Mesenteric Ischemia

Acute Mesenteric Ischemia

1. Anatomy
1. Arterial Circulation
• 3 main arteries supply the GI tract
• there is an extensive collateral network between these arteries


1. Celiac Artery (CA)
• supplies the foregut – distal esophagus, stomach, liver, spleen, duodenum
• arises from the ventral aspect of the infradiaphragmatic aorta
• surgical exposure requires division of the crura and median arcuate ligament


2. Superior Mesenteric Artery (SMA)
• supplies the midgut – from the jejunum to the mid transverse colon
• arises from the ventral infradiaphragmatic aorta just below the celiac artery
• on angiography, to fully visualize the origins of the CA and SMA, a lateral view must be obtained, as well as an AP view


3. Inferior Mesenteric Artery
• supplies the hindgut – from the mid transverse colon to the rectum
• arises from the lateral aspect of the of the infrarenal aorta


2. Collateral Circulation
• superior and inferior pancreaticoduodenal arteries are the primary collateral network between the CA and SMA



• the SMA and IMA are linked by the arc of Riolan centrally, the marginal artery of Drummond on the periphery of the colon, and unnamed retroperitoneal vessels



• the IMA and systemic circulation are linked by the internal iliac artery and the hemorrhoidal arteries


2. Pathophysiology
• in chronic mesenteric ischemia, gradual occlusion of 2 vessels is usually tolerated because the collateral network has time to grow and develop
• in acute mesenteric ischemia, occlusion of one main vessel is often sufficient to cause profound ischemia because the collateral network is undeveloped


1. SMA Embolus
• most common etiology of acute mesenteric ischemia (50%)
• most emboli originate from the heart as a result of atrial fibrillation or a myocardial infarction
• emboli preferentially lodge in the SMA because of its high flow rate, large diameter, and nearly parallel course to the aorta
• most emboli lodge 3 to 10 cm distal to the SMA origin
• proximal jejunum is usually spared


2. SMA Thrombosis
• accounts for 20% of cases
• occurs at the origin of the vessel
• chronic aortic and mesenteric atherosclerosis is present and patients often have a history of chronic mesenteric ischemia (acute-on-chronic ischemia)
• involvement of at least two of the major mesenteric arteries is required for symptom development
• patients often have significant collateral circulation
• entire midgut will be ischemic


3. Nonocclusive Mesenteric Ischemia
• 20% of cases
• caused by a low-flow state resulting from shock, vasopressor use, or hypovolemia
• mesenteric vessels may be normal
• mortality rate is high because of delay in diagnosis and the underlying medical condition of the patient


4. Mesenteric Venous Thrombosis
• 10% of cases
• SMV is most commonly involved (70%); the portal vein, splenic vein, and IMV account for the other 30%
• venous thrombosis leads to bowel wall edema and decreased perfusion
• primary risk factors include hypercoagulable states such as protein C or protein S deficiency
• secondary risk factors include malignancy or recent abdominal surgery (splenectomy)


3. Clinical Presentation
1. SMA Embolus
• sudden, severe periumbilical pain out of proportion to the physical exam
• nausea/vomiting and bowel emptying are common, but bloody diarrhea is rare
• many patients are in atrial fibrillation or have had a recent MI


2. SMA Thrombosis
• symptoms are often less dramatic than for an SMA embolus
• patients typically have a history of intestinal angina and weight loss from ‘food fear’
• patients may report worsening postprandial pain


3. Nonocclusive Mesenteric Ischemia
• ischemic symptoms may be overshadowed by the patient’s underlying medical condition
• ICU patients may present with progressive abdominal distention and acidosis


4. Mesenteric Venous Thrombosis
• slow to diagnose because the abdominal pain is often episodic and not very severe


4. Diagnosis
• requires a high index of suspicion
• EGD and colonoscopy are of no value in the acute setting
• barium enema is contraindicated if intestinal ischemia is suspected


