Approach Considerations
Various laboratory studies may be performed for suspected acute mesenteric ischemia (AMI), but in general, such studies will not establish the diagnosis. At most, they suggest the diagnosis; they do not exclude it.
If serious suspicion of AMI exists, the clinician should order diagnostic imaging studies (eg, plain radiography, classic angiography, computed tomography [CT] angiography [CTA], magnetic resonance angiography [MRA], or ultrasonography [US]) without waiting for laboratory results. Electrocardiography (ECG) and diagnostic peritoneal lavage (DPL) may also be considered.
A review of 180 consecutive malpractice claims at a Veterans Affairs Medical Center over a 12-year period ending in 1998 revealed seven cases involving AMI. Failure to make a timely diagnosis was alleged in five cases, and failure to administer anticoagulation was alleged in one. The remaining allegation was failure to prevent nonocclusive mesenteric ischemia (NOMI). Legal risk is reduced with early surgical consultation and the ordering of CT contrast (CTA) as soon as AMI is noted in the differential diagnosis. (See also Mesenteric Ischemia Imaging.)
Laboratory Studies
Laboratory findings in AMI are nonspecific and generally unreliable. No serum marker is sensitive or specific enough to establish or exclude the diagnosis of AMI. However, the following laboratory examinations should be ordered:
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Prothrombin time (PT)
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Activated partial thromboplastin time (aPTT)
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International normalized ratio (INR)
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Complete blood count (CBC), which may reveal leukocytosis (bandemia) or hemoconcentration
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Chemistry studies that may show metabolic acidosis, increased amylase levels, or increased lactate dehydrogenase (LDH) levels
The CBC may be within the reference range initially, but the white blood cell (WBC) count eventually rises as the disease progresses. Leukocytosis, a leftward shift, or both are observed in more than 50% of cases. The hematocrit is elevated initially from hemoconcentration due to third-spacing, but it decreases with gastrointestinal (GI) bleeding.
Metabolic acidosis is observed late in the disease course, but this is a nonspecific finding. Amylase levels are moderately elevated in more than 50% of patients, but this is also nonspecific. Lactate is elevated late in the clinical course. levels that are persistently within the reference normal range strongly indicate a diagnosis other than AMI (sensitivity 90-96%, specificity 60-87%). [58, 59, 60] Phosphate levels were initially thought to be sensitive, but later studies showed a sensitivity of only 25-33%.
If a hypercoagulable state is suggested, additional laboratory studies, such as tests for protein C and S deficiencies and antithrombin III deficiency, may be ordered. Testing for abnormalities in lupus anticoagulant, anticardiolipin antibody, and platelet aggregation may be considered.
Several studies have found that serum D-dimer may be used as an early marker for AMI, though it appears to be insensitive. [61, 62, 63, 64] Additional clinical experience will be required to validate the role of D-dimer in the screening and diagnosis of AMI.
Plain Abdominal Radiography
Although plain abdominal films can yield a presumptive diagnosis in 20-30% of patients with AMI, they often appear normal in this setting and therefore should not be used to rule out AMI. However, plain abdominal films are warranted for excluding identifiable causes of abdominal pain, such as perforated viscus with free intraperitoneal air.
Positive findings on plain abdominal radiography are usually late and nonspecific and include ileus, small-bowel obstruction, edematous or thickened bowel walls, and paucity of gas in the intestines. More specific signs, such as pneumatosis intestinalis (ie, submucosal gas), thumbprinting of the bowel wall, and portal vein gas are late findings (see the images below). In a study of 23 cases of bowel infarction, 30% of the patients demonstrated focally edematous bowel wall (thumbprinting) or pneumatosis intestinalis.
Pneumatosis intestinalis (black stripes of air) in advanced acute mesenteric ischemia (AMI) with gangrenous bowel.
Pneumatosis intestinalis, one of few radiographic findings in patients with mesenteric ischemia.
Conventional Angiography
Angiography (see the image below) has been the criterion standard for diagnosis and preoperative planning in AMI. Various studies have reported sensitivities of 74-100% and a specificity of 100% for acute arterial occlusion. [54, 65, 66, 67, 68, 69, 70] Anteroposterior views demonstrate collateral pathways, whereas lateral projections show the origins of visceral branches. Currently, however, angiography is less and less resorted to in clinical practice.
