Mesenteric Ischemia
Mesenteric ischemia offers a difficult and challenging problem in both diagnosis and treatment. Mortality associated with mesenteric ischemia remains high and in direct relationship to the extent of bowel infarction. Although time is of the essence, delays in diagnosis are common and usually result from failure to consider mesenteric ischemia in the differential diagnosis of abdominal pain. There is a wide spectrum of bowel ischemia, ranging from focal areas of ischemia to global transmural necrosis with a systemic inflammatory response resulting in bacterial translocation, endotoxemia, and sepsis. These processes do not necessarily subside when perfusion is restored. Reperfusion injures tissues both locally and systemically by producing harmful free radicals after oxygen is reintroduced to ischemic areas, which can lead to acute complications such as cardiac arrest or more gradual pathological activation of the immune response, resulting in multiorgan failure. Mesenteric ischemia has historically been divided into two broad categories: acute and chronic mesenteric ischemia.
Acute Mesenteric Ischemia
Acute mesenteric ischemia is caused by four distinct problems that compromise blood flow, including (1) acute embolic occlusion, (2) acute thrombotic occlusion, (3) nonocclusive mesenteric ischemia, and (4) mesenteric vein thrombosis. Early in the clinical course of an acute ischemic event, abdominal pain is typically severe and out of proportion to the findings on physical examination. This pain is associated with diffuse intestinal spasm that is manifest as vomiting, diarrhea, or both. On physical exam, the abdomen is soft and bowel sounds are hyperactive. As ischemia progresses bowel dilatation occurs, bowel activity decreases, and inflammation moves to the parietal peritoneum, resulting in peritoneal signs. At this point the ischemia is fairly advanced.
Diagnosis of Acute Forms of Mesenteric Ischemia
Because of the rarity of mesenteric ischemia, it may not be considered in the differential diagnosis of patients presenting with abdominal pain. Specific laboratory tests are not available, partly because of the ability of the liver to clear many of the products of ischemia released by the gut. High leukocytosis is usually present but is, obviously, nonspecific. During the past decade, technological improvements in duplex ultrasonography have made it more reliable in imaging the mesenteric circulation and identifying flow disturbances. When visualization is adequate, this is a useful screening test, especially in acute thrombotic ischemia and mesenteric venous thrombosis. Computed tomography also provides important clues to acute mesenteric ischemia and is diagnostic for mesenteric venous thrombosis. Atherosclerosis of the aorta and mesenteric ostia, lack of contrast in the proximal celiac or SMA, and bowel wall thickening may all suggest the diagnosis. Aortography is the gold standard for acute embolic, acute thrombotic, and nonocclusive mesenteric ischemia. Initial films should include lateral aortogram to view the ostia of the celiac and SMA. Catheters can be positioned to selectively infuse therapeutic levels of vasodilators into the SMA in patients with nonocclusive mesenteric ischemia.
Treatment of Acute Mesenteric Ischemia
Once the diagnosis is made, treatment consists of correcting any underlying medical conditions that may be contributing such as myocardial dysfunction or hypovolemia. Provided that the patient is not in heart failure, fluid resuscitation should be started and heparin anticoagulation began to prevent propagation of thrombus. There are nonsurgical options for treatment of mesenteric ischemia if the patient does not have signs of bowel infarction. Catheter-directed thrombolytic therapy has been used successfully in the treatment of early acute embolic occlusion. Angioplasty can also be performed at the time of diagnostic angiography to dilate ostial lesions of the celiac and superior mesenteric arteries.
Local infusion of vasodilators into the SMA using selective catheterization is also important. Even when perfusion can be restored, the duration of ischemia may result in infarcted bowel, requiring resection. Treatment of mesenteric venous thrombosis is nonsurgical and relies on anticoagulation as a means of reversing the hypercoaguability. There are several clear indications for surgical intervention for acute mesenteric ischemia: (1) mesenteric embolization requiring embolectomy, (2) peritonitis, (3) other clinical signs suggestive of bowel infarction, and (4) thrombotic occlusion requiring revascularization. The goals of surgery are to restore blood flow when possible and resect any bowel that is obviously necrotic. To this end, “second-look” operations may be advisable.
Chronic Mesenteric Ischemia
Chronic mesenteric ischemia is a rare disease, although the incidence may be rising with the increase in the aged population. The typical patient is elderly and vasculopathic, usually having had multiple procedures for coronary, cerebral, and peripheral vascular disease. The disease history includes weight loss, postprandial pain, and food fear. The etiology of chronic mesenteric ischemia is progressive atherosclerotic occlusion of the ostia of the mesenteric arteries near the aorta. The slowly progressive nature of this disease allows collateral circulation to compensate during the initial stages. Duplex ultrasound, the preferred screening test when the diagnosis is suspected, can usually demonstrate occlusion or stenosis of the ostia with collateral flow patterns. When duplex studies are positive or equivocal, angiography should be performed in anticipation of identifying occlusion or preocclusive stenosis of all mesenteric vessels. There are two treatment options for chronic mesenteric ischemia, with the decision based on the surgeon's estimated surgical risk for that patient. If the patient has extensive comorbidities and is not a surgical candidate, mesenteric angioplasty can be performed with the expectation of a good rate of initial technical success and relief of symptoms in the majority of patients. The downside of angioplasty is its lack of durability. Surgical revascularization is the treatment of choice.