1. Laboratory Tests
• common, but nonspecific, findings include leukocytosis, hemoconcentration, and an elevated serum lactate level
• normal lab findings do not exclude intestinal ischemia


2. Plain Abdominal X-rays
• useful for ruling out a perforated viscus or intestinal obstruction
• suspicious findings for infarcted bowel include bowel wall thickening, pneumatosis intestinalis, or portal venous gas


3. CT Angiography
• definitive diagnostic study and should be ordered promptly if mesenteric ischemia is suspected
• should be done without oral contrast, which can obscure the mesenteric vessels and obscure bowel wall enhancement
• emboli show an abrupt cutoff several centimeters from the SMA origin, typically at the middle colic orifice



• SMA thrombosis occurs at the origin of the vessel, and extensive collateral networks are often seen



• nonocclusive mesenteric ischemia will show a normal SMA with diffuse vasospasm of the mesenteric arcades
• catheter angiography also has a therapeutic role in infusion of catheter-directed vasodilator agents



• mesenteric venous thrombosis is diagnosed by the presence of venous filling defects on delayed imaging




5. Management
1. Initial Management
• fluid resuscitation
• correction of metabolic acidosis, with sodium bicarbonate if necessary
• antibiotics
• systemic heparinization to prevent thrombus propagation
• avoid vasopressors if possible


2. SMA Embolus
• embolectomy proceeds bowel resection
• SMA is found at the root of the mesentery where it passes over the third portion of the duodenum



• if the artery is not diseased, a transverse arteriotomy is made; otherwise a longitudinal arteriotomy is made
• a Fogarty catheter is passed proximally and distally to remove the embolus
• a transverse arteriotomy can be closed primarily; a longitudinal arteriotomy is closed with a vein or synthetic patch
• bowel viability is assessed once flow has been restored, and obviously necrotic bowel is resected
• Doppler assessment of antimesenteric pulsations or IV fluorescein followed by Wood lamp inspection can assist in determining bowel viability
• if bowel viability is questionable, it is left in discontinuity and a second-look procedure is done 24 – 48 hours later


3. SMA Thrombosis
• origins of the CA and SMA are severely atherosclerotic
• mesenteric bypass to a distal uninvolved segment is necessary for revascularization
• single-vessel or two-vessel bypass may be performed
• reversed saphenous vein or PTFE reinforced with rings may both be used as conduits
• may be done in an antegrade or retrograde fashion


1. Antegrade Bypass
• uses the supraceliac aorta, which is usually uninvolved by atherosclerosis
• allows the use of a short graft, which resists kinking when the bowel is returned to its normal position
• revascularization of the CA is usually performed to the common hepatic artery
• disadvantages include difficult exposure and the need for supraceliac aortic occlusion



2. Retrograde Bypass
• uses the infrarenal aorta or iliac artery for inflow
• advantages are the ease of exposure and no supraceliac aortic occlusion
• disadvantages include the use of inflow vessels that are usually heavily involved in atherosclerosis, and a long graft that is easily kinked



4. Nonocclusive Mesenteric Ischemia
• management is by catheter-directed infusion of a vasodilatory agent, usually papaverine
• patients are also heparinized
• all vasoconstricting agents must be stopped
• laparotomy will be required if peritonitis develops


5. SMV Thrombosis
• fluid resuscitation and anticoagulation are the primary treatment modalities
• venous thrombectomy has not been shown to be effective
• surgery is limited to bowel resection in patients with peritonitis
• patients with abdominal pain but who do not have peritonitis, may benefit from catheter-directed thrombolytic therapy
• the mesenteric venous circulation is reached by catheters placed in the splenic artery and SMA


6. Endovascular Treatment
• can only be used in cases where there is a very low suspicion for gangrenous bowel
• most useful in SMA thrombosis
• catheter-directed thrombolytic therapy is first used to dissolve the thrombus
• the diseased artery may then be treated with balloon angioplasty and stenting