Acute Mesenteric Ischemia Workup

In general, laboratory studies are not helpful in diagnosing acute mesenteric ischemia (AMI). No serum marker is sensitive or specific enough to establish or exclude the diagnosis of AMI. If serious suspicion of AMI exists, radiographic studies should be performed without waiting for laboratory results.

A review of 180 consecutive malpractice claims at a Veterans Affairs Medical Center in Virginia over a 12-year period ending in 1998 revealed 7 cases involving AMI. Failure to make a timely diagnosis was alleged in 5 cases, and failure to administer anticoagulation was alleged in 1 case. The remaining allegation was failure to prevent nonocclusive mesenteric ischemia (NOMI).

Legal risk is reduced with early surgical consultation and the ordering of computed tomography (CT) scanning with contrast (CT angiography) as soon as AMI is noted in the differential diagnosis.

Also see Mesenteric Ischemia Imaging.

The complete blood count (CBC) may be within the reference range initially, but the white blood cell (WBC) count eventually rises as the disease progresses. Leukocytosis and/or leftward shift are observed in over 50% of cases. The hematocrit is elevated initially from hemoconcentration due to third-spacing, but it decreases with gastrointestinal (GI) bleeding.

Amylase levels are moderately elevated in over 50% of patients, but this finding is nonspecific. Phosphate levels were initially thought to be sensitive, but later studies showed a sensitivity of only 25-33%.

Metabolic acidosis is observed late in disease course, but this is a nonspecific finding. Lactate is elevated late in the clinical course. levels that are persistently within the reference range strongly indicate a diagnosis other than AMI (sensitivity 96%, specificity 60%).

D-dimer has been suggested as possibly helpful on the basis of 1 small clinical study reported in 2001 and 1 experimental study in rats.^{[9, 10]} Clinical experience is lacking to validate the role of D-dimer in the screening and diagnosis of AMI.

Plain films of the abdomen often appear normal in the presence of AMI; however, they are warranted to exclude identifiable causes of abdominal pain, such as perforated viscus with free intraperitoneal air.

Positive findings 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; see the image below), thumbprinting of the bowel wall, and portal vein gas are late findings. In one study of 23 cases of bowel infarction, 30% of the patients demonstrated focally edematous bowel wall (thumbprinting) or pneumatosis intestinalis.

CT scanning helps exclude other causes of abdominal pain. It may show pneumatosis intestinalis, portal vein gas, bowel wall or mesenteric edema, abnormal gas patterns, thumbprinting, streaking of mesentery, and solid organ infarction. Bowel wall edema is the most common finding, representing submucosal infiltration of fluid or hemorrhage into ischemic bowel. Arterial occlusion may show nonenhancement of the vessels. Mesenteric venous thrombosis (MVT) usually shows a thrombus in the superior mesenteric vein (SMV) or portal vein.

CT angiography has a sensitivity of 71-96% and a specificity of 92-94% for AMI. In clinical practice, CT angiography is ordered much more frequently than classic angiography. CT angiography is noninvasive, readily available, and the preferred modality for MVT (90% sensitivity). Serial CT angiograms can be used to monitor patients treated nonsurgically with anticoagulation.

Angiography has been the criterion standard to aid in diagnosis and preoperative planning. It also plays an important role in pharmacologic infusion therapy. However, angiography is less and less resorted to in clinical practice. Sensitivity is reported to be 88% for AMI.

An embolus appears as a sharp cutoff of flow near the origin of the middle colic artery. Thrombus appears as a more tapered occlusion near the origin of the superior mesenteric artery (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 is actually a second-line study in patients with a strong suspicion of MVT because false-negative findings are common. Findings with MVT include thrombus in the 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.

Duplex ultrasonography is highly specific (92-100%) but is not as sensitive (70-89%) as angiography. The examination cannot detect clots beyond the proximal main vessels, nor can it be used to diagnose NOMI. Ultrasonography is considered a second-line study for AMI. It is often less useful in the presence of dilated loops of bowel.

In some studies, ultrasonography appears to be as useful as CT scanning if duplex scanning is performed for MVT. 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.

Magnetic resonance imaging (MRI) and magnetic resonance angiography (MRA) yield findings similar to those of CT scanning in AMI. MRA has a sensitivity of 100% and a specificity of 91%. It is particularly effective for evaluating MVT. The main drawbacks are the expense and the time required. In the future, rapid MRA may supplant angiography.

Echocardiography findings may confirm the source of embolization or show valvular pathology.

Electrocardiography (ECG) may show myocardial infarction or atrial fibrillation.

Nasogastric tube decompression helps relieve distention and allows evaluation for upper gastrointestinal (GI) bleeding.

Diagnostic peritoneal lavage (DPL) may recover the serosanguineous fluid associated with bowel infarction; with the availability of CT scanning or magnetic resonance angiography (MRA), DPL is not a preferred study if AMI is suspected.

Foley catheterization allows monitoring of urinary output as an indicator for minimal fluid resuscitation.

In patients with intestinal angina, percutaneous transluminal angioplasty and stenting of the celiac and/or mesenteric arteries have been reported with variable short- and long-term patency rates. A multi-institutional, randomized, controlled clinical trial is needed to define the optimal conditions for their application.