Etiology

OCCLUSIVE DISEASE

Emboli. The SMA is the most common site of embolic occlusion although the celiac artery can be affected. There is classically an underlying cardiac problem giving rise to the organized thrombus that embolizes. This is usually atrial fibrillation or less commonly a mural thrombus from an acute myocardial infarction. A history of previous embolic events is not uncommon. Other causes of emboli include iatrogenic intra-aortic manipulations, paradoxical emboli through a septal defect, atrial myxoma or primary aortic tumors.

The history is of constant severe epigastric or periumbilical pain of sudden onset. It is frequently followed by copious vomiting and explosive diarrhea.

Typically the patient has been previously well and asymptomatic. The abdominal signs are often lacking or nonspecific, with distension in association with absent or normal bowel sounds without any signs of peritonism. This combination of severe abdominal pain out of proportion to the clinical findings is typical. Peritonism or blood in the stool or vomitus indicates severe advanced intestinal ischemia with likely infarction and is generally a late clinical feature.

The presence of proximal SMA pulsation and the distribution of intestinal ischemia are intra-operative clues for an embolus. The occlusion in embolism is usually distal to the origin of the pancreaticoduodenal and middle colic branches, which allows some blood flow to the small intestine to be maintained. The stomach, duodenum, and proximal jejunum are normal with ischemia extending to the mid transverse colon.

Thrombosis.

Thrombosis of the superior mesenteric or celiac arteries is most often associated with a preexisting atherosclerotic lesion that already compromises flow. The most common preexisting pathology found in patients with acute mesenteric thrombosis is atherosclerosis.

Many patients present with histories consistent with chronic mesenteric ischemia. Wasting, postprandial pain, and phagophobia (fear of eating) are all common.

Typically, the atherosclerotic lesion gradually compromises flow to the gut, causing a progressive worsening of symptoms. During a period of low flow, the artery thromboses, and flow to the gut is compromised.

Unlike embolic events that occur in arterial branches and result in limited bowel ischemia, thrombosis occurs at the vessel origin, resulting in extensive bowel involvement.

Superior mesenteric arterial thrombosis may occur as the result of progression of SMA stenosis that had not previously been diagnosed or treated. There is often a history of intestinal or food fear with severe weight loss, the hallmark of chronic intestinal ischemia in about 65% of patients. The typical patient is female and a heavy smoker, often with evidence of widespread arterial disease including previous myocardial infarction or daudication. As with embolic occlusion, the combination of severe abdominal pain out of proportion to the clinical findings is typical. The thrombosis of the SMA occurs at the origin of the artery.

In contrast to embolic disease, the proximal SMA pulse is absent and the distribution of intestinal ischemia is more extensive. Only the stomach, duodenum and distal colon are spared.

In the young patients, fibromuscular dysplasia can cause mesenteric arterial thrombosis with equally devastating results. Intravenous cocaine abuse is another increasing problem accounting for intestinal ischemia in the young patients. The extent of intestinal ischemia and infarction tends to be foca and less than that seen with atherosclerotic thrombosis. The mechanism of ischemia appears to be occlusive rather than due to vasospasm. Mesenteric ischemia should be considered in the differential diagnosis when evaluating a young patient with a history of cocaine abuse presenting with an acute abdomen.

Some prothrombotic states such as hyperhomocysteinemia or the 20210 A prothrombin gene mutation have resulted in primary arterial thrombosis.

Mesenteric venous thrombosis (MVT) is rare and accounts for 5% to 15% of all acute mesenteric ischemia. It is classified as primary (where no cause is recognized) or secondary. Secondary MVT may follow hypercoagulable states, portal venous stasis and hypertension, intra-abdominal infection and inflammation or malignancy, use of oral contraceptives and splenectomy. Long-term anticoagulation is required for MVT, because of the high recurrence rates. The clinical presentation is usually less acute than that of arterial occlusion.

Severe but vague abdominal pain that tends to be colicky and slowly progressive is usually present. Few abdominal signs are present except tenderness, distension and decreased bowel sounds. The pain is out of proportion to the physical findings. Fecal occult blood is present in the majority of patients.

There is a pyrexia of greater than 38 °C in 25% to 50% of patients, and 20% have a tachycardia. Leucocytosis ranges from 12000 to 29000.

Frank peritonitis is seen only when transmural infarction or perforation has occurred.

Surgical findings include blood-stained free peritoneal fluid at laparotomy. The affected bowel is cyanotic and edematous with a rubbery texture.

Mesenteric arterial pulsations are present but the veins contain fresh thrombus that extrude when the veins are cut. Infarction is most common in the mid small bowel.

FIG. A - Schematic representation of the collateral circulation of the

intestine. B - Angiosraphic appearance of arch of Riolan from superior

mesenteric artery (stented at its origin). C - Angiographic appearance of

marginal artery of Drummond. D - Initial angiogram demonstrates occlu-

ded IMA. The delayed film shows the colonic supply.

Date: 2014-12-29; view: 1143