Babak J. Orandi
James H. Black III
Presentation
A 71-year-old woman with a remote but significant smoking history presents to the emergency department (ED) with 24 hours of abdominal pain, nausea, vomiting, and diarrhea. She was discharged from the hospital 1 week prior after an elective coronary artery bypass graft, which was significant only for an episode of newonset atrial fibrillation, which resolved after administration of an amiodarone bolus. In the ED, her vitals are as follows: temperature: 38.1°C, heart rate: 101, blood pressure: 148/67, respiratory rate: 16, and 98% saturation on 2 L of oxygen via nasal cannula. On physical examination, she is clearly uncomfortable. She has a number of well-healed incisions on her abdomen, including a right subcostal incision from an open cholecystectomy, a left lower-quadrant incision from an open appendectomy, a Pfannenstiel incision from two prior cesarean sections, and a lower midline incision from a total abdominal hysterectomy. Her abdomen is obese, distended, and diffusely and impressively tender to palpation throughout. Rectal exam reveals no gross blood, but a stool sample is guaiac positive.
Differential Diagnosis
Acute abdominal pain can pose a diagnostic challenge to clinicians as the presenting symptoms for a variety of etiologies are often nonspecific and overlapping. The differential diagnosis includes acute pancreatitis, abdominal aortic aneurysm, aortic dissection, myocardial infarction, acute diverticulitis, small bowel obstruction, peptic ulcer disease with perforation, and gastroenteritis. Though not applicable to this patient given her past surgical history, the differential also includes cholecystitis and appendicitis.
Workup
The patient undergoes CT angiography (CTA) for further evaluation of her abdominal pain, which reveals an abrupt cut-off in the superior mesenteric artery (SMA), approximately 4 cm distal to the vessel’s takeoff from the aorta. The small bowel is dilated, the walls are thickened, and there is trace free fluid in the pelvis. Laboratory testing is significant for a serum lactate level of 2.8 mmol/L, a leukocytosis of 16,200 cells/mL, a serum bicarbonate level of 19 mEq/L, an INR of 1.1, a partial thromboplastic time (PTT) of 22 seconds, and a prothrombin time (PT) of 13 seconds.
CTA has become the most important diagnostic test for acute mesenteric ischemia (AMI). While angiography has been the historic gold standard and has the benefit of also allowing for simultaneous endovascular revascularization options, CTA has the advantages of being more readily available and rapid. It also permits simultaneous evaluation of the bowel and the vasculature, as well as allowing for a more thorough evaluation of the abdominal cavity, which may rule out other etiologies, as the diagnosis is often in question prior to performing the CTA.
A number of laboratory abnormalities are variably present in AMI. Unfortunately, no specific, rapidly available serum marker exists for AMI in the way that cardiac enzymes are available for myocardial ischemia, for example. Worse yet, most of the markers that do rise in AMI only do so after transmural bowel infarction has already occurred. AMI causes a significant elevation in the white blood cell count, and lactic acidosis and serum amylasemia are often seen later in the course of the disease.
Diagnosis and Treatment
AMI is a relatively uncommon diagnosis, accounting for <1 in every 10,000 admissions. However, maintaining a high index of suspicion is critical because mortality is very high with this disease process and a delay in diagnosis and treatment can be fatal. Even if diagnosed within 24 hours, the mortality rate is 50%, and it climbs to over 70% if the diagnosis is established after that. AMI is typically the clinical manifestation of one of four processes: embolism, thrombosis, nonocclusive mesenteric ischemia (NOMI), or mesenteric venous thrombosis (MVT).
An embolic event, usually to the SMA, is the most common cause of AMI, accounting for approximately half of all cases (Figure 1). Risk factors include atrial fibrillation, congestive heart failure, a history of prior embolic events, and a recent myocardial infarction. Emboli tend to lodge in the SMA distal to the takeoff of the middle colic artery, which causes ischemia of the distal jejunum through the ascending colon, with sparing of the proximal jejunum and the transverse colon.
