Mesenteric Ischemia

I.  Overview of Mesenteric Ischemia and Vascular Supply of the Gut

• Mesenteric Ischemia can be divided into Acute vs Chronic, and Small Bowel vs Colonic Ischemia.  Acute Mesenteric Ischemia can be further divided into Occlusive vs Non-occlusive etiologies.

It's important to understand the the vasculature supply of the gut – the 3 major arteries below come directly off the aorta:

• Celiac axis – supplies the stomach (also supplies the common hepatic and splenic arteries)
• Superior Mesenteric Artery – supplies the entire small intestine and about 2/3 of the transverse colon
• Inferior Mesenteric Artery – supplies distal transverse colon, descending colon, and sigmoid colon
• (Rectal arteries come off of the internal iliac artery)
• Extensive collateral circulation exists between the systems, with the major weaknesses being the so-called “watershed” areas between the SMA and IMA (splenic flexure), and IMA and rectal arteries (recto-sigmoid junction).

II.  Acute Mesenteric Ischemia of the Small Intestine
This is the most feared form of mesenteric ischemia, which carries a >60% mortality rate.
Clinical presentation is classically that of acute onset of severe periumbilical pain that is out of proportion to physical exam findings, +/- nausea/vomiting.  Peritoneal signs and sepsis indicate bowel infarction.  It is important to note that in nonocclusive disease, ~25% of patients may have no abdominal pain.
Etiologies:
A.  Occlusive:

1. Mesenteric Artery Embolism – ~50% of cases, e.g. from A-fib, endocarditis, LV thrombus, etc.  The SMA is most anatomically predisposed, due to its large caliber and narrow take-off angle from the aorta.  The IMA is rarely affected due to its small caliber.  The most commonly involved segment is the middle of the jejunum, as this is the most distant from collateral circulation of the celiac axis and IMA.
2. Mesenteric Artery Thrombosis – 15-25% of cases - this pathogenesis resembles that of plaque stenosis and/or rupture as in MI.  They have bad preexisting atherosclerosis, and often have a history of “Intestinal Angina” representing chronic mesenteric ischemia where they have postprandial pain, nausea/vomiting, early satiety, and weight loss.
3. Mesenteric Venous Thrombosis – 5% of cases - causes increased resistance which backflows causing bowel wall edema, decreased arterial flow, and bowel infarction.  This usually occurs in patients with hypercoagulable states, as well as portal hypertension, abdominal infections, malignancy, trauma, and pancreatitis.

B. Nonocclusive Ischemia: 20-30% of cases - Pathogenesis involves splanchnic hypoperfusion and vasoconstriction, often with vasospasm.  This occurs in patients with bad atherosclerotic disease who are hypotensive (i.e. sepsis, CHF, etc), often on pressors, or other meds that cause decreased intestinal blood flow or vasoconstriction (Digoxin, Cocaine, Diuretics).

Diagnosis of Mesenteric Ischemia:  

• No lab is diagnostic, but should be suspicious in the appropriate setting with elevated WBC and metabolic acidosis, especially lactic acidosis.  In fact...  ** Acute abdominal pain + metabolic acidosis = Mesenteric Ischemia until proven otherwise !!**
•  Imaging: X-rays are generally insensitive.  CTA and MRA are better, and gold standard remains angiography.

Treatment:

• Involves aggressive IVFs and hemodynamic support, discontinuation of vasopressors if possible, broad spectrum antibiotics, NG tube for gastric decompression and NPO status.  Anticoagulation has a role for occlusive mesenteric ischemia.
• Early angiography is indicated both for diagnosis and therapy – intraarterial vasodilators or thrombolysis/angioplasty is possible.
• Surgical consult should be obtained early and is the only chance of survival if bowel infarction or perforation occurs; even then, mortality is extremely high.

III.  Ischemic Colitis, as compared to Acute Mesenteric Ischemia of the Small Bowel

• Ischemic colitis usually involves non-occlusive pathophysiology and affects the watershed areas (splenic flexure and recto-sigmoid junction), and typically presents in older patients with mild abdominal pain/cramping and bloody diarrhea.  Unlikely mesenteric ischemia, these patients are generally not severaly ill, and 85% will spontaneously resolve in several days with supportive treatment (NPO, IVFs, Abxs).  Only ~15% progress to life-threatening gangrenous ischemia.  Diagnosis is typically made by history and colonoscopy, which shows pale mucosa with petechial bleeding. 
• So remember, on average, Acute Mesenteric Ischemia of the Small Bowel = VERY VERY BAD.  Ischemic Colitis = NOT SO BAD.

(Chanu Rhee MD, 5/16/11)