarrhoea, PR bleeding. Pain is not as severe as that of mesenteric ischaemia.
  • Risk factors are similar to those for mesenteric ischaemia
• Management
  • As above

Omental infarction

• Omental infarction is a rare cause of abdominal pain
• 85% of all reported cases of omental infarction have occurred in adults, most frequently in the age group of 40–50 years and twice more commonly in males than in female. 
• occur more commonly on the right
• associated with
  • obesity
  • local trauma
  • occupational vibration
  • heavy food intake
  • excessive exercise
  • coughing
  • excessive strain
  • sudden changes in position
  • use of laxatives.
• Secondary infarction can be induced by the following causes:
  • torsion due to adhesion between the omentum and pathologic foci such as
    • surgical scars
    • cysts
    • tumours
    • hernias
  • thrombosis due to various causes such as
    • hypercoagulopathy
    • vascular abnormalities;
  • congestion of the mesenteric vein due to systemic diseases such as right-sided heart failure.
• Omental infarction may be diagnosed by:
  • CT: triangular or oval-shaped fatty mass or an interspersed area with hyperattenuating streaky infiltration is pathognomonic for omental infarction.
  • Ultrasound is less useful, it may misinterpret the infarcted fatty lesion as normal intra-abdominal fat.
  • Omental infarction has previously been diagnosed on exploratory surgery, however this diagnostic modality is now infrequently used with the widespread availability of CT.