Given the vague presenting symptomatology, lack of clear physical exam findings and the high mortality associated with a delayed diagnosis of acute mesenteric ischemia (AMI), one must maintain a “high index of suspicion” in anyone beyond the sixth decade of life who presents with diffuse abdominal pain out of proportion to the physical exam.

The management of an acute superior mesenteric artery (SMA) thrombosis is a surgical emergency that, if delayed in treatment, has a mortality of 50% to 80%. In 1951, Klass described an SMA embolectomy with two cases of SMA embolization; both patients died postoperatively from cardiac failure and hemorrhage, but this brought to light the importance of an aggressive approach to AMI. These patients present with an acute abdomen and prompt diagnosis, early resuscitation/anticoagulation, laparotomy without delay, and SMA embolectomy are associated with favorable outcomes. Oftentimes, a second look laparotomy within 24 hours is needed to survey segments of small intestine that may have progressed to necrosis.

Early in the clinical presentation, patients with an acute SMA embolus present with development of a continuous severe epigastric abdominal pain. During this initial intestinal ischemia period, the patient may also note nausea and loss of appetite. On physical exam, diminished bowel sounds are present. No rebound tenderness may be elicited, leading to the classic early clinical findings of AMI presenting as “pain out of proportion to the exam.” Early in the presentation, there are generally no major abnormalities in standard blood work. The absence of acidosis, elevated lactate levels, or leukocytosis should not reassure the clinician in the presence of marked abdominal pain. If not promptly recognized and treated, the intestinal ischemia progresses to frank bowel necrosis and possible perforation. Patients in this late stage present with diffuse abdominal pain and peritonitis, hypotension, fever, and leukocytosis. A recent analysis of 861 patients from the American College of Surgeons’ National Surgical Quality Improvement Program database (2007 to 2008) demonstrated that bowel resection for AMI was associated with a 30-day morbidity and mortality of 56.6% and 27.9%, respectively.

Because early in AMI laboratory values are nonspecific and the physical exam may not be very revealing, successful early diagnosis of AMI relies on the clinician maintaining a “high index of suspicion.” Prompt early diagnosis of an acute SMA embolus remains the best surgical therapy.

The SMA arises from the aorta at the level of the first lumbar vertebra at a sharp caudal angle; due to the near parallel course to the abdominal aorta, this is a favorable angle that allows emboli to preferentially lodge within the SMA, compared to the other visceral
vessels. An acute embolus usually lodges in the proximal segment of the SMA distal to its origin right at or just beyond the middle colic branch orifice, which is the first branch a few centimeters from the origin of the SMA. As a result, a patient with an SMA embolus will present with a proximal segment of jejunum that is spared from ischemia; the rest of the small bowel and proximal transverse colon is affected (Fig. 14.1A). However, in patients with chronic mesenteric ischemia (CMI), who have an in situ thrombosis of an origin atherosclerotic plaque that has a high-grade stenosis, the entire small bowel, right colon, and proximal transverse colon are affected (Fig. 14.1B). Hence, these two clinical scenarios can often be distinguished on laparotomy by the relative sparring of ischemia in the proximal jejunum. In modern day practice, preoperative imaging (see below in Preoperative planning) can generally reveal the difference between an SMA embolus and in situ thrombosis due the presence of chronic severe atherosclerotic disease in the latter scenario.

Computed tomography angiography (CTA) is an invaluable preoperative imaging modality in AMI.

Imaging with a CTA of the abdomen and pelvis is an ideal noninvasive imaging modality in the patient who has no significant underlying renal insufficiency or serious contrast allergy. Even in these cases, with proper prompt premedication against contrast nephropathy or allergy, an urgent CTA may be undertaken. CTA not only reveals the patency and presence of any atherosclerotic disease of the SMA and other mesenteric vessels, but also gives the clinician a good sense of the “health” of the small bowel wall; that is, whether it is thin and normal or thickened/congested, or whether any pneumatosis, consistent with bowel wall necrosis, is present. It of course also screens the patient for other explanations of patient signs and symptoms.

A duplex ultrasound may occasionally be obtained for the preoperative imaging. This could be done while the patient is being resuscitated so that no delay in definitive therapy occurs. An advantage is that it avoids a contrast bolus dose; hence, there is no risk of contrast nephropathy or allergy. However, a good examination is highly dependent on a skilled technician; patient’s obesity and gas in the bowel may significantly impair clear visualization of the mesenteric vessels. Moreover, this service may not be available during nonworking hours. All these factors make broad utilization of this noninvasive imaging modality unreliable.

The vascular surgeon may encounter instances in which a patient with an acute abdomen has been taken for an exploratory laparotomy by a general surgeon and he or she is called in by the general surgeon as an intraoperative consult when AMI is discovered. Clearly in these cases, no CTA or duplex ultrasound may be available preoperatively. In such cases, an SMA embolectomy should be undertaken (as described in “Technique”). If there is poor inflow or back bleeding noted after the embolectomy, either an aorto-SMA bypass is done or a mesenteric angiogram is obtained postoperatively (discussed in “Complications”).