Patient Presentation and Diagnosis

Early diagnosis of this condition is essential because it may decrease mortality in patients with AMI. ^{5 , 15} A prompt diagnosis itself represents a major challenge in AMI (of any cause) not only because of the rarity of the condition but also because of the relatively nonspecific nature of its presentation. Because the diagnosis is obscure, it is often not considered on initial presentation. ¹⁶

In patients with AMI caused by arterial embolic occlusion, rapid onset of periumbilical abdominal pain is common. The pain may be described as cramplike and continuous, is generally moderate to severe, and may even be resistant to opiates in some patients. In 30 to 50% of patients, the onset of abdominal pain is sudden; in another one-third of patients, the pain begins acutely over less than 1 hour. ^{16 , 17} With the severity of the abdominal pain, evidence of peritonitis might be expected on physical examination; however, this is generally not an early finding (hence the classic term pain out of proportion to the physical examination). ¹⁸ Following the onset of abdominal pain in many patients, a rapid evacuation of the bowel comes in the form of diarrhea (42%), which may be bloody, as well as vomiting (71%). ¹⁷ The abdominal pain and gastrointestinal symptoms usually occur as components of a triad of symptoms in these patients: (1) pain out of proportion to the physical examination, (2) vomiting and diarrhea, and (3) an obvious source of an embolus such as atrial fibrillation or recent myocardial infarction. ¹⁷ Given the overlap with cardiovascular disease, patients with mesenteric ischemia generally present in their sixth to eighth decade of life.

Because atherosclerosis is generally the cause in patients with AMI caused by arterial thrombosis, the thrombosis generally occurs with longstanding and progressive narrowing of the artery. Therefore, immediate symptoms may occur in the setting of insidious symptoms of CMI (discussed below).

Laboratory abnormalities are nearly always present in patients with this condition. Patients may demonstrate evidence of metabolic acidosis, with high lactate levels and leukocytosis. ¹⁹ Assessment of the D-dimer level may be useful, as a normal value may exclude the diagnosis of mesenteric ischemia. ²⁰ Unfortunately, however, there exists no rapid laboratory test that is pathognomonic or sufficient to establish the diagnosis. ¹²

In cases of embolic occlusion, the thromboembolus lodges several centimeters past the SMA origin as the vessel narrows distal to the takeoff of the first jejunal branches and middle colic artery. The SMA may be more prone to embolic occlusion because of its relatively acute takeoff from the aorta. CT images in these patients may demonstrate a discrete filling defect (thromboembolus) within the SMA (▶ Fig. 10.3); this filling defect may show a convex proximal surface and can be partially or totally occlusive. Because the jejunal branches and the middle colic artery may be spared, the corresponding bowel may have a normal appearance at CT. Most emboli are cardiogenic; therefore, intracardiac thrombi may also be identified on CT when patients are evaluated for mesenteric ischemia. Synchronous emboli are evident in approximately a quarter of patients. ¹⁷

Treatment

The most common management for acute embolic mesenteric ischemia is surgery. Patients with advanced disease and infarcted bowel who present with peritonitis or shock may proceed to laparotomy even if the diagnosis is not fully established preoperatively. Although invasive, surgical revascularization affords the benefit of direct visual inspection of the bowel to assess for viability. The bowel is evaluated at laparotomy; if the bowel is deemed nonviable, it is resected. In addition to visual inspection and clinical judgment, several other techniques are used for the assessment of viability, including palpation or Doppler ultrasound of distal arterial arcades and injection of fluorescein. Unfortunately, the sensitivity of such clinical assessment is relatively poor, and “second-look” laparotomy may be necessary to resect more nonviable bowel. ²¹ Revascularization with a bypass graft can be performed via an antegrade (using the supraceliac aorta as inflow) or retrograde (using the common iliac artery as inflow) approach. Balloon embolectomy is an alternative to bypass that can be performed with or without patch angioplasty of the superior mesenteric arteriotomy. ²² Retrograde stenting after open exposure of the SMA has also been used. ²³ Supportive measures may include fluid resuscitation to counteract fluid shifts from third spacing and initiation of broad-spectrum antibiotics to combat bacterial translocation across the ischemic intestinal wall. Anticoagulation with intravenous unfractionated heparin helps to prevent further clot propagation; this medication can be titrated off quickly if surgery becomes necessary.

Endovascular techniques for mesenteric revascularization in acute embolic mesenteric ischemia have gained interest in recent years because of reports of relatively favorable outcomes. ^{23 , 24 , 25} Catheter-directed thrombolysis, aspiration embolectomy, and angioplasty have all been used with good technical success. Vasodilators may be of benefit for cases in which poor flow is compounded by arterial vasospasm (▶ Fig. 10.3d). However, the literature describing the endovascular treatment of acute occlusive mesenteric ischemia remains limited. Relatively few comparative studies have been performed, and none of these studies were randomized. ^{24 , 25 , 26 , 27} Nonrandomized comparisons between endovascular and open approaches have demonstrated lower rates of complications such as respiratory failure (27 vs. 64%) and renal failure (27 vs. 50%) with endovascular techniques. ²⁷ In one study, mortality was significantly lower (36 vs. 50%) in the endovascular cohort if patients were able to avoid surgical revascularization compared to the group initially treated with open surgical revascularization. There is also some evidence that if bowel resection is required, endovascular treatment before resection may decrease the length of bowel resected. ²⁷ However, these results must be interpreted with caution given the quality of the data.