Mesenteric Ischemia and Other Vascular Lesions

Pathophysiology

Ischemic injury to the intestine occurs when it is deprived of oxygen and other nutrients necessary to maintain cellular metabolism and integrity. Reduced blood flow may reflect poor systemic perfusion or may result from local changes in the splanchnic vasculature.

Mesenteric ischemia is a function of the following:

• State of the systemic circulation and extent of collateral blood flow to the intestine.

• Number and caliber of vessels affected, and the ability to supply the needs of the dependent segment of bowel.

• Response of the vascular bed to diminished perfusion, vasoactive substances, local humoral factors, products of cellular metabolism, and the response of the mesenteric vasculature to autonomic stimuli.

• Duration of the insult.

• Metabolic needs of the dependent segment, as dictated by its function and bacterial population.

Acute intestinal ischemia may be classified as occlusive or nonocclusive.

Occlusive mesenteric ischemia most commonly results from an arterial thrombus or an embolus of the celiac or superior mesenteric artery. Although less common, it can also be a consequence of inferior mesenteric artery occlusion or from mesenteric venous occlusion in the same distribution.

• 1.

  Arterial embolism accounts for 50% to 60% of cases and is most frequently due to a dislodged left atrial or ventricular mural thrombi, or from a valvular or proximal aortic lesion.

• 2.

  Arterial thrombosis accounts for 20% of cases and usually occurs as a superimposed phenomenon in patients with a history of chronic intestinal ischemia from atherosclerotic disease.

• 3.

  Mesenteric venous thrombosis accounts for 10% of cases and can be either idiopathic or from hypercoagulable states (e.g. hereditary, malignancy, abdominal surgery).

Nonocclusive mesenteric ischemia is thought to occur as a result of splanchnic hypoperfusion and vasoconstriction. This “low-flow” state is commonly seen in patients after an acute coronary syndrome, cardiogenic or septic shock, and in patients requiring vasopressors. It commonly affects the “watershed” areas of the colon, such as the splenic flexure and rectosigmoid junction.

Clinical Picture

The most common presentation of acute mesenteric ischemia is severe abdominal pain, initially colicky and periumbilical, then becoming diffuse and constant. One of the distinctive findings in mesenteric ischemia is that of abdominal pain that is out of proportion to examination, and initially with minimal peritoneal signs. Some patients have surprisingly normal physical findings on abdominal examination despite the severe pain.

Vomiting, anorexia, diarrhea, and constipation also occur frequently but are of little diagnostic help. Examination of the abdomen may also reveal distention; bowel sounds are often normal, even in patients with severe infarction. Gross gastrointestinal bleeding is unusual except in ischemic colitis. Leukocytosis is typical. Later in the disease course, gangrene of the bowel occurs, with diffuse peritonitis, sepsis, and shock.

Diagnosis

Different imaging modalities can be utilized to help in diagnosis. Abdominal plain films in patients with mesenteric ischemia may reveal air-fluid levels and distention. Pneumatosis intestinalis, portal venous gas, and pneumoperitoneum may also be seen with advanced disease.

Although the gold standard for diagnosis is angiography, the most common diagnostic methods of choice are computed tomography (CT) angiography and magnetic resonance (MR) angiography. CT is preferred over MR because of its lower costs, speed, and availability. However, MR angiography may be more sensitive for the diagnosis of mesenteric venous thrombosis and an option for those with an allergy to iodinated contrast.