Mesenteric ischemia - Schwartz

Mesenteric Ischemia

Mesenteric ischemia can present as one of two distinct clinical syndromes: acute mesenteric ischemia and chronic mesenteric ischemia.

Four distinct pathophysiologic mechanisms can lead to acute mesenteric ischemia:

1. Arterial embolus 2. Arterial thrombosis 3. Vasospasm (also known as nonocclusive mesenteric ischemia) 4. Venous thrombosis

Embolus is the most common cause of acute mesenteric ischemia, and is responsible for more than 50% of cases. The embolic source is usually in the heart; most often the left atrial or ventricular thrombi or valvular lesions. Indeed, up to 95% of patients with acute mesenteric ischemia due to emboli will have a documented history of cardiac disease. Embolism to the superior mesenteric artery accounts for 50% of cases; most of these emboli become wedged and cause occlusion at branch points in the mid- to distal superior mesenteric artery, usually distal to the origin of the middle colic artery. In contrast, acute occlusions due to thrombosis tend to occur in the proximal mesenteric arteries, near their origins. Acute thrombosis is usually superimposed on pre-existing atherosclerotic lesions at these sites. Nonocclusive mesenteric ischemia is the result of vasospasm and usually is diagnosed in critically ill patients receiving vasopressor agents.

Mesenteric venous thrombosis accounts for 5 to 15% of cases of acute mesenteric ischemia and involves the superior mesenteric vein in 95% of cases.⁶⁰ The inferior mesenteric vein is only rarely involved. Mesenteric venous thrombosis is classified as primary if no etiologic factor is identifiable, or as secondary if an etiologic factor, such as heritable or acquired coagulation disorders, is identified.

Regardless of the pathophysiologic mechanism, acute mesenteric ischemia can lead to intestinal mucosal sloughing within 3 hours of onset and full-thickness intestinal infarction by 6 hours.

In contrast, chronic mesenteric ischemia develops insidiously, allowing for development of collateral circulation, and, therefore, rarely leads to intestinal infarction. Chronic mesenteric arterial ischemia results from atherosclerotic lesions in the main splanchnic arteries (celiac, superior mesenteric, and inferior mesenteric arteries). In most patients with symptoms attributable to chronic mesenteric ischemia, at least two of these arteries are either occluded or severely stenosed. A chronic form of mesenteric venous thrombosis can involve the portal or splenic veins and may lead to portal hypertension, with resulting esophagogastric varices, splenomegaly, and hypersplenism.

Severe abdominal pain, out of proportion to the degree of tenderness on examination, is the hallmark of acute mesenteric ischemia, regardless of the pathophysiologic mechanism. The pain typically is perceived to be colicky and most severe in the midabdomen. Associated symptoms can include nausea, vomiting, and diarrhea. Physical findings are characteristically absent early in the course of ischemia. With the onset of bowel infarction, abdominal distention, peritonitis, and passage of bloody stools occur.

Chronic mesenteric ischemia presents insidiously. Postprandial abdominal pain is the most prevalent symptom, producing a characteristic aversion to food ("food-fear") and weight loss. These patients are often thought to have a malignancy and suffer a prolonged period of symptoms before the correct diagnosis is made.

Most patients with chronic mesenteric venous thrombosis are asymptomatic because of the presence of extensive collateral venous drainage routes; this condition is usually discovered as an incidental finding on imaging studies. However, some patients with chronic mesenteric venous thrombosis present with bleeding from esophagogastric varices.