Acute Mesenteric (Small Bowel) Ischemia

Etiology and PathogenesisTop

Acute mesenteric (small bowel) ischemia results from a sudden occlusion and/or insufficiency of the mesenteric arteries or veins that reduce intestinal perfusion and thus threaten the viability of the small intestine.

Causes include:

1) Mesenteric artery embolism (MAE; 45%-50%). The emboli mostly originate from the heart and sometimes from an atherosclerotic aorta. They typically lodge 3 to 10 cm distally to the origin of the superior mesenteric artery (SMA) given its relatively large diameter and low angle of takeoff. Twenty percent of SMA emboli occur with concurrent emboli in other arterial beds such as the spleen or kidney.

2) Nonocclusive mesenteric ischemia (NOMI; 20%-30%). Reduced blood flow due to low-flow states including shock; or splanchnic vasoconstriction from drugs including cocaine, ergotamine, vasopressin analogues, and norepinephrine.

3) Mesenteric arterial thrombosis (MAT; 15%-25%). This is often associated with preexisting chronic atherosclerotic disease at the origin of visceral arteries, which progresses to critical stenosis. Vasculitis, mesenteric dissection, and mycotic aneurysms may also lead to thrombus formation and occlusion.

4) Mesenteric venous thrombosis (MVT; 5%-15%): Etiologies associated with Virchow triad (stagnant blood flow, hypercoagulability, endothelial damage) can lead to mesenteric venous occlusion, which in turn may result in impaired microvascular perfusion.

Clinical Features and Natural HistoryTop

Mesenteric ischemia most commonly occurs in the elderly.

1. Signs and symptoms: Because symptoms are nonspecific, diagnosis requires a high degree of clinical suspicion. The dominant feature is severe abdominal pain that is out of proportion to physical examination and is most frequently located in the periumbilical area. Nausea and vomiting are frequent. Diarrhea, which may be bloody in transmural infarction, may also occur. In critically ill and ventilated patients, unexplained abdominal distention or gastrointestinal bleeding associated with increasing vasopressor support may be the only signs.

In the early stages mucosal ischemia leads to significant symptomatic abdominal pain with a fairly unremarkable physical examination due to lack of serosal/peritoneal irritation (ie, pain out of proportion). Bowel sounds may be hyperactive but can be reduced in later stages to complete cessation. Fever, diffuse peritonitis, and shock develop subsequently if intestinal necrosis or perforation occurs.

2. Risk factors: Risk factors differ by etiology. MAEs occur in patients with atrial fibrillation, left ventricle dysfunction, cardiac valvular disease, or recent myocardial infarction. MAT frequently occurs in patients with underlying chronic intestinal ischemia from atherosclerosis (ie, acute on chronic mesenteric ischemia; see Chronic Mesenteric (Small Bowel) Ischemia. MVT tends to occur in patients with acquired (eg, secondary to malignancy, oral contraceptive use) or hereditary hypercoagulable states (eg, factor V Leiden mutation, protein C or S deficiency, polycythemia). Trauma, recent abdominal surgery, and inflammatory conditions such as pancreatitis or diverticulitis are also risk factors for MVT. NOMI occurs frequently in critically ill ventilated patients.

DiagnosisTop

Diagnostic Tests

1. Laboratory tests: The most common findings are hemoconcentration and leukocytosis (usually >20×109/L). In patients developing intestinal necrosis, metabolic lactic acidosis, hyperphosphatemia, elevated levels of serum liver and pancreatic enzymes, elevate D-dimer levels, and elevated creatine kinase levels may be seen as early as in the first few hours.

2. Imaging studies: Abdominal radiographs are nonspecific, and >25% radiographs are normal. Radiographic features of intestinal necrosis (including pneumatosis [the presence of gas in the intestinal wall] or portal venous gas) appear late; they are only helpful when findings suggest perforation or a different diagnosis, such as obstruction. Biphasic computed tomography angiography (CTA) (without oral contrast) should be performed without delay (even in the presence of an acute kidney injury) and has replaced conventional angiography with its high sensitivity and specificity (94% and 95%, respectively). CTA helps identify the etiology of acute mesenteric ischemia (using pre-contrast, arterial, and venous phases) and the likely viability of the small bowel segment affected. However, arteriography is still considered by some to be the definitive diagnostic method if there is uncertainty (with sensitivity ~90%) or if required for therapeutic purposes. It should not delay surgery, if indicated.

TreatmentTop

Although specific treatment depends on etiology, the initial management for all patients focuses on resuscitation and prevention of complications.

1. Appropriate treatment of shock, including fluid resuscitation with crystalloid products (see Shock).

2. Conservative management: Broad-spectrum antibiotics should be initiated following diagnosis.Evidence 1Strong recommendation (benefits outweigh downsides; right action for all or almost all patients). Moderate Quality of Evidence (moderate confidence that we know true effects of the intervention). Quality of Evidence lowered due to significant indirectness and heterogeneity of the interventions and their results. Bala M, Catena F, Kashuk J, et al. Acute mesenteric ischemia: updated guidelines of the World Society of Emergency Surgery. World J Emerg Surg. 2022 19 Oct;17:54. doi: - 10.1186/s13017-022-00443-x. PMID:36261857; PMCID: PMC9580452 Medications that may exacerbate mesenteric ischemia, such as vasoconstricting agents, should be avoided. If required, vasopressors with a relatively smaller effect on mesenteric circulation, such as dobutamine, low-dose dopamine, and milrinone, are preferred. Electrolyte abnormalities (eg, hyperkalemia and metabolic acidosis) should be corrected. Consider nasogastric decompression.