Patients with embolization to the superior mesenteric artery (SMA)—that is, acute mesenteric arterial embolism (AMAE)—have an aortogram that demonstrates filling of the proximal SMA vessels to a sharp cutoff with no visualization of the distal vessels. Such an abrupt cutoff with the absence of collateral circulation is diagnostic, with nearly 100% sensitivity in acute embolic occlusion. [71]
Unlike patients with AMAE, those with acute mesenteric arterial thrombosis (AMAT) have well-developed collateral circulation as a consequence of long-standing, chronic ischemia. Thrombosis of the SMA generally appears as a more tapered occlusion near to or flush with the aortic origin of the vessel, resulting in an aortogram that fails to demonstrate any visualization of the SMA.
NOMI is characterized by narrowing of the origins of multiple SMA branches, alternating dilation and narrowing of the intestinal branches (ie, the “string of sausages” sign), spasm of the mesenteric arcades, and impaired filling of the intramural vessels.
Angiography has the added advantage of offering therapeutic as well as diagnostic options, including administration of intra-arterial thrombolytic agents for acute arterial thrombosis [72] and intra-arterial infusion of papaverine for all types of arterial ischemia.
The disadvantages of angiography are that it is highly invasive and unsuitable for critically ill patients; that it often is not readily available and may delay surgical management; and that nephrotoxicity may occur because of the effects of intravenous (IV) contrast on the kidneys. Angiography also has a relatively high false-negative rate in patients presenting early in the course of AMI. [73] Finally, arteriography can precipitate acute ischemia; thus, it is important to make sure that the patient is well hydrated.
Despite the disadvantages, if suspicion for AMI is high in an emergency setting, the treating physician should aggressively pursue conventional angiography if it is easily accessible. Prompt laparotomy is indicated if AMI is suspected but expeditious angiography is not available. If the case is not an emergency, it may be worthwhile to perform a dipyridamole-thallium scan to evaluate for coronary artery disease (CAD).
If mesenteric venous thrombosis (MVT) is strongly suspected, as in a patient with a history of hypercoagulability, angiography is considered a second-line study because of the high false-negative rate; abdominal CTA is preferable. Findings with angiography in MVT include thrombus in the superior mesenteric vein (SMV), reflux of contrast into the aorta, prolonged arterial phase with accumulation of contrast and thickened bowel walls, extravasation of contrast into bowel lumen, and filling defect in the portal vein or complete lack of venous phase.
CT and CTA
Contrast CT has proved very valuable for the assessment of mesenteric ischemia; current multiarray spiral scanners allow detailed examination of both the small bowel and the mesenteric vessels. [74, 75, 76] Multiple studies have cited sensitivities of 96-100% and specificities of 89-94%.
CT findings with a specificity greater than 95% for AMI include SMA or SMV thrombosis, intestinal pneumatosis, portal venous gas, lack of bowel-wall enhancement, and ischemia of other organs [77] ; less specific findings include distended bowel, absence of intestinal gas, thickened bowel wall, mesenteric or perienteric fat stranding, ascites, pneumoperitoneum, and air-fluid levels. [78, 79] (See the images below.)
Mesenteric venous air (red arrows) and free intraperitoneal air (white arrow.) Mesenteric venous air is common finding in advanced acute mesenteric ischemia. Courtesy of Brandon Dessecker, MD, Melanie Nukula, MD, and Robert Marx, DO.
Pronounced portal venous air seen within liver (red arrow), as well as free intraperitoneal air (white arrow) and pneumatosis intestinalis throughout bowel wall, which are classic findings for advanced acute mesenteric ischemia. Courtesy of Brandon Dessecker, MD, Melanie Nukula, MD, and Robert Marx, DO.
Bowel-wall edema (see the image below) is the most common finding, representing submucosal infiltration of fluid or hemorrhage into ischemic bowel. Arterial occlusion may show nonenhancement of the vessels. MVT usually shows a thrombus in the SMV or the portal vein.
CT scan (with contrast) of nonocclusive mesenteric ischemia with resulting bowel wall edema (arrows).
CTA has a sensitivity of 71-96% and a specificity of 92-94% for AMI. In current clinical practice, CTA is ordered much more frequently than classic angiography. CTA is noninvasive and readily available, and serial CT angiograms can be used to monitor patients treated nonsurgically with anticoagulation.
Abdominal CTA is considered by many to be the diagnostic test of choice if the index of suspicion for MVT is high and the patient is stable enough to undergo the procedure; sensitivities are greater than 90%. [80, 34, 81] CT findings include enlargement of the SMV or portal vein, a sharply defined vein wall with a rim of increased density, and low density (representing thrombus) within the vein (see the images below). [36]
CT scan demonstrating cavernous change of superior mesenteric vein as consequence of venous thrombosis.
