AMI has many causes, depending on the underlying pathology. Emboli can come from cardiac thrombi that develop after myocardial infarction or in association with atrial fibrillation or valvular disease. Thrombi can come from atherosclerotic disease, aortic dissection or aneurysm, and decreased output from heart failure.
NOMI occurs in the setting of any low-flow state, such as congestive heart failure, sepsis, or cocaine use (vasoconstriction). Additionally, hypercoagulable states, such as protein-C or -S deficiency, pregnancy, cancer, or intra-abdominal infection, also predispose patients to developing thrombosis.
The most common presenting symptom in patients with AMI present is abdominal pain, which is typically severe, constant, and poorly localized.[1,2] Pain onset varies depending on the underlying pathology; for example, embolic disease usually results in abrupt pain onset, whereas NOMI may cause subacute symptom progression over several days.
The hallmark finding of AMI is pain out of proportion to physical findings, as patients rarely manifest the peritoneal signs or focal tenderness normally associated with abdominal emergencies. Diarrhea occurs in up to 50% of patients; nausea and vomiting are also frequently present. Similar to patients with cardiac ischemia, 20%-50% of patients may experience abdominal angina prior to developing AMI. Abdominal angina is a clinical syndrome of postprandial abdominal pain lasting up to 3 hours after meals, and is associated with a change in bowel movements, early satiety, and weight loss. However, postprandial pain and weight loss are more often seen in patients with chronic mesenteric ischemia, as opposed to patients with AMI.
Patients may also present with concurrent disease causing their AMI. For example, a congestive heart failure exacerbation or dehydration secondary to gastroenteritis may trigger patients to develop NOMI.
Diagnosing AMI involves a combination of clinical suspicion and imaging. Laboratory tests are typically nonspecific and not helpful in diagnosing AMI. Leukocytosis often develops as the disease progresses, and more than 75% of patients eventually develop white blood cell counts greater than 15,000 cells/mm3. Lactate levels generally rise late in the clinical course; levels that remain persistently normal suggest another diagnosis is more likely than AMI. However, levels within the reference range are frequently seen in the acute phase of the disease. Lactate levels also have poor specificity, at 42%-77%, even when positive.
Elevated serum amylase and potassium levels may be noted because the intestine produces amylase in response to injury and ischemia releases intracellular potassium. Metabolic acidosis and elevated LDH may also be present. Radiography is usually not helpful in diagnosing AMI; however, they may be helpful in ruling out other causes of abdominal catastrophes. In someone with excruciating pain associated with AMI, plain radiography can rule out a perforated viscus may be warranted.
The most important test for identifying mesenteric ischemia is angiography. CT angiography has largely supplanted traditional angiography due to its increased ease of use and minimally invasive approach. CT angiography has sensitivities for AMI reported between 96%-100%, and specificities between 89%-94%. Ultrasonography has shown good potential for identifying and evaluating mesenteric ischemia. It has a high specificity of 92%-100%, but suffers poor sensitivities of 70%-89%. Like other abdominal ultrasonography, the results vary depending on operator skill, patient body habitus, and bowel gas patterns. Magnetic resonance (MR) angiography has been shown to have sensitivities and specificities similar to CT angiography; however, CT angiography has higher spatial resolution and faster acquisition times, and better identification of calcified plaques. In addition, MRA is limited by both time and expense constraints, and it cannot be recommended as a first-line diagnostic choice, except in cases where CT contrast cannot be tolerated.[1,2,4] Traditional angiography are seldom used for diagnosing AMI in an ED setting, but when it is used, a lateral aortogram is the best view to image the mesenteric vessels as they arise from the aorta anteriorly.
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Cite this: Craig A. Goolsby. A 76-Year-Old Woman With Abdominal Pain - Medscape - Sep 24, 2010.