FIGURE 1 • Acute embolus to the SMA usually lodges in the region of the first branches of the SMA, with the length of small bowel ischemia less affected, usually only portions of the distal jejunum, ileum, and colon. (From Cameron JL, Sandone C. Atlas of Gastrointestinal Surgery. Vol. 2, 2nd ed. 2011, with permission from PMPH-USA, Ltd.)
Arterial thrombosis accounts for 20% of AMI cases. Many of these patients have extensive atherosclerotic disease in the mesenteric vasculature. A thorough history will often reveal abdominal pain after meals, weight loss, and food avoidance. Unlike in SMA embolism, which tends to occur slightly more distal, SMA thrombosis typically occurs at the origin of the vessel, which creates ischemia from the midduodenum to the splenic flexure (Figure 2).
FIGURE 2 • Acute thrombotic occlusion of the SMA usually occurs secondary to a proximal atherosclerotic plaque, yielding extensive ischemia of the entire small bowel and large bowel. (From Cameron JL, Sandone C. Atlas of Gastrointestinal Surgery. Vol. 2, 2nd ed. 2011, with permission from PMPH-USA, Ltd.)
NOMI results from a relative low-flow state and vasoconstriction, creating an imbalance between mesenteric oxygen supply and demand. These patients typically have a heavy burden of atherosclerotic disease and are critically ill, which predisposes them to NOMI, though the celiac, superior mesenteric, and inferior mesenteric arteries are all usually patent. These patients’ CTAs usually demonstrate narrowing of multiple branches of the SMA, alternating dilation and narrowing of the vessel, spasm of the SMA arcades, and impaired filling of intramural vessels. CTA often will show a flattened inferior vena cava, consistent with the low-flow etiology of NOMI.
MVT is a less common cause of AMI. The ischemia is secondary to venous engorgement, which eventually impairs arterial inflow, which is why MVT has a more subtle onset than do arterial thrombotic and embolic events. Significant ascites is often appreciated on imaging. Many patients with superior mesenteric vein (SMV) thrombosis are asymptomatic, but patients with an occluded SMV on CT and abdominal findings on physical exam may have AMI.
In general, the cornerstones of treatment for AMI include fluid resuscitation, correction of electrolyte abnormalities, intravenous antibiotics, anticoagulation, immediate revascularization, and resection of irreversibly necrotic bowel. Patients with NOMI and MVT typically only require surgery for necrotic bowel. NOMI patients may benefit from intra-arterial infusion of arterial vasodilators, including papaverine. MVT patients require immediate anticoagulation. Given this patient’s recent history of atrial fibrillation, cardiac disease, leukocytosis, low-grade fever, tachycardia, lactic acidemia, and pain out of proportion to physical exam, an emergent exploratory laparotomy is indicated.
Surgical Approach
Historically, open surgical exploration and revascularization, either catheter embolectomy or retrograde SMA bypass (Table 1), was the sole treatment for AMI. While it remains the gold standard, a number of authors advocate endovascular approaches to the treatment of AMI as the collective experience with catheter-based interventions grows. No prospective, randomized data exist as to the optimal approach, nor is it likely that they ever will, given how infrequently this disease process presents. The minimally invasive nature of endovascular surgery may be beneficial to patients who are often already critically ill; however, the strongest argument against this approach is that it does not permit the surgeon to assess bowel viability and perform bowel resections when indicated. If an endovascular approach is to be pursued, patient selection must be stringent and intensive monitoring for signs and symptoms of peritonitis is mandatory. The surgeon must have a low threshold for conversion to an open operation. More often than not, even with successful endovascular revascularization, a laparotomy is still required to assess the bowel (Figures 3–8).
TABLE 1. Key Technical Steps and Potential Pitfalls to SMA Revascularization (Figures 3–8)
FIGURE 3 • A long midline incision is best used to evaluate the viscera and provide ample exposure to identify suitable targets for revascularization. The towel rolls lifts the costal margin upward and facilitates exposure of the supraceliac aorta, which is usually spared from significant plaque burden and thus may provide suitable inflow for SMA revascularization. (From Cameron JL, Sandone C. Atlas of Gastrointestinal Surgery. Vol. 2, 2nd ed. 2011, with permission from PMPH-USA, Ltd.)