MRI and MRA
Magnetic resonance imaging (MRI) and MRA yield findings similar to those of CT in AMI. MRA has a sensitivity of 100% and a specificity of 91%. It is particularly effective for evaluating MVT.
Despite its high sensitivity, MRI is not yet as practical as CT in the setting of suspected AMI, because of the cost and the time required for the examination. If these drawbacks can be eliminated or mitigated, rapid MRA may eventually supplant CTA.
Ultrasonography
Duplex US is highly specific (92-100%), but its sensitivity (70-89%) does not match that of angiography. It cannot detect clots beyond the proximal main vessels, nor can it be used to diagnose NOMI. US is considered a second-line study for AMI. It is often less useful in the presence of dilated loops of bowel.
In some studies, US appears to be as useful as CT if duplex scanning is performed for MVT at an early stage. It may show a thrombus or absent flow in the involved arteries or veins. Other possible findings include portal vein gas, biliary disease, free peritoneal fluid, thickened bowel wall, and intramural gas. Some researchers believe that duplex scans should be used as a first-line diagnostic tool in any patient thought to have MVT.
Echocardiography may confirm the source of embolization or show valvular pathology.
Other Tests
ECG may show myocardial infarction or atrial fibrillation.
Nasogastric tube decompression is diagnostically useful, both for helping to relieve distention and for facilitating evaluation for upper gastrointestinal bleeding.
DPL may recover the serosanguineous fluid associated with bowel infarction; with the availability of CTA or MRA, DPL is now very rarely (if ever) used when AMI is suspected.
Foley catheterization allows monitoring of urinary output as an indicator for minimal fluid resuscitation. Placement of a central line may be useful in hemodynamically unstable patients.
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Pneumatosis intestinalis (black stripes of air) in advanced acute mesenteric ischemia (AMI) with gangrenous bowel.
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CT scan (with contrast) of nonocclusive mesenteric ischemia with resulting bowel wall edema (arrows).
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Aortogram showing narrowing of superior mesenteric artery.
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Radiograph showing bowel spasm (early sign of ischemia).
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Gas in colon wall (typical of advanced ischemia).
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Ischemia stricture.
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Thumbprinting of bowel, characteristic of mesenteric artery ischemia.
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Pathologic findings 2 hours after bowel ischemia starts.
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Microscopic findings 24 hours after ischemia starts.
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Gross specimen showing hemorrhagic dead bowel after resection from patient with acute mesenteric ischemia.
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Pneumatosis intestinalis, one of few radiographic findings in patients with mesenteric ischemia.
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Diagnosis and treatment of intestinal ischemia (mesenteric venous thrombosis and major nonembolic arterial occlusion). Solid lines indicate accepted management plan; dashed lines indicate alternate management plan. DVT=deep vein thrombosis; SMA=superior mesenteric artery. Adapted from Gastroenterology. 2000 May;118(5):954-68.
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Diagnosis and treatment of intestinal ischemia (minor arterial occlusion or embolus, major embolus, and splanchnic vasoconstriction without occlusion). Solid lines indicate accepted management plan; dashed lines indicate alternate management plan. DVT=deep vein thrombosis; SMA=superior mesenteric artery. Adapted from Gastroenterology. 2000 May;118(5):954-68.
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Management of colon ischemia. Solid lines indicate accepted management plan; dashed lines indicate alternative management plan. BE=barium enema; NPO—nil per os (nothing by mouth); PLC=protein-losing colopathy; IBD=inflammatory bowel disease. Adapted from Gastroenterology. 2000 May;118(5):954-68.
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Complete aortic occlusion (Leriche syndrome) with acute embolism of superior mesenteric artery.
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Gross specimen of dead bowel.
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Meandering artery (radiographic sign of preexisting bowel ischemia).
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CT scan demonstrating thrombosis of superior mesenteric vein.
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CT scan demonstrating thrombosis of portal vein.
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CT scan demonstrating cavernous change of superior mesenteric vein as consequence of venous thrombosis.
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Mesenteric venous air (red arrows) and free intraperitoneal air (white arrow.) Mesenteric venous air is common finding in advanced acute mesenteric ischemia. Courtesy of Brandon Dessecker, MD, Melanie Nukula, MD, and Robert Marx, DO.
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Pronounced portal venous air seen within liver (red arrow), as well as free intraperitoneal air (white arrow) and pneumatosis intestinalis throughout bowel wall, which are classic findings for advanced acute mesenteric ischemia. Courtesy of Brandon Dessecker, MD, Melanie Nukula, MD, and Robert Marx, DO.