3. Anticoagulation: Continuous infusion of unfractionated heparin for occlusive etiologies (MAT, MAE, MVT) is strongly recommended in the absence of contraindications to therapeutic anticoagulation. In the absence of signs of bowel necrosis, it may be the only treatment required for patients with MVT, with clinical improvement seen in 24 to 48 hours.Evidence 2Strong recommendation (benefits clearly outweigh downsides; right action for all or almost all patients). Moderate Quality of Evidence (moderate confidence that we know true effects of the intervention). Quality of Evidence lowered due to significant indirectness and heterogeneity of the interventions and their results. Bala M, Catena F, Kashuk J, et al. Acute mesenteric ischemia: updated guidelines of the World Society of Emergency Surgery. World J Emerg Surg. 2022 19 Oct;17:54. doi: - 10.1186/s13017-022-00443-x. PMID:36261857; PMCID: PMC9580452. Bala M, Kashuk J, Moore EE, et al. Acute mesenteric ischemia: guidelines of the World Society of Emergency Surgery. World J Emerg Surg. 2017 Aug 7;12:38. doi: 10.1186/s13017-017-0150-5. PMID: 28794797; PMCID: PMC5545843.  Unfractionated heparin infusion is particularly useful as it is easy to initiate, titrate, and cease therapy. Systemic thrombolytic therapy, understanding the risks of bleeding, may be considered but is rarely needed. Anticoagulation may be considered in NOMI although there are no clinical trials to support this practice.

4. Endovascular treatment (local thrombolysis, percutaneous thrombectomy or embolectomy, balloon angioplasty, stenting) may be considered and preferred when available in a timely manner for patients with MAE or MAT and without signs of intestinal necrosis.Evidence 3Weak recommendation (benefits likely outweigh downsides, but the balance is close or uncertain; an alternative course of action may be better for some patients). Moderate Quality of Evidence (moderate confidence that we know true effects of the intervention). Quality of Evidence lowered due to significant indirectness and heterogeneity of the interventions and their results. Bala M, Catena F, Kashuk J, et al. Acute mesenteric ischemia: updated guidelines of the World Society of Emergency Surgery. World J Emerg Surg. 2022 19 Oct;17:54. doi: - 10.1186/s13017-022-00443-x. PMID:36261857; PMCID: PMC9580452. There are no prospective randomized controlled trials comparing endovascular approaches and surgical management. Based on retrospective and large database studies, endovascular treatment may have lower associated mortality and bowel resection rate compared with surgical management; however, this may be due to selection bias, as patients undergoing surgery often have more advanced disease. In the case of arterial vasospasm or NOMI found on arteriography, intra-arterial administration of vasodilators (eg, papaverine, prostaglandins, nitroglycerin) may be considered if the patient is not responding to conservative management based on case reports and small series. There are no high-quality studies assessing endovascular treatment for MVT; however, options may exist including transarterial and transvenous thrombolysis in patients not responding to anticoagulation.

5. Early surgical treatment is indicated if perforation or necrosis is suspected. Otherwise, surgical treatment is usually reserved for failure of nonoperative management, as endovascular procedures are increasingly performed for revascularization. The benefit of early surgery in patients with suspected necrosis is direct visualization of bowel viability. The goals of surgery include assessment of bowel viability, resection of the nonviable bowel, and revascularization. Although laparotomy is often required, diagnostic laparoscopy can be considered if pneumoperitoneum can be safely achieved to assess bowel viability and to possibly avoid the need for laparotomy if no surgical intervention is warranted based on laparoscopic evaluation. Revascularization options include embolectomy and angioplasty in patients with embolism and thrombectomy or bypass anastomosis in patients with mesenteric artery thrombosis. Intraoperative arteriography (hybrid endovascular and operative procedures) or duplex ultrasound are useful adjuncts when there is diagnostic uncertainty regarding appropriate vascularization. Temporary abdominal wall closure for take-back laparotomy is a useful adjunct in patients requiring reassessment of bowel viability and in damage control situations.

6. Postoperative care: Close intensive care monitoring, vasopressor support, renal replacement therapy, and/or ongoing antibiotics may be required due to reperfusion injury and sepsis response. Therapeutic systemic anticoagulation with heparin or low-molecular-weight heparin (if no repeat surgical intervention is planned) is considered in the acute phase for all patients with occlusive etiology. Long-term anticoagulation or antiplatelet therapy may be necessary to prevent relapse. Nutrition with enteral or parenteral feeding can be tailored based on individual circumstances. Surveillance imaging with repeat CTA or duplex ultrasound is required for patients undergoing revascularization.

PrognosisTop

Every 6-hour delay in the diagnosis of acute mesenteric ischemia is associated with a 2-fold increase in the risk of short-term mortality. In patients with intestinal necrosis, short-term postoperative mortality may be as high as 90%. However, with early surgical intervention, mortality decreases to ~10% if the patient has an operation within 24 hours. Comorbidities, age, and etiology also affect mortality. Patients with arterial occlusions have worse survival rates.