FIGURE 4 • Palpation of the root of the mesentery is the first step to determine the status of the mesenteric circulation. In thrombotic occlusion from proximal plaque, the pulse will be absent and a revascularization will be necessary. In acute embolus, a pulse will be palpable via transmission through fresh clot. (From Cameron JL, Sandone C. Atlas of Gastrointestinal Surgery. Vol. 2, 2nd ed. 2011, with permission from PMPH-USA, Ltd.)
FIGURE 5 • A transverse arteriotomy on the mid-SMA in the region of the middle colic can be used to extract the clot. (From Cameron JL, Sandone C. Atlas of Gastrointestinal Surgery. Vol. 2, 2nd ed. 2011, with permission from PMPH-USA, Ltd.)
FIGURE 6 • A four to five French embolectomy catheter is used to clear the clot and restore inflow from the SMA. Distally, careful passes of a three to four French embolectomy catheter are used for clot extraction. (From Cameron JL, Sandone C. Atlas of Gastrointestinal Surgery. Vol. 2, 2nd ed. 2011, with permission from PMPH-USA, Ltd.)
FIGURE 7 • Distant clot in the mesenteric arcade can be milked back to the arteriotomy to extract the clot burden. (From Cameron JL, Sandone C. Atlas of Gastrointestinal Surgery. Vol. 2, 2nd ed. 2011, with permission from PMPH-USA, Ltd.)
FIGURE 8 • Interrupted, nonabsorbable, monofilament closure of the arteriotomy is preferred to avoid narrowing the closed SMA. (From Cameron JL, Sandone C. Atlas of Gastrointestinal Surgery. Vol. 2, 2nd ed. 2011, with permission from PMPH-USA, Ltd.)
Special Intraoperative Considerations
Many surgeons advocate a mandatory “second look” operation within 24 to 36 hours with minimal bowel resection at the time of the first surgery. While no data support or refute this approach, its supporters suggest that it minimizes the amount of bowel that must be removed as this waiting period after revascularization renders salvageable bowel that at first glance might have been removed. In addition, avoiding bowel resection and leaving the patient with an open abdomen permits a rapid return to the ICU for resuscitation and stabilization, as many of the patients are critically ill at the time of their operation. Other authors have reported the use of a selective second-look strategy.
Postoperative Management
Patients should be continued on systemic heparin therapy in the immediate postoperative period. All patients will require anticoagulation and/or anti-platelet therapy. Because these patients tend to have systemic vascular disease, they require aggressive risk factor modification as much as possible.
Case Conclusion
The patient undergoes an exploratory laparotomy, which reveals extensively threatened bowel from the jejunum through the ascending colon. An embolectomy is performed, which reestablishes blood flow after the removal of a large embolus. The patient’s abdomen is left open with a wound vacuum system left in place. She is returned to the Surgical Intensive Care Unit for aggressive resuscitation. Approximately 36 hours later, she is returned to the operating room for a second look. Much of the previously threatened bowel has demonstrated significant improvement, though she still requires resection of the distal ileum. At that time, her abdomen is definitively closed. She is discharged to a rehabilitation facility on postoperative day seven, with warfarin anticoagulation, aspirin, a statin, and monthly vitamin B12 injections to compensate for the distal ileum resection.
TAKE HOME POINTS
· AMI is a relatively rare disease, but its high mortality rate mandates a low threshold for action.
· CTA is typically the diagnostic test of choice.
· The four most common causes of AMI are arterial embolus or thrombosis, NOMI, and MVT.
· Patients should receive intravenous heparin, fluid resuscitation, and broad-spectrum antibiotics as soon as possible.
· Open revascularization with bowel resection is the gold standard treatment for AMI. A second-look operation may minimize the amount of bowel needing resection.
· Endovascular revascularization has been used successfully in a number of cases, though its use does not preclude the need for a laparotomy to assess bowel viability and to resect if necessary.
SUGGESTED READINGS
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